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![Kirill A. Shutemov](/assets/img/avatar_default.png)
commita0e6e0831c
upstream modify_ldt(2) leaves the old LDT mapped after switching over to the new one. The old LDT gets freed and the pages can be re-used. Leaving the mapping in place can have security implications. The mapping is present in the userspace page tables and Meltdown-like attacks can read these freed and possibly reused pages. It's relatively simple to fix: unmap the old LDT and flush TLB before freeing the old LDT memory. This further allows to avoid flushing the TLB in map_ldt_struct() as the slot is unmapped and flushed by unmap_ldt_struct() or has never been mapped at all. [ tglx: Massaged changelog and removed the needless line breaks ] Fixes:f55f0501cb
("x86/pti: Put the LDT in its own PGD if PTI is on") Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com> Signed-off-by: Thomas Gleixner <tglx@linutronix.de> Cc: bp@alien8.de Cc: hpa@zytor.com Cc: dave.hansen@linux.intel.com Cc: luto@kernel.org Cc: peterz@infradead.org Cc: boris.ostrovsky@oracle.com Cc: jgross@suse.com Cc: bhe@redhat.com Cc: willy@infradead.org Cc: linux-mm@kvack.org Cc: stable@vger.kernel.org Link: https://lkml.kernel.org/r/20181026122856.66224-3-kirill.shutemov@linux.intel.com Signed-off-by: Sasha Levin <sashal@kernel.org>
497 lines
12 KiB
C
497 lines
12 KiB
C
// SPDX-License-Identifier: GPL-2.0
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/*
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* Copyright (C) 1992 Krishna Balasubramanian and Linus Torvalds
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* Copyright (C) 1999 Ingo Molnar <mingo@redhat.com>
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* Copyright (C) 2002 Andi Kleen
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*
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* This handles calls from both 32bit and 64bit mode.
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*
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* Lock order:
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* contex.ldt_usr_sem
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* mmap_sem
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* context.lock
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*/
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#include <linux/errno.h>
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#include <linux/gfp.h>
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#include <linux/sched.h>
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#include <linux/string.h>
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#include <linux/mm.h>
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#include <linux/smp.h>
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#include <linux/syscalls.h>
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#include <linux/slab.h>
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#include <linux/vmalloc.h>
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#include <linux/uaccess.h>
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#include <asm/ldt.h>
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#include <asm/tlb.h>
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#include <asm/desc.h>
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#include <asm/mmu_context.h>
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#include <asm/syscalls.h>
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static void refresh_ldt_segments(void)
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{
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#ifdef CONFIG_X86_64
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unsigned short sel;
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/*
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* Make sure that the cached DS and ES descriptors match the updated
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* LDT.
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*/
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savesegment(ds, sel);
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if ((sel & SEGMENT_TI_MASK) == SEGMENT_LDT)
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loadsegment(ds, sel);
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savesegment(es, sel);
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if ((sel & SEGMENT_TI_MASK) == SEGMENT_LDT)
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loadsegment(es, sel);
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#endif
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}
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/* context.lock is held by the task which issued the smp function call */
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static void flush_ldt(void *__mm)
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{
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struct mm_struct *mm = __mm;
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if (this_cpu_read(cpu_tlbstate.loaded_mm) != mm)
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return;
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load_mm_ldt(mm);
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refresh_ldt_segments();
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}
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/* The caller must call finalize_ldt_struct on the result. LDT starts zeroed. */
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static struct ldt_struct *alloc_ldt_struct(unsigned int num_entries)
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{
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struct ldt_struct *new_ldt;
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unsigned int alloc_size;
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if (num_entries > LDT_ENTRIES)
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return NULL;
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new_ldt = kmalloc(sizeof(struct ldt_struct), GFP_KERNEL);
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if (!new_ldt)
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return NULL;
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BUILD_BUG_ON(LDT_ENTRY_SIZE != sizeof(struct desc_struct));
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alloc_size = num_entries * LDT_ENTRY_SIZE;
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/*
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* Xen is very picky: it requires a page-aligned LDT that has no
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* trailing nonzero bytes in any page that contains LDT descriptors.
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* Keep it simple: zero the whole allocation and never allocate less
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* than PAGE_SIZE.
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*/
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if (alloc_size > PAGE_SIZE)
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new_ldt->entries = vzalloc(alloc_size);
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else
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new_ldt->entries = (void *)get_zeroed_page(GFP_KERNEL);
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if (!new_ldt->entries) {
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kfree(new_ldt);
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return NULL;
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}
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/* The new LDT isn't aliased for PTI yet. */
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new_ldt->slot = -1;
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new_ldt->nr_entries = num_entries;
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return new_ldt;
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}
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/*
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* If PTI is enabled, this maps the LDT into the kernelmode and
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* usermode tables for the given mm.
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*/
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static int
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map_ldt_struct(struct mm_struct *mm, struct ldt_struct *ldt, int slot)
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{
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#ifdef CONFIG_PAGE_TABLE_ISOLATION
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bool is_vmalloc, had_top_level_entry;
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unsigned long va;
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spinlock_t *ptl;
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int i, nr_pages;
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pgd_t *pgd;
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if (!static_cpu_has(X86_FEATURE_PTI))
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return 0;
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/*
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* Any given ldt_struct should have map_ldt_struct() called at most
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* once.
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*/
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WARN_ON(ldt->slot != -1);
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/*
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* Did we already have the top level entry allocated? We can't
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* use pgd_none() for this because it doens't do anything on
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* 4-level page table kernels.
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*/
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pgd = pgd_offset(mm, LDT_BASE_ADDR);
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had_top_level_entry = (pgd->pgd != 0);
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is_vmalloc = is_vmalloc_addr(ldt->entries);
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nr_pages = DIV_ROUND_UP(ldt->nr_entries * LDT_ENTRY_SIZE, PAGE_SIZE);
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for (i = 0; i < nr_pages; i++) {
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unsigned long offset = i << PAGE_SHIFT;
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const void *src = (char *)ldt->entries + offset;
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unsigned long pfn;
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pte_t pte, *ptep;
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va = (unsigned long)ldt_slot_va(slot) + offset;
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pfn = is_vmalloc ? vmalloc_to_pfn(src) :
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page_to_pfn(virt_to_page(src));
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/*
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* Treat the PTI LDT range as a *userspace* range.
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* get_locked_pte() will allocate all needed pagetables
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* and account for them in this mm.
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*/
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ptep = get_locked_pte(mm, va, &ptl);
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if (!ptep)
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return -ENOMEM;
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/*
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* Map it RO so the easy to find address is not a primary
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* target via some kernel interface which misses a
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* permission check.
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*/
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pte = pfn_pte(pfn, __pgprot(__PAGE_KERNEL_RO & ~_PAGE_GLOBAL));
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set_pte_at(mm, va, ptep, pte);
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pte_unmap_unlock(ptep, ptl);
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}
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if (mm->context.ldt) {
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/*
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* We already had an LDT. The top-level entry should already
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* have been allocated and synchronized with the usermode
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* tables.
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*/
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WARN_ON(!had_top_level_entry);
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if (static_cpu_has(X86_FEATURE_PTI))
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WARN_ON(!kernel_to_user_pgdp(pgd)->pgd);
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} else {
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/*
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* This is the first time we're mapping an LDT for this process.
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* Sync the pgd to the usermode tables.
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*/
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WARN_ON(had_top_level_entry);
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if (static_cpu_has(X86_FEATURE_PTI)) {
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WARN_ON(kernel_to_user_pgdp(pgd)->pgd);
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set_pgd(kernel_to_user_pgdp(pgd), *pgd);
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}
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}
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ldt->slot = slot;
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#endif
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return 0;
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}
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static void unmap_ldt_struct(struct mm_struct *mm, struct ldt_struct *ldt)
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{
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#ifdef CONFIG_PAGE_TABLE_ISOLATION
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unsigned long va;
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int i, nr_pages;
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if (!ldt)
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return;
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/* LDT map/unmap is only required for PTI */
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if (!static_cpu_has(X86_FEATURE_PTI))
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return;
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nr_pages = DIV_ROUND_UP(ldt->nr_entries * LDT_ENTRY_SIZE, PAGE_SIZE);
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for (i = 0; i < nr_pages; i++) {
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unsigned long offset = i << PAGE_SHIFT;
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spinlock_t *ptl;
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pte_t *ptep;
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va = (unsigned long)ldt_slot_va(ldt->slot) + offset;
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ptep = get_locked_pte(mm, va, &ptl);
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pte_clear(mm, va, ptep);
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pte_unmap_unlock(ptep, ptl);
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}
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va = (unsigned long)ldt_slot_va(ldt->slot);
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flush_tlb_mm_range(mm, va, va + nr_pages * PAGE_SIZE, 0);
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#endif /* CONFIG_PAGE_TABLE_ISOLATION */
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}
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static void free_ldt_pgtables(struct mm_struct *mm)
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{
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#ifdef CONFIG_PAGE_TABLE_ISOLATION
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struct mmu_gather tlb;
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unsigned long start = LDT_BASE_ADDR;
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unsigned long end = start + (1UL << PGDIR_SHIFT);
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if (!static_cpu_has(X86_FEATURE_PTI))
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return;
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tlb_gather_mmu(&tlb, mm, start, end);
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free_pgd_range(&tlb, start, end, start, end);
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tlb_finish_mmu(&tlb, start, end);
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#endif
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}
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/* After calling this, the LDT is immutable. */
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static void finalize_ldt_struct(struct ldt_struct *ldt)
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{
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paravirt_alloc_ldt(ldt->entries, ldt->nr_entries);
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}
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static void install_ldt(struct mm_struct *mm, struct ldt_struct *ldt)
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{
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mutex_lock(&mm->context.lock);
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/* Synchronizes with READ_ONCE in load_mm_ldt. */
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smp_store_release(&mm->context.ldt, ldt);
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/* Activate the LDT for all CPUs using currents mm. */
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on_each_cpu_mask(mm_cpumask(mm), flush_ldt, mm, true);
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mutex_unlock(&mm->context.lock);
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}
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static void free_ldt_struct(struct ldt_struct *ldt)
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{
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if (likely(!ldt))
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return;
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paravirt_free_ldt(ldt->entries, ldt->nr_entries);
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if (ldt->nr_entries * LDT_ENTRY_SIZE > PAGE_SIZE)
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vfree_atomic(ldt->entries);
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else
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free_page((unsigned long)ldt->entries);
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kfree(ldt);
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}
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/*
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* Called on fork from arch_dup_mmap(). Just copy the current LDT state,
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* the new task is not running, so nothing can be installed.
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*/
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int ldt_dup_context(struct mm_struct *old_mm, struct mm_struct *mm)
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{
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struct ldt_struct *new_ldt;
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int retval = 0;
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if (!old_mm)
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return 0;
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mutex_lock(&old_mm->context.lock);
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if (!old_mm->context.ldt)
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goto out_unlock;
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new_ldt = alloc_ldt_struct(old_mm->context.ldt->nr_entries);
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if (!new_ldt) {
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retval = -ENOMEM;
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goto out_unlock;
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}
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memcpy(new_ldt->entries, old_mm->context.ldt->entries,
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new_ldt->nr_entries * LDT_ENTRY_SIZE);
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finalize_ldt_struct(new_ldt);
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retval = map_ldt_struct(mm, new_ldt, 0);
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if (retval) {
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free_ldt_pgtables(mm);
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free_ldt_struct(new_ldt);
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goto out_unlock;
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}
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mm->context.ldt = new_ldt;
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out_unlock:
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mutex_unlock(&old_mm->context.lock);
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return retval;
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}
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/*
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* No need to lock the MM as we are the last user
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*
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* 64bit: Don't touch the LDT register - we're already in the next thread.
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*/
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void destroy_context_ldt(struct mm_struct *mm)
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{
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free_ldt_struct(mm->context.ldt);
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mm->context.ldt = NULL;
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}
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void ldt_arch_exit_mmap(struct mm_struct *mm)
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{
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free_ldt_pgtables(mm);
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}
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static int read_ldt(void __user *ptr, unsigned long bytecount)
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{
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struct mm_struct *mm = current->mm;
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unsigned long entries_size;
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int retval;
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down_read(&mm->context.ldt_usr_sem);
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if (!mm->context.ldt) {
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retval = 0;
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goto out_unlock;
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}
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if (bytecount > LDT_ENTRY_SIZE * LDT_ENTRIES)
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bytecount = LDT_ENTRY_SIZE * LDT_ENTRIES;
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entries_size = mm->context.ldt->nr_entries * LDT_ENTRY_SIZE;
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if (entries_size > bytecount)
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entries_size = bytecount;
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if (copy_to_user(ptr, mm->context.ldt->entries, entries_size)) {
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retval = -EFAULT;
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goto out_unlock;
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}
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if (entries_size != bytecount) {
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/* Zero-fill the rest and pretend we read bytecount bytes. */
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if (clear_user(ptr + entries_size, bytecount - entries_size)) {
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retval = -EFAULT;
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goto out_unlock;
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}
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}
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retval = bytecount;
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out_unlock:
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up_read(&mm->context.ldt_usr_sem);
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return retval;
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}
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static int read_default_ldt(void __user *ptr, unsigned long bytecount)
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{
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/* CHECKME: Can we use _one_ random number ? */
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#ifdef CONFIG_X86_32
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unsigned long size = 5 * sizeof(struct desc_struct);
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#else
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unsigned long size = 128;
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#endif
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if (bytecount > size)
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bytecount = size;
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if (clear_user(ptr, bytecount))
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return -EFAULT;
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return bytecount;
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}
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static int write_ldt(void __user *ptr, unsigned long bytecount, int oldmode)
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{
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struct mm_struct *mm = current->mm;
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struct ldt_struct *new_ldt, *old_ldt;
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unsigned int old_nr_entries, new_nr_entries;
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struct user_desc ldt_info;
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struct desc_struct ldt;
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int error;
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error = -EINVAL;
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if (bytecount != sizeof(ldt_info))
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goto out;
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error = -EFAULT;
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if (copy_from_user(&ldt_info, ptr, sizeof(ldt_info)))
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goto out;
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error = -EINVAL;
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if (ldt_info.entry_number >= LDT_ENTRIES)
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goto out;
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if (ldt_info.contents == 3) {
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if (oldmode)
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goto out;
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if (ldt_info.seg_not_present == 0)
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goto out;
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}
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if ((oldmode && !ldt_info.base_addr && !ldt_info.limit) ||
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LDT_empty(&ldt_info)) {
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/* The user wants to clear the entry. */
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memset(&ldt, 0, sizeof(ldt));
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} else {
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if (!IS_ENABLED(CONFIG_X86_16BIT) && !ldt_info.seg_32bit) {
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error = -EINVAL;
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goto out;
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}
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fill_ldt(&ldt, &ldt_info);
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if (oldmode)
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ldt.avl = 0;
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}
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if (down_write_killable(&mm->context.ldt_usr_sem))
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return -EINTR;
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old_ldt = mm->context.ldt;
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old_nr_entries = old_ldt ? old_ldt->nr_entries : 0;
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new_nr_entries = max(ldt_info.entry_number + 1, old_nr_entries);
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error = -ENOMEM;
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new_ldt = alloc_ldt_struct(new_nr_entries);
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if (!new_ldt)
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goto out_unlock;
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if (old_ldt)
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memcpy(new_ldt->entries, old_ldt->entries, old_nr_entries * LDT_ENTRY_SIZE);
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new_ldt->entries[ldt_info.entry_number] = ldt;
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finalize_ldt_struct(new_ldt);
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/*
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* If we are using PTI, map the new LDT into the userspace pagetables.
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* If there is already an LDT, use the other slot so that other CPUs
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* will continue to use the old LDT until install_ldt() switches
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* them over to the new LDT.
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*/
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error = map_ldt_struct(mm, new_ldt, old_ldt ? !old_ldt->slot : 0);
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if (error) {
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/*
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* This only can fail for the first LDT setup. If an LDT is
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* already installed then the PTE page is already
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* populated. Mop up a half populated page table.
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*/
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if (!WARN_ON_ONCE(old_ldt))
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free_ldt_pgtables(mm);
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free_ldt_struct(new_ldt);
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goto out_unlock;
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}
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install_ldt(mm, new_ldt);
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unmap_ldt_struct(mm, old_ldt);
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free_ldt_struct(old_ldt);
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error = 0;
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out_unlock:
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up_write(&mm->context.ldt_usr_sem);
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out:
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return error;
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}
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SYSCALL_DEFINE3(modify_ldt, int , func , void __user * , ptr ,
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unsigned long , bytecount)
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{
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int ret = -ENOSYS;
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switch (func) {
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case 0:
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ret = read_ldt(ptr, bytecount);
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break;
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case 1:
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ret = write_ldt(ptr, bytecount, 1);
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break;
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case 2:
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ret = read_default_ldt(ptr, bytecount);
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break;
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case 0x11:
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ret = write_ldt(ptr, bytecount, 0);
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break;
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}
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/*
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* The SYSCALL_DEFINE() macros give us an 'unsigned long'
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* return type, but tht ABI for sys_modify_ldt() expects
|
|
* 'int'. This cast gives us an int-sized value in %rax
|
|
* for the return code. The 'unsigned' is necessary so
|
|
* the compiler does not try to sign-extend the negative
|
|
* return codes into the high half of the register when
|
|
* taking the value from int->long.
|
|
*/
|
|
return (unsigned int)ret;
|
|
}
|