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![Thomas Gleixner](/assets/img/avatar_default.png)
commit 9f5cb6b32d
upstream.
Now that the LDT mapping is in a known area when PAGE_TABLE_ISOLATION is
enabled its a primary target for attacks, if a user space interface fails
to validate a write address correctly. That can never happen, right?
The SDM states:
If the segment descriptors in the GDT or an LDT are placed in ROM, the
processor can enter an indefinite loop if software or the processor
attempts to update (write to) the ROM-based segment descriptors. To
prevent this problem, set the accessed bits for all segment descriptors
placed in a ROM. Also, remove operating-system or executive code that
attempts to modify segment descriptors located in ROM.
So its a valid approach to set the ACCESS bit when setting up the LDT entry
and to map the table RO. Fixup the selftest so it can handle that new mode.
Remove the manual ACCESS bit setter in set_tls_desc() as this is now
pointless. Folded the patch from Peter Ziljstra.
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Josh Poimboeuf <jpoimboe@redhat.com>
Cc: Juergen Gross <jgross@suse.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
316 lines
7.5 KiB
C
316 lines
7.5 KiB
C
// SPDX-License-Identifier: GPL-2.0
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#include <linux/kernel.h>
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#include <linux/errno.h>
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#include <linux/sched.h>
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#include <linux/user.h>
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#include <linux/regset.h>
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#include <linux/syscalls.h>
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#include <linux/uaccess.h>
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#include <asm/desc.h>
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#include <asm/ldt.h>
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#include <asm/processor.h>
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#include <asm/proto.h>
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#include "tls.h"
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/*
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* sys_alloc_thread_area: get a yet unused TLS descriptor index.
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*/
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static int get_free_idx(void)
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{
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struct thread_struct *t = ¤t->thread;
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int idx;
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for (idx = 0; idx < GDT_ENTRY_TLS_ENTRIES; idx++)
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if (desc_empty(&t->tls_array[idx]))
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return idx + GDT_ENTRY_TLS_MIN;
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return -ESRCH;
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}
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static bool tls_desc_okay(const struct user_desc *info)
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{
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/*
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* For historical reasons (i.e. no one ever documented how any
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* of the segmentation APIs work), user programs can and do
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* assume that a struct user_desc that's all zeros except for
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* entry_number means "no segment at all". This never actually
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* worked. In fact, up to Linux 3.19, a struct user_desc like
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* this would create a 16-bit read-write segment with base and
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* limit both equal to zero.
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*
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* That was close enough to "no segment at all" until we
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* hardened this function to disallow 16-bit TLS segments. Fix
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* it up by interpreting these zeroed segments the way that they
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* were almost certainly intended to be interpreted.
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*
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* The correct way to ask for "no segment at all" is to specify
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* a user_desc that satisfies LDT_empty. To keep everything
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* working, we accept both.
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*
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* Note that there's a similar kludge in modify_ldt -- look at
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* the distinction between modes 1 and 0x11.
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*/
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if (LDT_empty(info) || LDT_zero(info))
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return true;
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/*
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* espfix is required for 16-bit data segments, but espfix
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* only works for LDT segments.
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*/
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if (!info->seg_32bit)
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return false;
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/* Only allow data segments in the TLS array. */
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if (info->contents > 1)
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return false;
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/*
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* Non-present segments with DPL 3 present an interesting attack
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* surface. The kernel should handle such segments correctly,
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* but TLS is very difficult to protect in a sandbox, so prevent
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* such segments from being created.
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*
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* If userspace needs to remove a TLS entry, it can still delete
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* it outright.
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*/
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if (info->seg_not_present)
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return false;
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return true;
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}
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static void set_tls_desc(struct task_struct *p, int idx,
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const struct user_desc *info, int n)
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{
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struct thread_struct *t = &p->thread;
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struct desc_struct *desc = &t->tls_array[idx - GDT_ENTRY_TLS_MIN];
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int cpu;
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/*
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* We must not get preempted while modifying the TLS.
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*/
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cpu = get_cpu();
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while (n-- > 0) {
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if (LDT_empty(info) || LDT_zero(info))
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memset(desc, 0, sizeof(*desc));
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else
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fill_ldt(desc, info);
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++info;
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++desc;
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}
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if (t == ¤t->thread)
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load_TLS(t, cpu);
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put_cpu();
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}
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/*
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* Set a given TLS descriptor:
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*/
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int do_set_thread_area(struct task_struct *p, int idx,
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struct user_desc __user *u_info,
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int can_allocate)
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{
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struct user_desc info;
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unsigned short __maybe_unused sel, modified_sel;
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if (copy_from_user(&info, u_info, sizeof(info)))
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return -EFAULT;
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if (!tls_desc_okay(&info))
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return -EINVAL;
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if (idx == -1)
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idx = info.entry_number;
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/*
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* index -1 means the kernel should try to find and
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* allocate an empty descriptor:
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*/
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if (idx == -1 && can_allocate) {
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idx = get_free_idx();
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if (idx < 0)
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return idx;
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if (put_user(idx, &u_info->entry_number))
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return -EFAULT;
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}
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if (idx < GDT_ENTRY_TLS_MIN || idx > GDT_ENTRY_TLS_MAX)
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return -EINVAL;
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set_tls_desc(p, idx, &info, 1);
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/*
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* If DS, ES, FS, or GS points to the modified segment, forcibly
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* refresh it. Only needed on x86_64 because x86_32 reloads them
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* on return to user mode.
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*/
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modified_sel = (idx << 3) | 3;
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if (p == current) {
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#ifdef CONFIG_X86_64
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savesegment(ds, sel);
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if (sel == modified_sel)
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loadsegment(ds, sel);
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savesegment(es, sel);
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if (sel == modified_sel)
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loadsegment(es, sel);
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savesegment(fs, sel);
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if (sel == modified_sel)
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loadsegment(fs, sel);
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savesegment(gs, sel);
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if (sel == modified_sel)
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load_gs_index(sel);
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#endif
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#ifdef CONFIG_X86_32_LAZY_GS
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savesegment(gs, sel);
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if (sel == modified_sel)
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loadsegment(gs, sel);
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#endif
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} else {
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#ifdef CONFIG_X86_64
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if (p->thread.fsindex == modified_sel)
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p->thread.fsbase = info.base_addr;
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if (p->thread.gsindex == modified_sel)
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p->thread.gsbase = info.base_addr;
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#endif
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}
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return 0;
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}
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SYSCALL_DEFINE1(set_thread_area, struct user_desc __user *, u_info)
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{
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return do_set_thread_area(current, -1, u_info, 1);
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}
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/*
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* Get the current Thread-Local Storage area:
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*/
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static void fill_user_desc(struct user_desc *info, int idx,
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const struct desc_struct *desc)
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{
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memset(info, 0, sizeof(*info));
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info->entry_number = idx;
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info->base_addr = get_desc_base(desc);
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info->limit = get_desc_limit(desc);
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info->seg_32bit = desc->d;
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info->contents = desc->type >> 2;
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info->read_exec_only = !(desc->type & 2);
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info->limit_in_pages = desc->g;
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info->seg_not_present = !desc->p;
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info->useable = desc->avl;
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#ifdef CONFIG_X86_64
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info->lm = desc->l;
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#endif
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}
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int do_get_thread_area(struct task_struct *p, int idx,
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struct user_desc __user *u_info)
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{
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struct user_desc info;
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if (idx == -1 && get_user(idx, &u_info->entry_number))
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return -EFAULT;
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if (idx < GDT_ENTRY_TLS_MIN || idx > GDT_ENTRY_TLS_MAX)
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return -EINVAL;
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fill_user_desc(&info, idx,
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&p->thread.tls_array[idx - GDT_ENTRY_TLS_MIN]);
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if (copy_to_user(u_info, &info, sizeof(info)))
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return -EFAULT;
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return 0;
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}
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SYSCALL_DEFINE1(get_thread_area, struct user_desc __user *, u_info)
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{
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return do_get_thread_area(current, -1, u_info);
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}
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int regset_tls_active(struct task_struct *target,
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const struct user_regset *regset)
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{
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struct thread_struct *t = &target->thread;
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int n = GDT_ENTRY_TLS_ENTRIES;
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while (n > 0 && desc_empty(&t->tls_array[n - 1]))
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--n;
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return n;
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}
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int regset_tls_get(struct task_struct *target, const struct user_regset *regset,
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unsigned int pos, unsigned int count,
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void *kbuf, void __user *ubuf)
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{
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const struct desc_struct *tls;
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if (pos >= GDT_ENTRY_TLS_ENTRIES * sizeof(struct user_desc) ||
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(pos % sizeof(struct user_desc)) != 0 ||
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(count % sizeof(struct user_desc)) != 0)
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return -EINVAL;
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pos /= sizeof(struct user_desc);
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count /= sizeof(struct user_desc);
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tls = &target->thread.tls_array[pos];
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if (kbuf) {
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struct user_desc *info = kbuf;
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while (count-- > 0)
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fill_user_desc(info++, GDT_ENTRY_TLS_MIN + pos++,
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tls++);
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} else {
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struct user_desc __user *u_info = ubuf;
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while (count-- > 0) {
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struct user_desc info;
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fill_user_desc(&info, GDT_ENTRY_TLS_MIN + pos++, tls++);
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if (__copy_to_user(u_info++, &info, sizeof(info)))
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return -EFAULT;
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}
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}
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return 0;
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}
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int regset_tls_set(struct task_struct *target, const struct user_regset *regset,
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unsigned int pos, unsigned int count,
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const void *kbuf, const void __user *ubuf)
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{
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struct user_desc infobuf[GDT_ENTRY_TLS_ENTRIES];
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const struct user_desc *info;
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int i;
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if (pos >= GDT_ENTRY_TLS_ENTRIES * sizeof(struct user_desc) ||
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(pos % sizeof(struct user_desc)) != 0 ||
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(count % sizeof(struct user_desc)) != 0)
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return -EINVAL;
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if (kbuf)
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info = kbuf;
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else if (__copy_from_user(infobuf, ubuf, count))
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return -EFAULT;
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else
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info = infobuf;
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for (i = 0; i < count / sizeof(struct user_desc); i++)
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if (!tls_desc_okay(info + i))
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return -EINVAL;
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set_tls_desc(target,
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GDT_ENTRY_TLS_MIN + (pos / sizeof(struct user_desc)),
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info, count / sizeof(struct user_desc));
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return 0;
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}
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