From 670f928ba09b06712da34a3c44be6c8fa561fb19 Mon Sep 17 00:00:00 2001 From: Ricardo Neri Date: Fri, 27 Oct 2017 13:25:41 -0700 Subject: [PATCH] x86/insn-eval: Add utility function to get segment descriptor The segment descriptor contains information that is relevant to how linear addresses need to be computed. It contains the default size of addresses as well as the base address of the segment. Thus, given a segment selector, we ought to look at segment descriptor to correctly calculate the linear address. In protected mode, the segment selector might indicate a segment descriptor from either the global descriptor table or a local descriptor table. Both cases are considered in this function. This function is a prerequisite for functions in subsequent commits that will obtain the aforementioned attributes of the segment descriptor. Improvements-by: Borislav Petkov Signed-off-by: Ricardo Neri Signed-off-by: Thomas Gleixner Reviewed-by: Borislav Petkov Cc: "Michael S. Tsirkin" Cc: Peter Zijlstra Cc: Dave Hansen Cc: ricardo.neri@intel.com Cc: Adrian Hunter Cc: Paul Gortmaker Cc: Huang Rui Cc: Qiaowei Ren Cc: Shuah Khan Cc: Kees Cook Cc: Jonathan Corbet Cc: Jiri Slaby Cc: Dmitry Vyukov Cc: "Ravi V. Shankar" Cc: Chris Metcalf Cc: Brian Gerst Cc: Arnaldo Carvalho de Melo Cc: Andy Lutomirski Cc: Colin Ian King Cc: Chen Yucong Cc: Adam Buchbinder Cc: Vlastimil Babka Cc: Lorenzo Stoakes Cc: Masami Hiramatsu Cc: Paolo Bonzini Cc: Andrew Morton Cc: Thomas Garnier Link: https://lkml.kernel.org/r/1509135945-13762-15-git-send-email-ricardo.neri-calderon@linux.intel.com --- arch/x86/lib/insn-eval.c | 57 ++++++++++++++++++++++++++++++++++++++++ 1 file changed, 57 insertions(+) diff --git a/arch/x86/lib/insn-eval.c b/arch/x86/lib/insn-eval.c index 6a902b155f5d..d85e84091ddc 100644 --- a/arch/x86/lib/insn-eval.c +++ b/arch/x86/lib/insn-eval.c @@ -6,9 +6,13 @@ #include #include #include +#include +#include +#include #include #include #include +#include #include #undef pr_fmt @@ -468,6 +472,59 @@ static int get_reg_offset(struct insn *insn, struct pt_regs *regs, return regoff[regno]; } +/** + * get_desc() - Obtain pointer to a segment descriptor + * @sel: Segment selector + * + * Given a segment selector, obtain a pointer to the segment descriptor. + * Both global and local descriptor tables are supported. + * + * Returns: + * + * Pointer to segment descriptor on success. + * + * NULL on error. + */ +static struct desc_struct *get_desc(unsigned short sel) +{ + struct desc_ptr gdt_desc = {0, 0}; + unsigned long desc_base; + +#ifdef CONFIG_MODIFY_LDT_SYSCALL + if ((sel & SEGMENT_TI_MASK) == SEGMENT_LDT) { + struct desc_struct *desc = NULL; + struct ldt_struct *ldt; + + /* Bits [15:3] contain the index of the desired entry. */ + sel >>= 3; + + mutex_lock(¤t->active_mm->context.lock); + ldt = current->active_mm->context.ldt; + if (ldt && sel < ldt->nr_entries) + desc = &ldt->entries[sel]; + + mutex_unlock(¤t->active_mm->context.lock); + + return desc; + } +#endif + native_store_gdt(&gdt_desc); + + /* + * Segment descriptors have a size of 8 bytes. Thus, the index is + * multiplied by 8 to obtain the memory offset of the desired descriptor + * from the base of the GDT. As bits [15:3] of the segment selector + * contain the index, it can be regarded as multiplied by 8 already. + * All that remains is to clear bits [2:0]. + */ + desc_base = sel & ~(SEGMENT_RPL_MASK | SEGMENT_TI_MASK); + + if (desc_base > gdt_desc.size) + return NULL; + + return (struct desc_struct *)(gdt_desc.address + desc_base); +} + /** * insn_get_modrm_rm_off() - Obtain register in r/m part of the ModRM byte * @insn: Instruction containing the ModRM byte