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x86/insn-eval: Fix use-after-free access to LDT entry

get_desc() computes a pointer into the LDT while holding a lock that
protects the LDT from being freed, but then drops the lock and returns the
(now potentially dangling) pointer to its caller.

Fix it by giving the caller a copy of the LDT entry instead.

Fixes: 670f928 ("x86/insn-eval: Add utility function to get segment descriptor")
Cc: stable@vger.kernel.org
Signed-off-by: Jann Horn <jannh@google.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
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thejh authored and torvalds committed Jun 2, 2019
1 parent 1e1d926 commit de9f869616dd95e95c00bdd6b0fcd3421e8a4323
Showing with 24 additions and 23 deletions.
  1. +24 −23 arch/x86/lib/insn-eval.c
@@ -557,39 +557,42 @@ static int get_reg_offset_16(struct insn *insn, struct pt_regs *regs,
}

/**
* get_desc() - Obtain pointer to a segment descriptor
* get_desc() - Obtain contents of a segment descriptor
* @out: Segment descriptor contents on success
* @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.
* True on success, false on failure.
*
* NULL on error.
*/
static struct desc_struct *get_desc(unsigned short sel)
static bool get_desc(struct desc_struct *out, 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;
bool success = false;
struct ldt_struct *ldt;

/* Bits [15:3] contain the index of the desired entry. */
sel >>= 3;

mutex_lock(&current->active_mm->context.lock);
ldt = current->active_mm->context.ldt;
if (ldt && sel < ldt->nr_entries)
desc = &ldt->entries[sel];
if (ldt && sel < ldt->nr_entries) {
*out = ldt->entries[sel];
success = true;
}

mutex_unlock(&current->active_mm->context.lock);

return desc;
return success;
}
#endif
native_store_gdt(&gdt_desc);
@@ -604,9 +607,10 @@ static struct desc_struct *get_desc(unsigned short sel)
desc_base = sel & ~(SEGMENT_RPL_MASK | SEGMENT_TI_MASK);

if (desc_base > gdt_desc.size)
return NULL;
return false;

return (struct desc_struct *)(gdt_desc.address + desc_base);
*out = *(struct desc_struct *)(gdt_desc.address + desc_base);
return true;
}

/**
@@ -628,7 +632,7 @@ static struct desc_struct *get_desc(unsigned short sel)
*/
unsigned long insn_get_seg_base(struct pt_regs *regs, int seg_reg_idx)
{
struct desc_struct *desc;
struct desc_struct desc;
short sel;

sel = get_segment_selector(regs, seg_reg_idx);
@@ -666,11 +670,10 @@ unsigned long insn_get_seg_base(struct pt_regs *regs, int seg_reg_idx)
if (!sel)
return -1L;

desc = get_desc(sel);
if (!desc)
if (!get_desc(&desc, sel))
return -1L;

return get_desc_base(desc);
return get_desc_base(&desc);
}

/**
@@ -692,7 +695,7 @@ unsigned long insn_get_seg_base(struct pt_regs *regs, int seg_reg_idx)
*/
static unsigned long get_seg_limit(struct pt_regs *regs, int seg_reg_idx)
{
struct desc_struct *desc;
struct desc_struct desc;
unsigned long limit;
short sel;

@@ -706,8 +709,7 @@ static unsigned long get_seg_limit(struct pt_regs *regs, int seg_reg_idx)
if (!sel)
return 0;

desc = get_desc(sel);
if (!desc)
if (!get_desc(&desc, sel))
return 0;

/*
@@ -716,8 +718,8 @@ static unsigned long get_seg_limit(struct pt_regs *regs, int seg_reg_idx)
* not tested when checking the segment limits. In practice,
* this means that the segment ends in (limit << 12) + 0xfff.
*/
limit = get_desc_limit(desc);
if (desc->g)
limit = get_desc_limit(&desc);
if (desc.g)
limit = (limit << 12) + 0xfff;

return limit;
@@ -741,7 +743,7 @@ static unsigned long get_seg_limit(struct pt_regs *regs, int seg_reg_idx)
*/
int insn_get_code_seg_params(struct pt_regs *regs)
{
struct desc_struct *desc;
struct desc_struct desc;
short sel;

if (v8086_mode(regs))
@@ -752,19 +754,18 @@ int insn_get_code_seg_params(struct pt_regs *regs)
if (sel < 0)
return sel;

desc = get_desc(sel);
if (!desc)
if (!get_desc(&desc, sel))
return -EINVAL;

/*
* The most significant byte of the Type field of the segment descriptor
* determines whether a segment contains data or code. If this is a data
* segment, return error.
*/
if (!(desc->type & BIT(3)))
if (!(desc.type & BIT(3)))
return -EINVAL;

switch ((desc->l << 1) | desc->d) {
switch ((desc.l << 1) | desc.d) {
case 0: /*
* Legacy mode. CS.L=0, CS.D=0. Address and operand size are
* both 16-bit.

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