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x86_64, entry: Fix out of bounds read on sysenter

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commit 653bc77af6 upstream.

Rusty noticed a Really Bad Bug (tm) in my NT fix.  The entry code
reads out of bounds, causing the NT fix to be unreliable.  But, and
this is much, much worse, if your stack is somehow just below the
top of the direct map (or a hole), you read out of bounds and crash.

Excerpt from the crash:

[    1.129513] RSP: 0018:ffff88001da4bf88  EFLAGS: 00010296

  2b:*    f7 84 24 90 00 00 00     testl  $0x4000,0x90(%rsp)

That read is deterministically above the top of the stack.  I
thought I even single-stepped through this code when I wrote it to
check the offset, but I clearly screwed it up.

Fixes: 8c7aa698ba ("x86_64, entry: Filter RFLAGS.NT on entry from userspace")
Reported-by: Rusty Russell <rusty@ozlabs.org>
Signed-off-by: Andy Lutomirski <luto@amacapital.net>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
This commit is contained in:
Andy Lutomirski 2014-10-31 18:08:45 -07:00 committed by Greg Kroah-Hartman
parent b1a9c1e796
commit 621be26198
1 changed files with 1 additions and 1 deletions
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2
arch/x86/ia32/ia32entry.S
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@ -157,7 +157,7 @@ ENTRY(ia32_sysenter_target)
* ourselves. To save a few cycles, we can check whether
* NT was set instead of doing an unconditional popfq.
*/
testl $X86_EFLAGS_NT,EFLAGS(%rsp) /* saved EFLAGS match cpu */
testl $X86_EFLAGS_NT,EFLAGS-ARGOFFSET(%rsp)
jnz sysenter_fix_flags
sysenter_flags_fixed: