From 7aeaaad7fb98f1c68b77bd9dee0ad0e6e92cd0ff Mon Sep 17 00:00:00 2001

From: Noah Goldstein <goldstein.w.n@gmail.com>

Date: Tue, 15 Feb 2022 20:27:21 -0600

Subject: [PATCH] x86: Fix bug in strncmp-evex and strncmp-avx2 [BZ #28895]

Logic can read before the start of `s1` / `s2` if both `s1` and `s2`

are near the start of a page. To avoid having the result contimated by

these comparisons the `strcmp` variants would mask off these

comparisons. This was missing in the `strncmp` variants causing

the bug. This commit adds the masking to `strncmp` so that out of

range comparisons don't affect the result.

test-strcmp, test-strncmp, test-wcscmp, and test-wcsncmp all pass as

well a full xcheck on x86_64 linux.

Reviewed-by: H.J. Lu <hjl.tools@gmail.com>

(cherry picked from commit e108c02a5e23c8c88ce66d8705d4a24bb6b9a8bf)

---

 string/test-strncmp.c                  | 23 +++++++++++++++++++++++

 sysdeps/x86_64/multiarch/strcmp-avx2.S |  1 +

 sysdeps/x86_64/multiarch/strcmp-evex.S |  1 +

 3 files changed, 25 insertions(+)

diff --git a/string/test-strncmp.c b/string/test-strncmp.c

index 927a6daa..e61fffd9 100644

--- a/string/test-strncmp.c

+++ b/string/test-strncmp.c

@@ -403,6 +403,28 @@ check2 (void)

   free (s2);

 }

+static void

+check4 (void)

+{

+  /* To trigger bug 28895; We need 1) both s1 and s2 to be within 32 bytes of

+     the end of the page. 2) For there to be no mismatch/null byte before the

+     first page cross. 3) For length (`n`) to be large enough for one string to

+     cross the page. And 4) for there to be either mismatch/null bytes before

+     the start of the strings.  */

+

+  size_t size = 10;

+  size_t addr_mask = (getpagesize () - 1) ^ (sizeof (CHAR) - 1);

+  CHAR *s1 = (CHAR *)(buf1 + (addr_mask & 0xffa));

+  CHAR *s2 = (CHAR *)(buf2 + (addr_mask & 0xfed));

+  int exp_result;

+

+  STRCPY (s1, L ("tst-tlsmod%"));

+  STRCPY (s2, L ("tst-tls-manydynamic73mod"));

+  exp_result = SIMPLE_STRNCMP (s1, s2, size);

+  FOR_EACH_IMPL (impl, 0)

+  check_result (impl, s1, s2, size, exp_result);

+}

+

 static void

 check3 (void)

 {

@@ -445,6 +467,7 @@ test_main (void)

   check1 ();

   check2 ();

   check3 ();

+  check4 ();

   printf ("%23s", "");

   FOR_EACH_IMPL (impl, 0)

diff --git a/sysdeps/x86_64/multiarch/strcmp-avx2.S b/sysdeps/x86_64/multiarch/strcmp-avx2.S

index 04675aa4..179cc0e3 100644

--- a/sysdeps/x86_64/multiarch/strcmp-avx2.S

+++ b/sysdeps/x86_64/multiarch/strcmp-avx2.S

@@ -661,6 +661,7 @@ L(ret8):

 # ifdef USE_AS_STRNCMP

 	.p2align 4,, 10

 L(return_page_cross_end_check):

+	andl	%r10d, %ecx

 	tzcntl	%ecx, %ecx

 	leal	-VEC_SIZE(%rax, %rcx), %ecx

 	cmpl	%ecx, %edx

diff --git a/sysdeps/x86_64/multiarch/strcmp-evex.S b/sysdeps/x86_64/multiarch/strcmp-evex.S

index ed56af8e..0dfa62bd 100644

--- a/sysdeps/x86_64/multiarch/strcmp-evex.S

+++ b/sysdeps/x86_64/multiarch/strcmp-evex.S

@@ -689,6 +689,7 @@ L(ret8):

 # ifdef USE_AS_STRNCMP

 	.p2align 4,, 10

 L(return_page_cross_end_check):

+	andl	%r10d, %ecx

 	tzcntl	%ecx, %ecx

 	leal	-VEC_SIZE(%rax, %rcx, SIZE_OF_CHAR), %ecx

 #  ifdef USE_AS_WCSCMP

-- 

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