[PATCH v2] string: Fix OOB read on generic strncmp

[Adhemerval Zanella <adhemerval.zanella@linaro.org>]

For unaligned case, reading ahead can only be done if parting reads
matches the aligned input.

Also extend the stratcliff tests to check such cases.

Checked on x86_64-linux-gnu, i686-linux-gnu, powerpc64-linux-gnu,
and powerpc-linux-gnu by removing the arch-specific assembly
implementation and disabling multi-arch (it covers both LE and BE
for 64 and 32 bits).
---
 string/stratcliff.c | 17 ++++++++++++++++-
 string/strncmp.c    | 13 ++++++++++++-
 2 files changed, 28 insertions(+), 2 deletions(-)

diff --git a/string/stratcliff.c b/string/stratcliff.c
index 74d64cc03d..88ac787088 100644
--- a/string/stratcliff.c
+++ b/string/stratcliff.c
@@ -401,12 +401,27 @@ do_test (void)
 		result = 1;
 	      }
 
-	    if (STRNCMP (dest + nchars - outer, adr + middle, outer) <= 0)
+	    /* Also check for size larger than the string.  */
+	    if (STRNCMP (adr + middle, dest + nchars - outer, outer + 99) >= 0)
 	      {
 		printf ("%s 2 flunked for outer = %zu, middle = %zu, full\n",
+			STRINGIFY (STRNCMP), outer + 99, middle);
+		result = 1;
+	      }
+
+	    if (STRNCMP (dest + nchars - outer, adr + middle, outer) <= 0)
+	      {
+		printf ("%s 3 flunked for outer = %zu, middle = %zu, full\n",
 			STRINGIFY (STRNCMP), outer, middle);
 		result = 1;
 	      }
+
+	    if (STRNCMP (dest + nchars - outer, adr + middle, outer + 99) <= 0)
+	      {
+		printf ("%s 4 flunked for outer = %zu, middle = %zu, full\n",
+			STRINGIFY (STRNCMP), outer + 99, middle);
+		result = 1;
+	      }
 	  }
 
       /* strncpy/wcsncpy tests */
diff --git a/string/strncmp.c b/string/strncmp.c
index 4c8bf36bb9..751bf53d55 100644
--- a/string/strncmp.c
+++ b/string/strncmp.c
@@ -73,7 +73,11 @@ strncmp_unaligned_loop (const op_t *x1, const op_t *x2, op_t w1, uintptr_t ofs,
   uintptr_t sh_2 = sizeof(op_t) * CHAR_BIT - sh_1;
 
   op_t w2 = MERGE (w2a, sh_1, (op_t)-1, sh_2);
-  if (!has_zero (w2) && n > (sizeof (op_t) - ofs))
+
+  /* Reading ahead is wrong if w1 and w2 already differs.  */
+  op_t w1a = MERGE (w1, 0, (op_t)-1, sh_2);
+
+  if (!has_zero (w2) && w2 == w1a && n >= (sizeof (op_t) - ofs))
     {
       op_t w2b;
 
@@ -90,6 +94,13 @@ strncmp_unaligned_loop (const op_t *x1, const op_t *x2, op_t w1, uintptr_t ofs,
 	  if (has_zero (w2b) || n <= (sizeof (op_t) - ofs))
 	    break;
 	  w1 = *x1++;
+
+	  /* Reading ahead is wrong if w1 and w2 already differs.  */
+	  w2 = MERGE (w2b, sh_1, (op_t)-1, sh_2);
+	  w1a = MERGE (w1, 0, (op_t)-1, sh_2);
+	  if (w2 != w1a)
+	    return final_cmp (w1a, w2, n);
+
 	  w2a = w2b;
 	}
 
-- 
2.34.1