string: Fix OOB read on generic strncmp

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).

Reviewed-by: Szabolcs Nagy <szabolcs.nagy@arm.com>

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;
 	}