[glibc/azanella/strncmp-fix] string: Fix OOB read on generic strncmp

[Adhemerval Zanella <azanella@sourceware.org>]

https://sourceware.org/git/gitweb.cgi?p=glibc.git;h=52cb8571a851e954830a5f4b3c80f3b0f705ce1d

commit 52cb8571a851e954830a5f4b3c80f3b0f705ce1d
Author: Adhemerval Zanella <adhemerval.zanella@linaro.org>
Date:   Tue Feb 21 16:03:19 2023 -0300

    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>

Diff:
---
 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;
 	}