lib/test_printf.c: check for out-of-bound writes

Add a few padding bytes on either side of the test buffer, and check
that these (and the part of the buffer not used) are untouched by
vsnprintf.

Signed-off-by: Rasmus Villemoes <linux@rasmusvillemoes.dk>
Acked-by: Kees Cook <keescook@chromium.org>
Cc: Al Viro <viro@ZenIV.linux.org.uk>
Cc: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Joe Perches <joe@perches.com>
Cc: Maurizio Lombardi <mlombard@redhat.com>
Cc: Tejun Heo <tj@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 19 insertions, 5 deletions

diff --git a/lib/test_printf.c b/lib/test_printf.c
index 9232a2add28c..1ce1a1dd8faf 100644
--- a/lib/test_printf.c
+++ b/lib/test_printf.c
@@ -16,6 +16,7 @@
 #include <linux/in.h>
 
 #define BUF_SIZE 256
+#define PAD_SIZE 16
 #define FILL_CHAR '$'
 
 #define PTR1 ((void*)0x01234567)
@@ -39,6 +40,7 @@
 static unsigned total_tests __initdata;
 static unsigned failed_tests __initdata;
 static char *test_buffer __initdata;
+static char *alloced_buffer __initdata;
 
 static int __printf(4, 0) __init
 do_test(int bufsize, const char *expect, int elen,
@@ -49,7 +51,7 @@ do_test(int bufsize, const char *expect, int elen,
 
         total_tests++;
 
-        memset(test_buffer, FILL_CHAR, BUF_SIZE);
+        memset(alloced_buffer, FILL_CHAR, BUF_SIZE + 2*PAD_SIZE);
         va_copy(aq, ap);
         ret = vsnprintf(test_buffer, bufsize, fmt, aq);
         va_end(aq);
@@ -60,8 +62,13 @@ do_test(int bufsize, const char *expect, int elen,
                 return 1;
         }
 
+        if (memchr_inv(alloced_buffer, FILL_CHAR, PAD_SIZE)) {
+                pr_warn("vsnprintf(buf, %d, \"%s\", ...) wrote before buffer\n", bufsize, fmt);
+                return 1;
+        }
+
         if (!bufsize) {
-                if (memchr_inv(test_buffer, FILL_CHAR, BUF_SIZE)) {
+                if (memchr_inv(test_buffer, FILL_CHAR, BUF_SIZE + PAD_SIZE)) {
                         pr_warn("vsnprintf(buf, 0, \"%s\", ...) wrote to buffer\n",
                                 fmt);
                         return 1;
@@ -76,6 +83,12 @@ do_test(int bufsize, const char *expect, int elen,
                 return 1;
         }
 
+        if (memchr_inv(test_buffer + written + 1, FILL_CHAR, BUF_SIZE + PAD_SIZE - (written + 1))) {
+                pr_warn("vsnprintf(buf, %d, \"%s\", ...) wrote beyond the nul-terminator\n",
+                        bufsize, fmt);
+                return 1;
+        }
+
         if (memcmp(test_buffer, expect, written)) {
                 pr_warn("vsnprintf(buf, %d, \"%s\", ...) wrote '%s', expected '%.*s'\n",
                         bufsize, fmt, test_buffer, written, expect);
@@ -342,16 +355,17 @@ test_pointer(void)
 static int __init
 test_printf_init(void)
 {
-        test_buffer = kmalloc(BUF_SIZE, GFP_KERNEL);
-        if (!test_buffer)
+        alloced_buffer = kmalloc(BUF_SIZE + 2*PAD_SIZE, GFP_KERNEL);
+        if (!alloced_buffer)
                 return -ENOMEM;
+        test_buffer = alloced_buffer + PAD_SIZE;
 
         test_basic();
         test_number();
         test_string();
         test_pointer();
 
-        kfree(test_buffer);
+        kfree(alloced_buffer);
 
         if (failed_tests == 0)
                 pr_info("all %u tests passed\n", total_tests);