linux-kernel - [PATCH v4 1/3] lib/vsprintf.c: Fix potential NULL deref in hex

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Date:	Wed,  4 Mar 2015 00:20:33 +0100
From:	Rasmus Villemoes <linux@...musvillemoes.dk>
To:	Andrew Morton <akpm@...ux-foundation.org>,
	Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
	Rasmus Villemoes <linux@...musvillemoes.dk>,
	"Peter Zijlstra (Intel)" <peterz@...radead.org>,
	Tejun Heo <tj@...nel.org>
Cc:	linux-kernel@...r.kernel.org
Subject: [PATCH v4 1/3] lib/vsprintf.c: Fix potential NULL deref in hex_string

The helper hex_string() is broken in two ways. First, it doesn't
increment buf regardless of whether there is room to print, so callers
such as kasprintf() that try to probe the correct storage to allocate
will get a too small return value. But even worse, kasprintf() (and
likely anyone else trying to find the size of the result) pass NULL
for buf and 0 for size, so we also have end == NULL. But this means
that the end-1 in hex_string() is (char*)-1, so buf < end-1 is true
and we get a NULL pointer deref. I double-checked this with a trivial
kernel module that just did a kasprintf(GFP_KERNEL, "%14ph",
"CrashBoomBang").

Nobody seems to be using %ph with kasprintf, but we might as well fix
it before it hits someone.

Acked-by: Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
Signed-off-by: Rasmus Villemoes <linux@...musvillemoes.dk>
---
 lib/vsprintf.c | 16 ++++++++++++----
 1 file changed, 12 insertions(+), 4 deletions(-)

diff --git a/lib/vsprintf.c b/lib/vsprintf.c
index b235c96167d3..a1b6487c6710 100644
--- a/lib/vsprintf.c
+++ b/lib/vsprintf.c
@@ -783,11 +783,19 @@ char *hex_string(char *buf, char *end, u8 *addr, struct printf_spec spec,
 	if (spec.field_width > 0)
 		len = min_t(int, spec.field_width, 64);

-	for (i = 0; i < len && buf < end - 1; i++) {
-		buf = hex_byte_pack(buf, addr[i]);
+	for (i = 0; i < len; ++i) {
+		if (buf < end)
+			*buf = hex_asc_hi(addr[i]);
+		++buf;
+		if (buf < end)
+			*buf = hex_asc_lo(addr[i]);
+		++buf;

-		if (buf < end && separator && i != len - 1)
-			*buf++ = separator;
+		if (separator && i != len - 1) {
+			if (buf < end)
+				*buf = separator;
+			++buf;
+		}
 	}

 	return buf;
-- 
2.1.3

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