linux-kernel - [PATCH v2 1/3] lib/vsprintf.c: Fix potential NULL deref in hex

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Date:	Thu, 29 Jan 2015 11:03:18 +0100
From:	Rasmus Villemoes <linux@...musvillemoes.dk>
To:	Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Jiri Kosina <jkosina@...e.cz>,
	Randy Dunlap <rdunlap@...radead.org>,
	Fabian Frederick <fabf@...net.be>,
	Bjorn Helgaas <bhelgaas@...gle.com>,
	Ryan Mallon <rmallon@...il.com>,
	Masanari Iida <standby24x7@...il.com>
Cc:	Rasmus Villemoes <linux@...musvillemoes.dk>,
	linux-kernel@...r.kernel.org
Subject: [PATCH v2 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.

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 ec337f64f52d..3568e3906777 100644
--- a/lib/vsprintf.c
+++ b/lib/vsprintf.c
@@ -782,11 +782,19 @@ 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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