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@rasmusvillemoes.dk>
Acked-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

diff --git a/lib/vsprintf.c b/lib/vsprintf.c
index 3ab8c9cf3980dc0228d3c5a879da86cb88d67361..4da1e7aaf9d55a8c873311f0cff13e9404325113 100644 (file)
--- a/lib/vsprintf.c
+++ b/lib/vsprintf.c
@@ -777,11 +777,19 @@ 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;