linux-kernel - Re: [PATCH 1/2] lib/vsprintf.c: Fix potential NULL deref in hex

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Date:	Wed, 28 Jan 2015 16:53:44 +0200
From:	Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
To:	Rasmus Villemoes <linux@...musvillemoes.dk>
Cc:	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>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] lib/vsprintf.c: Fix potential NULL deref in
 hex_string

On Wed, 2015-01-28 at 14:25 +0100, Rasmus Villemoes wrote:
> 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").

Good catch, though I don't like the implementation of fix.

What about the following?

diff --git a/lib/vsprintf.c b/lib/vsprintf.c
index 8690798..47b36ddd 100644
--- a/lib/vsprintf.c
+++ b/lib/vsprintf.c
@@ -783,11 +783,20 @@ 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;


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


-- 
Andy Shevchenko <andriy.shevchenko@...el.com>
Intel Finland Oy

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