linux-kernel - [PATCH] vsprintf: Remove accidental VLA usage

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Date:   Wed, 7 Mar 2018 15:07:14 -0800
From:   Kees Cook <keescook@...omium.org>
To:     Andrew Morton <akpm@...ux-foundation.org>
Cc:     "Tobin C. Harding" <me@...in.cc>, Jonathan Corbet <corbet@....net>,
        Pantelis Antoniou <pantelis.antoniou@...sulko.com>,
        "Steven Rostedt (VMware)" <rostedt@...dmis.org>,
        kernel-hardening@...ts.openwall.com, linux-kernel@...r.kernel.org,
        "Gustavo A. R. Silva" <gustavo@...eddedor.com>
Subject: [PATCH] vsprintf: Remove accidental VLA usage

The "sym" calculation is actually a fixed size, but since the max()
macro uses some extensive tricks for safety, it ends up looking like a
variable size. This replaces max() with a simple max macro which is
sufficient for the calculation of the array size.

Seen with -Wvla. Fixed as part of the directive to remove all VLAs from
the kernel: https://lkml.org/lkml/2018/3/7/621

Signed-off-by: Kees Cook <keescook@...omium.org>
---
 lib/vsprintf.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/lib/vsprintf.c b/lib/vsprintf.c
index d7a708f82559..f420ab1477cb 100644
--- a/lib/vsprintf.c
+++ b/lib/vsprintf.c
@@ -744,8 +744,9 @@ char *resource_string(char *buf, char *end, struct resource *res,
 #define FLAG_BUF_SIZE		(2 * sizeof(res->flags))
 #define DECODED_BUF_SIZE	sizeof("[mem - 64bit pref window disabled]")
 #define RAW_BUF_SIZE		sizeof("[mem - flags 0x]")
-	char sym[max(2*RSRC_BUF_SIZE + DECODED_BUF_SIZE,
-		     2*RSRC_BUF_SIZE + FLAG_BUF_SIZE + RAW_BUF_SIZE)];
+#define SIMPLE_MAX(x, y)	((x) > (y) ? (x) : (y))
+	char sym[SIMPLE_MAX(2*RSRC_BUF_SIZE + DECODED_BUF_SIZE,
+			    2*RSRC_BUF_SIZE + FLAG_BUF_SIZE + RAW_BUF_SIZE)];
 
 	char *p = sym, *pend = sym + sizeof(sym);
 	int decode = (fmt[0] == 'R') ? 1 : 0;
-- 
2.7.4


-- 
Kees Cook
Pixel Security