netdev - [patch] isdn: avm: potential signedness issue in loading firmware

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Message-ID: <20140509115437.GA31764@mwanda>
Date:	Fri, 9 May 2014 14:54:37 +0300
From:	Dan Carpenter <dan.carpenter@...cle.com>
To:	Karsten Keil <isdn@...ux-pingi.de>
Cc:	netdev@...r.kernel.org, security@...nel.org,
	Henry Hoggard <henry@...ryhoggard.co.uk>
Subject: [patch] isdn: avm: potential signedness issue in loading firmware

The concern here is that a negative value for "len" could lead underflow
the "while (left > FWBUF_SIZE) {" test and lead to memory corruption.
I do not believe this is possible because we test for negatives in
old_capi_manufacturer(), "if (ldef.t4file.len <= 0) {".

But it sort of makes the code nicer to only deal with positive lengths
so I have made it unsigned.  The length is still not capped and so if we
get a too large value, we keep on writing it out to the firmware byte by
byte until the writes start failing.

Loading the firmware requires CAP_SYS_ADMIN.

Reported-by: Henry Hoggard <henry@...ryhoggard.co.uk>
Signed-off-by: Dan Carpenter <dan.carpenter@...cle.com>

diff --git a/drivers/isdn/hardware/avm/b1.c b/drivers/isdn/hardware/avm/b1.c
index 4d9b195..7f2ed49 100644
--- a/drivers/isdn/hardware/avm/b1.c
+++ b/drivers/isdn/hardware/avm/b1.c
@@ -153,7 +153,8 @@ int b1_load_t4file(avmcard *card, capiloaddatapart *t4file)
 {
 	unsigned char buf[FWBUF_SIZE];
 	unsigned char *dp;
-	int i, left;
+	unsigned int left;
+	int i;
 	unsigned int base = card->port;

 	dp = t4file->data;
--
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