Re: [PATCH] net: 6pack: fix slab-out-of-bounds in decode_data

On Sat, Aug 14, 2021 at 05:17:44PM +0300, Pavel Skripkin wrote:

On 8/14/21 3:23 AM, Kevin Dawson wrote:

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On Fri, Aug 13, 2021 at 05:58:34PM +0300, Dan Carpenter wrote:

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On Fri, Aug 13, 2021 at 02:28:55PM +0300, Pavel Skripkin wrote:

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Syzbot reported slab-out-of bounds write in decode_data().
The problem was in missing validation checks.

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Syzbot's reproducer generated malicious input, which caused

decode_data() to be called a lot in sixpack_decode(). Since
rx_count_cooked is only 400 bytes and noone reported before,
that 400 bytes is not enough, let's just check if input is malicious
and complain about buffer overrun.

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...

diff --git a/drivers/net/hamradio/6pack.c

b/drivers/net/hamradio/6pack.c

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index fcf3af76b6d7..f4ffc2a80ab7 100644

--- a/drivers/net/hamradio/6pack.c
+++ b/drivers/net/hamradio/6pack.c

@@ -827,6 +827,12 @@ static void decode_data(struct sixpack *sp, unsigned char inbyte)
 		return;
 	}
 > +	if (sp->rx_count_cooked + 3 >= sizeof(sp->cooked_buf)) {

It should be + 2 instead of + 3.

We write three bytes.  idx, idx + 1, idx + 2.  Otherwise, good fix!

I would suggest that the statement be:

	if (sp->rx_count_cooked + 3 > sizeof(sp->cooked_buf)) {

or even, because it's a buffer overrun test:

	if (sp->rx_count_cooked > sizeof(sp->cooked_buf) - 3) {

Hmm, I think, it will be more straightforward for someone not aware about
driver details.

@Dan, can I add your Reviewed-by tag to v3 and what do you think about
Kevin's suggestion?

I don't care.  Sure.  I'm also fine with leaving it as is.  I've been
using "idx + 2 >= sizeof()" enough recently that it has become an idiom
for me.  But that's probably a bias on my part.

I guess "idx + 3 > sizeof()" is probably the most readable.  Moving
the + 3 to the other side would prevent integer overflows but we're not
concerned about that here and no need to over engineer things if it
hurts readability.

regards,
dan carpenter