[PATCH 3/3] connection: Prevent pointer overflow from large lengths.

[Pekka Paalanen]

On Tue, 14 Aug 2018 13:07:53 +0200
Michal Srb <msrb at suse.com> wrote:

> If the remote side sends sufficiently large `length` field, it will
> overflow the `p` pointer. Technically it is undefined behavior, in
> practice it makes `p < end`, so the length check passes. Attempts to
> access the data later causes crashes.
> 
> This issue manifests only on 32bit systems, but the behavior is
> undefined everywhere.
> ---
>  src/connection.c | 12 +++++++-----
>  1 file changed, 7 insertions(+), 5 deletions(-)

Hi,

this looks correct to me and should address Jann's concerns too. I also
checked that (end - p) cannot become negative.

Reviewed-by: Pekka Paalanen <pekka.paalanen at collabora.co.uk>

I'm still playing with the tests patch a bit.


Thanks,
pq

> 
> diff --git a/src/connection.c b/src/connection.c
> index 31396bc..a0f10ae 100644
> --- a/src/connection.c
> +++ b/src/connection.c
> @@ -683,7 +683,7 @@ wl_connection_demarshal(struct wl_connection *connection,
>  			struct wl_map *objects,
>  			const struct wl_message *message)
>  {
> -	uint32_t *p, *next, *end, length, id;
> +	uint32_t *p, *next, *end, length, length_in_u32, id;
>  	int fd;
>  	char *s;
>  	int i, count, num_arrays;
> @@ -739,8 +739,8 @@ wl_connection_demarshal(struct wl_connection *connection,
>  				break;
>  			}
>  
> -			next = p + div_roundup(length, sizeof *p);
> -			if (next > end) {
> +			length_in_u32 = div_roundup(length, sizeof *p);
> +			if ((uint32_t) (end - p) < length_in_u32) {
>  				wl_log("message too short, "
>  				       "object (%d), message %s(%s)\n",
>  				       closure->sender_id, message->name,
> @@ -748,6 +748,7 @@ wl_connection_demarshal(struct wl_connection *connection,
>  				errno = EINVAL;
>  				goto err;
>  			}
> +			next = p + length_in_u32;
>  
>  			s = (char *) p;
>  
> @@ -798,8 +799,8 @@ wl_connection_demarshal(struct wl_connection *connection,
>  		case 'a':
>  			length = *p++;
>  
> -			next = p + div_roundup(length, sizeof *p);
> -			if (next > end) {
> +			length_in_u32 = div_roundup(length, sizeof *p);
> +			if ((uint32_t) (end - p) < length_in_u32) {
>  				wl_log("message too short, "
>  				       "object (%d), message %s(%s)\n",
>  				       closure->sender_id, message->name,
> @@ -807,6 +808,7 @@ wl_connection_demarshal(struct wl_connection *connection,
>  				errno = EINVAL;
>  				goto err;
>  			}
> +			next = p + length_in_u32;
>  
>  			array_extra->size = length;
>  			array_extra->alloc = 0;

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