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

[Derek Foreman]

On 2018-08-17 05:53 AM, Pekka Paalanen wrote:
> 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>

Also
Reviewed-by: Derek Foreman <derek.foreman.samsung at gmail.com>

I'll land this and the 2nd patch in the series today before rolling the
RC2 releases.

Looks like the test needs a follow up, so that can land later.

Thanks,
Derek

> 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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> 


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