@@ -222,6 +222,16 @@ static int __strp_recv(read_descriptor_t *desc, struct sk_buff *orig_skb,
if (!stm->strp.full_len) {
ssize_t len;
+ /* Can only parse if there is no offset */
+ if (unlikely(stm->strp.offset)) {
+ if (!pskb_pull(skb, stm->strp.offset)) {
+ STRP_STATS_INCR(strp->stats.mem_fail);
+ strp_parser_err(strp, -ENOMEM, desc);
+ break;
+ }
+ stm->strp.offset = 0;
+ }
+
len = (*strp->cb.parse_msg)(strp, head);
if (!len) {
@@ -249,8 +259,7 @@ static int __strp_recv(read_descriptor_t *desc, struct sk_buff *orig_skb,
STRP_STATS_INCR(strp->stats.msg_too_big);
strp_parser_err(strp, -EMSGSIZE, desc);
break;
- } else if (len <= (ssize_t)head->len -
- skb->len - stm->strp.offset) {
+ } else if (len <= (ssize_t)head->len - skb->len) {
/* Length must be into new skb (and also
* greater than zero)
*/
Offset is not well handled by strparser users right now. Out of the current strparser users, we have: - tls, that handles offset properly in parse and rcv callbacks - kcm, that handles offset in rcv but not in parse - bpf sockmap, that does not seem to handle offset anywhere Calling pskb_pull() on the skb before parsing ensures that the offset will be 0 everywhere in practice unless the user modifies it themselves like tls, as a workaround for the other two protocols. This fixes a bug whilch can be exhibited by implementing a simpe kcm parser that looks for the packet size in the first word of the packet, and sending two such packets in a single write() call on the other side: the second message will be cut at the length of the first message. Since this is a stream protocol, all the following messages will also be corrupt since it will start looking for the next offset at a wrong position. Signed-off-by: Dominique Martinet <asmadeus@codewreck.org> --- Discussions on the bug along with a (bad) reproducer can be found here: http://lkml.kernel.org/m/20180803182830.GB29193@nautica (now the problem is better understood though it's much simpler to send two messages at once than to spam and wait for tcp aggregation to do it) Two notes: - I've marked this patch v0 as we could move the pskb_pull() up to where strp.offset is set, and just always leave it at 0 in the strparser code. This will let applications that are fine dealing with a non-zero offset deal with it as they seem fit (tls writes into the offset and full_len fields behind the back of the stream parser), while still being safe for kcm/sockmap - Even with that modification I'm not totally happy with single-handedly eating the offset for strparser users which could handle it, but I'm not really familiar with the cost this really has in practice... A better fix would be to handle the offset properly in the callbacks, but frankly at least for kcm I don't see how (maybe because I'm not familiar with how bpf programs work) Another idea I had would be to write flags when registering the protocol e.g. strp->cb.flags & STRP_CAN_PARSE_WITH_OFFSET or something like that, but without an idea of the cost of that pull I don't know if it's worth doing. Anyway, comments welcome. net/strparser/strparser.c | 13 +++++++++++-- 1 file changed, 11 insertions(+), 2 deletions(-)