[PATCH v0] strparser: remove any offset before parsing messages

From: Dominique Martinet
Date: Thu Aug 09 2018 - 18:40:37 EST

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

Signed-off-by: Dominique Martinet <asmadeus@xxxxxxxxxxxxx>

Discussions on the bug along with a (bad) reproducer can be found here:
(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
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

- 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
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

Anyway, comments welcome.

net/strparser/strparser.c | 13 +++++++++++--
1 file changed, 11 insertions(+), 2 deletions(-)

diff --git a/net/strparser/strparser.c b/net/strparser/strparser.c
index 625acb27efcc..d7a3b81c3481 100644
--- a/net/strparser/strparser.c
+++ b/net/strparser/strparser.c
@@ -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_parser_err(strp, -EMSGSIZE, desc);
- } 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)