Jakub Kicinski wrote:
> sk_msg_trim() tries to only update curr pointer if it falls into
> the trimmed region. The logic, however, does not take into the
> account pointer wrapping that sk_msg_iter_var_prev() does.
> This means that when the message was trimmed completely, the new
> curr pointer would have the value of MAX_MSG_FRAGS - 1, which is
> neither smaller than any other value, nor would it actually be
> correct.
>
> Special case the trimming to 0 length a little bit.
>
> This bug caused the TLS code to not copy all of the message, if
> zero copy filled in fewer sg entries than memcopy would need.
>
> Big thanks to Alexander Potapenko for the non-KMSAN reproducer.
>
> Fixes: d829e9c4112b ("tls: convert to generic sk_msg interface")
> Reported-by: syzbot+f8495bff23a879a6d0bd@xxxxxxxxxxxxxxxxxxxxxxxxx
> Reported-by: syzbot+6f50c99e8f6194bf363f@xxxxxxxxxxxxxxxxxxxxxxxxx
> Signed-off-by: Jakub Kicinski <jakub.kicinski@xxxxxxxxxxxxx>
> ---
> Daniel, John, does this look okay?
Thanks for the second ping!
>
> CC: Eric Biggers <ebiggers@xxxxxxxxxx>
> CC: herbert@xxxxxxxxxxxxxxxxxxx
> CC: glider@xxxxxxxxxx
> CC: linux-crypto@xxxxxxxxxxxxxxx
>
> net/core/skmsg.c | 5 ++++-
> 1 file changed, 4 insertions(+), 1 deletion(-)
>
> diff --git a/net/core/skmsg.c b/net/core/skmsg.c
> index cf390e0aa73d..c42c145216b1 100644
> --- a/net/core/skmsg.c
> +++ b/net/core/skmsg.c
> @@ -276,7 +276,10 @@ void sk_msg_trim(struct sock *sk, struct sk_msg *msg, int len)
> * However trimed data that has not yet been used in a copy op
> * does not require an update.
> */
> - if (msg->sg.curr >= i) {
> + if (!msg->sg.size) {
> + msg->sg.curr = 0;
> + msg->sg.copybreak = 0;
> + } else if (msg->sg.curr >= i) {
> msg->sg.curr = i;
> msg->sg.copybreak = msg->sg.data[i].length;
> }
> --
Its actually not sufficient. We can't directly do comparisons against curr
like this. msg->sg is a ring buffer so we have to be careful for these
types of comparisons.
Examples hopefully help explian. Consider the case with a ring layout on
entering sk_msg_trim,
0 1 2 N = MAX_MSG_FRAGS
|_|_|_|...|_|_|_|...|_|_|_|_|....|_|_|
^ ^ ^
curr end start
Start trimming from end
0 1 2 N = MAX_MSG_FRAGS
|X|X|X|...|X|X|_|...|_|_|i|X|....|X|X|
^ ^ ^
curr end start
We trim backwards through ring with sk_msg_iter_var_prev(). And its
possible to end with the result of above where 'i' is greater than curr
and greater than start leaving scatterlist elements so size != 0.
i > curr && i > start && sg.size != 0
but we wont catch it with this condition
if (msg->sg.curr >= i)
So we won't reset curr and copybreak so we have a potential issue now
where curr is pointing at data that has been trimmed.
I'll put together a fix but the correct thing to do here is a proper
ring greater than op which is not what we have there. Although, your patch
is also really a good one to have because reseting curr = 0 and
copybreak = 0 when possible keeps the ring from being fragmented which
avoids chaining when we push scatterlists down to crypto layer. So for
your patch,
Acked-By: John Fastabend <john.fastabend@xxxxxxxxx>
If it should go to net or net-next I think is probably up for debate
Nice catch!!! Can you send me the reproducer?
Thanks,
John
