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MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Sender: linux-crypto-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-crypto@vger.kernel.org On Thu, 31 Oct 2019 15:05:53 -0700, John Fastabend wrote: > 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@syzkaller.appspotmail.com > > Reported-by: syzbot+6f50c99e8f6194bf363f@syzkaller.appspotmail.com > > Signed-off-by: Jakub Kicinski > > --- > > Daniel, John, does this look okay? > > Thanks for the second ping! No problem, I was worried the patch got categorized as TLS and therefore lower prio ;) > > CC: Eric Biggers > > CC: herbert@gondor.apana.org.au > > CC: glider@google.com > > CC: linux-crypto@vger.kernel.org > > > > 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 see, that makes sense and explains some of the complexity! Perhaps the simplest way to go would be to adjust the curr as we go then? The comparison logic could get a little hairy. So like this: diff --git a/net/core/skmsg.c b/net/core/skmsg.c index cf390e0aa73d..c2b0f9cb589c 100644 --- a/net/core/skmsg.c +++ b/net/core/skmsg.c @@ -261,25 +261,29 @@ void sk_msg_trim(struct sock *sk, struct sk_msg *msg, int len) msg->sg.size = len; while (msg->sg.data[i].length && trim >= msg->sg.data[i].length) { + bool move_curr = msg->sg.curr == i; + trim -= msg->sg.data[i].length; sk_msg_free_elem(sk, msg, i, true); sk_msg_iter_var_prev(i); + if (move_curr) { + msg->sg.curr = i; + msg->sg.copybreak = msg->sg.data[i].length; + } if (!trim) goto out; } msg->sg.data[i].length -= trim; sk_mem_uncharge(sk, trim); -out: /* If we trim data before curr pointer update copybreak and current * so that any future copy operations start at new copy location. * However trimed data that has not yet been used in a copy op * does not require an update. */ - if (msg->sg.curr >= i) { - msg->sg.curr = i; + if (msg->sg.curr == i && msg->sg.copybreak > msg->sg.data[i].length) msg->sg.copybreak = msg->sg.data[i].length; - } +out: sk_msg_iter_var_next(i); msg->sg.end = i; } > 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 > > If it should go to net or net-next I think is probably up for debate > > Nice catch!!! Can you send me the reproducer? I was using the repro from the syzbot report: https://syzkaller.appspot.com/bug?extid=6f50c99e8f6194bf363f plus this hack from Alexander to avoid the need for KMSAN: https://lkml.kernel.org/linux-crypto/CAG_fn=UGCoDk04tL2vB981JmXgo6+-RUPmrTa3dSsK5UbZaTjA@mail.gmail.com/