Jakub Kicinski wrote:
> 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@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!
>
> No problem, I was worried the patch got categorized as TLS and therefore
> lower prio ;)
Nope close to the top of the list here.
>
> > > 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 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:
I don't think the comparison is too bad. Working it out live here. First
do a bit of case analysis, We have 3 pointers so there are 3!=6 possible
arrangements (permutations),
1. S,C,E 6. S,E,C
5. C,S,E 2. C,E,S
3. E,S,C 4. E,C,S
Case 1: Normal case start < curr < end
0 1 2 N = MAX_MSG_FRAGS
|_|_|_|...|_|_|_|...|_|_|_|_|....|_|_|
^ ^ ^
start curr end
if (start < end && i < curr)
curr = i;
Case 2: curr < end < start (in absolute index terms)
0 1 2 N = MAX_MSG_FRAGS
|_|_|_|...|_|_|_|...|_|_|_|_|....|_|_|
^ ^ ^
curr end start
if (end < start && (i < curr || i > start))
curr = i
Case 3: end < start < curr
0 1 2 N = MAX_MSG_FRAGS
|_|_|_|...|_|_|_|...|_|_|_|_|....|_|_|
^ ^ ^
end start curr
if (end < start && (i < curr)
curr = i;
Case 4: end < curr < start
0 1 2 N = MAX_MSG_FRAGS
|_|_|_|...|_|_|_|...|_|_|_|_|....|_|_|
^ ^ ^
end curr start
(nonsense curr would be invalid here it must be between the start and end)
Case 5: curr < start < end
0 1 2 N = MAX_MSG_FRAGS
|_|_|_|...|_|_|_|...|_|_|_|_|....|_|_|
^ ^ ^
curr start end
(nonsense curr would be invalid here it must be between the start and end)
Case 6: start < end < curr
0 1 2 N = MAX_MSG_FRAGS
|_|_|_|...|_|_|_|...|_|_|_|_|....|_|_|
^ ^ ^
start end curr
(nonsense curr would be invalid here it must be between the start and end)
So I think the following would suffice,
if (msg->sg.start < msg->sg.end && i < msg->sg.curr) {
msg->sg.curr = i;
msg->sg.copybreak = msg->sg.data[i].length;
} else if (msg->sg.end < msg->sg.start && (i < msg->sg.curr || i > msg->sg.start))
msg->sg.curr = i;
msg->sg.copybreak = msg->sg.data[i].length;
} else if (msg->sg.end < msg->sg.start && (i < msg->sg.curr) {
curr = i;
msg->sg.copybreak = msg->sg.data[i].length;
}
Finally fold the last two cases into one so we get
if (msg->sg.start < msg->sg.end && i < msg->sg.curr) {
msg->sg.curr = i;
msg->sg.copybreak = msg->sg.data[i].length;
} else if (msg->sg.end < msg->sg.start && (i < msg->sg.curr || i > msg->sg.start))
msg->sg.curr = i;
msg->sg.copybreak = msg->sg.data[i].length;
So not so bad. Put that in a helper in sk_msg.h and use it in trim. I can check
logic in the AM and draft a patch but I think that should be correct. Also will
need to audit to see if there are other cases this happens. In general I tried
to always use i == msg->sg.{start|end|curr} to avoid this.
Hopefully it wasn't too verbose above but figured it couldn't hurt.
.John
