* UAF in ip6_do_table on 4.19 kernel
@ 2019-11-11 20:49 stranche
2019-11-11 22:09 ` Eric Dumazet
0 siblings, 1 reply; 2+ messages in thread
From: stranche @ 2019-11-11 20:49 UTC (permalink / raw)
To: fw, netfilter-devel; +Cc: Subashab
Hi all,
We recently had a crash reported to us on the 4.19 kernel where
ip6_do_table() appeared to be referencing a jumpstack that had already
been freed.
Based on the dump, it appears that the scenario was a concurrent use of
iptables-restore and active data transfer. The kernel has Florian's
commit
to wait in xt_replace_table instead of get_counters(), 80055dab5de0
("netfilter: x_tables: make xt_replace_table wait until old rules are
not used
anymore"), so it appears that xt_replace_table is somehow returning
prematurely, allowing __do_replace() to free the table while it is still
in use.
After reviewing the code, we had a question about the following section:
/* ... so wait for even xt_recseq on all cpus */
for_each_possible_cpu(cpu) {
seqcount_t *s = &per_cpu(xt_recseq, cpu);
u32 seq = raw_read_seqcount(s);
if (seq & 1) {
do {
cond_resched();
cpu_relax();
} while (seq == raw_read_seqcount(s));
}
}
Based on the other uses of seqcount locks, there should be a paired
read_seqcount_retry() to mark the end of the read section like below:
for_each_possible_cpu(cpu) {
seqcount_t *s = &per_cpu(xt_recseq, cpu);
u32 seq;
do {
seq = raw_read_seqcount(s);
if (seq & 1) {
cond_resched();
cpu_relax();
}
} while (read_seqcount_retry(s, seq);
}
These two snippets are very similar, as the original seems like it
attempted to open-code this retry() helper, but there is a slight
difference in
the smp_rmb() placement relative to the "retry" read of the sequence
value.
Original:
READ_ONCE(s->sequence);
smp_rmb();
... //check and resched
READ_ONCE(s->sequence);
smp_rmb();
... //compare the two sequence values
Modified using read_seqcount_retry():
READ_ONCE(s->sequence);
smp_rmb();
... //check and and resched
smp_rmb();
READ_ONCE(s->sequence);
... //compare the two sequence values
Is it possible that this difference in ordering could lead to an
incorrect read of the sequence in certain neurotic scenarios?
Alternatively, is there
some other place that this jumpstack could be freed while still in use?
Thanks,
Sean
---
The Qualcomm Innovation Center, Inc. is a member of the Code Aurora
Forum,
a Linux Foundation Collaborative Project
^ permalink raw reply [flat|nested] 2+ messages in thread
* Re: UAF in ip6_do_table on 4.19 kernel
2019-11-11 20:49 UAF in ip6_do_table on 4.19 kernel stranche
@ 2019-11-11 22:09 ` Eric Dumazet
0 siblings, 0 replies; 2+ messages in thread
From: Eric Dumazet @ 2019-11-11 22:09 UTC (permalink / raw)
To: stranche, fw, netfilter-devel; +Cc: Subashab
On 11/11/19 12:49 PM, stranche@codeaurora.org wrote:
> Hi all,
>
> We recently had a crash reported to us on the 4.19 kernel where ip6_do_table() appeared to be referencing a jumpstack that had already been freed.
> Based on the dump, it appears that the scenario was a concurrent use of iptables-restore and active data transfer. The kernel has Florian's commit
> to wait in xt_replace_table instead of get_counters(), 80055dab5de0 ("netfilter: x_tables: make xt_replace_table wait until old rules are not used
> anymore"), so it appears that xt_replace_table is somehow returning prematurely, allowing __do_replace() to free the table while it is still in use.
>
> After reviewing the code, we had a question about the following section:
> /* ... so wait for even xt_recseq on all cpus */
> for_each_possible_cpu(cpu) {
> seqcount_t *s = &per_cpu(xt_recseq, cpu);
> u32 seq = raw_read_seqcount(s);
>
> if (seq & 1) {
> do {
> cond_resched();
> cpu_relax();
> } while (seq == raw_read_seqcount(s));
> }
> }
The intent of this code is to check that each cpu went through a phase where the seq was even at least once.
>
> Based on the other uses of seqcount locks, there should be a paired read_seqcount_retry() to mark the end of the read section like below:
> for_each_possible_cpu(cpu) {
> seqcount_t *s = &per_cpu(xt_recseq, cpu);
> u32 seq;
>
> do {
> seq = raw_read_seqcount(s);
> if (seq & 1) {
> cond_resched();
> cpu_relax();
> }
> } while (read_seqcount_retry(s, seq);
This would loop possibly more times, since you exit if the count is _currently_ even.
If we are unlucky this could loop for a very long time.
> }
>
> These two snippets are very similar, as the original seems like it attempted to open-code this retry() helper, but there is a slight difference in
> the smp_rmb() placement relative to the "retry" read of the sequence value.
> Original:
> READ_ONCE(s->sequence);
> smp_rmb();
> ... //check and resched
> READ_ONCE(s->sequence);
> smp_rmb();
> ... //compare the two sequence values
>
> Modified using read_seqcount_retry():
> READ_ONCE(s->sequence);
> smp_rmb();
> ... //check and and resched
> smp_rmb();
> READ_ONCE(s->sequence);
> ... //compare the two sequence values
>
> Is it possible that this difference in ordering could lead to an incorrect read of the sequence in certain neurotic scenarios? Alternatively, is there
> some other place that this jumpstack could be freed while still in use?
>
4.19 has many bugs really. Please upgrade to v4.19.83
^ permalink raw reply [flat|nested] 2+ messages in thread
end of thread, other threads:[~2019-11-11 22:09 UTC | newest]
Thread overview: 2+ messages (download: mbox.gz / follow: Atom feed)
-- links below jump to the message on this page --
2019-11-11 20:49 UAF in ip6_do_table on 4.19 kernel stranche
2019-11-11 22:09 ` Eric Dumazet
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).