LinuxLists.cc - slab corruption with current -git (was Re: [git pull] vfs pile 1 (splice))

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Subject: Re: slab corruption with current -git (was Re: [git pull] vfs pile 1 (splice))

Florian Westphal <[email protected]> writes:

> Linus Torvalds <[email protected]> wrote:
>> On Sun, Oct 9, 2016 at 12:11 PM, Linus Torvalds
>> <[email protected]> wrote:
>> >
>> > Anyway, I don't think I can bisect it, but I'll try to narrow it down
>> > a *bit* at least.
>> >
>> > Not doing any more pulls on this unstable base, I've been puttering
>> > around in trying to clean up some stupid printk logging issues
>> > instead.
>>
>> So I finally got a oops with slub debugging enabled. It doesn't really
>> narrow things down, though, it kind of extends on the possible
>> suspects. Now adding David Miller and Pablo, because it looks like it
>> may be netfilter that does something bad and corrupts memory.
>
> Quite possible, the netns interactions are not nice :-/
>
>> Without further ado, here's the new oops:
>>
>> general protection fault: 0000 [#1] SMP
>> CPU: 7 PID: 169 Comm: kworker/u16:7 Not tainted
>> 4.8.0-11288-gb66484cd7470 #1
>> Hardware name: System manufacturer System Product Name/Z170-K, BIOS
> ..
>> Call Trace:
>> netfilter_net_exit+0x2f/0x60
>> ops_exit_list.isra.4+0x38/0x60
>> cleanup_net+0x1ba/0x2a0
>> process_one_work+0x1f1/0x480
>> worker_thread+0x48/0x4d0
>> ? process_one_work+0x480/0x480
>
> ..
>
>> like it's a pointer loaded from a free'd allocation.
>>
>> The code disassembles to
>>
>> 0: 0f b6 ca movzbl %dl,%ecx
>> 3: 48 8d 84 c8 00 01 00 lea 0x100(%rax,%rcx,8),%rax
>> a: 00
>> b: 49 8b 5c c5 00 mov 0x0(%r13,%rax,8),%rbx
>> 10: 48 85 db test %rbx,%rbx
>> 13: 0f 84 cb 00 00 00 je 0xe4
>> 19: 4c 3b 63 40 cmp 0x40(%rbx),%r12
>> 1d: 48 8b 03 mov (%rbx),%rax
>> 20: 0f 84 e9 00 00 00 je 0x10f
>> 26: 48 85 c0 test %rax,%rax
>> 29: 74 26 je 0x51
>> 2b:* 4c 3b 60 40 cmp 0x40(%rax),%r12 <-- trapping instruction
>> 2f: 75 08 jne 0x39
>> 31: e9 ef 00 00 00 jmpq 0x125
>> 36: 48 89 d8 mov %rbx,%rax
>> 39: 48 8b 18 mov (%rax),%rbx
>> 3c: 48 85 db test %rbx,%rbx
>>
>> and that oopsing instruction seems to be the compare of
>> "hooks_entry->orig_ops" from hooks_entry in this expression:
>>
>> if (hooks_entry && hooks_entry->orig_ops == reg) {
>>
>> so hooks_entry() is bogus. It was gotten from
>>
>> hooks_entry = nf_hook_entry_head(net, reg);
>>
>> but that's as far as I dug. And yes, I do have
>> CONFIG_NETFILTER_INGRESS=y in case that matters.
>>
>> And all this code has changed pretty radically in commit e3b37f11e6e4
>> ("netfilter: replace list_head with single linked list"), and there
>> was clearly already something wrong with that code, with commit
>> 5119e4381a90 ("netfilter: Fix potential null pointer dereference")
>> adding the test against NULL. But I suspect that only hid the "oops,
>> it's actually not NULL, it loaded some uninitialized value" problem.
>>
>> Over to the networking guys.. Ideas?
>
> Sorry, not off the top of my head.
> Pablo is currently travelling back home from netdev 1.2 in Tokyo,
> I can help starting Wednesday when I am back.
>
> One shot in the dark (not even compile tested; wonder if we can end up
> zapping bogus hook ...)
>

I was just about to build and test something similar:

diff --git a/net/netfilter/core.c b/net/netfilter/core.c
index c9d90eb..e84103f 100644
--- a/net/netfilter/core.c
+++ b/net/netfilter/core.c
@@ -189,7 +189,7 @@ void nf_unregister_net_hook(struct net *net, const struct nf_hook_ops *reg)

unlock:
mutex_unlock(&nf_hook_mutex);
- if (!hooks_entry) {
+ if (!hooks_entry || hooks_entry->orig_ops != reg) {
WARN(1, "nf_unregister_net_hook: hook not found!\n");
return;
}

2016-10-10 02:49:47

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Subject: Re: slab corruption with current -git (was Re: [git pull] vfs pile 1 (splice))

On Sun, Oct 9, 2016 at 6:35 PM, Aaron Conole <[email protected]> wrote:
>
> I was just about to build and test something similar:

So I haven't actually tested that one, but looking at the code, it
really looks very bogus. In fact, that code just looks like crap. It
does *not* do a proper "remove singly linked list entry". It's exactly
the kind of code that I rail against, and that people should never
write.

Any code that can't even traverse a linked list is not worth looking at.

There is one *correct* way to remove an entry from a singly linked
list, and it looks like this:

struct entry **pp, *p;

pp = &head;
while ((p = *pp) != NULL) {
if (right_entry(p)) {
*pp = p->next;
break;
}
pp = &p->next;
}

and that's it. Nothing else. The above code exits the loop with "p"
containing the entry that was removed, or NULL if nothing was. It
can't get any simpler than that, but more importantly, anything more
complicated than that is WRONG.

Seriously, nothing else is acceptable. In particular, any linked list
traversal that makes a special case of the first entry or the last
entry should not be allowed to exist. Note how there is not a single
special case in the above correct code. It JustWorks(tm).

That nf_unregister_net_hook() code has all the signs of exactly that
kind of broken list-handling code: special-casing the head of the
loop, and having the loop condition test both current and that odd
"next to next" pointer etc. It's all very very wrong.

So I really see two options:

- do that singly-linked list traversal right (and I'm serious:
nothing but the code above can ever be right)

- don't make up your own list handling code at all, and use the
standard linux list code.

So either e3b37f11e6e4 needs to be reverted, or it needs to be taught
to use real list handling. If the code doesn't want to use the
regular list.h (either the doubly linked one, or the hlist one), it
needs to at least learn to do list removal right.

Linus

2016-10-10 03:41:21

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Subject: Re: slab corruption with current -git (was Re: [git pull] vfs pile 1 (splice))

On Sun, Oct 9, 2016 at 7:49 PM, Linus Torvalds
<[email protected]> wrote:
>
> There is one *correct* way to remove an entry from a singly linked
> list, and it looks like this:
>
> struct entry **pp, *p;
>
> pp = &head;
> while ((p = *pp) != NULL) {
> if (right_entry(p)) {
> *pp = p->next;
> break;
> }
> pp = &p->next;
> }
>
> and that's it. Nothing else.

This COMPLETELY UNTESTED patch tries to fix the nf_hook_entry code to do this.

I repeat: it's ENTIRELY UNTESTED. I just converted the insertion and
deletion to the proper pattern, but I could easily have gotten the
insertion priority test the wrong way around entirely, for example. Or
it could simply have some other completely broken bug in it. It
compiles for me, but that's all I actually checked.

Note that the "correct way" of doing list operations also almost
inevitably is the shortest way by far, since it gets rid of all the
special cases. So the patch looks nice. It gets rid of the magic
"nf_set_hooks_head()" thing too, because once you do list following
right, the head is no different from any other pointer in the list.

So the patch stats look good:

net/netfilter/core.c | 108 ++++++++++++++++-----------------------------------
1 file changed, 33 insertions(+), 75 deletions(-)

but again, it's entirely *entirely* untested. Please consider this
just a "this is generally how list insert/delete operations should be
done, avoiding special cases for the first entry".

ALSO NOTE! The code assumes that the "nf_hook_mutex" locking only
protects the actual *lists*, and that the address to the list can be
looked up without holding the lock. That's generally how things are
done, and it simplifies error handling (because you can do the "there
is no such list at all" test before you do anything else. But again, I
don't actually know the code, and if there is something that actually
expands the number of lists etc that depends on that mutex, then the
list head lookup may need to be inside the lock too.

Linus

Attachments:

patch.diff (4.89 kB)

2016-10-10 03:57:55

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Subject: Re: slab corruption with current -git

From: Linus Torvalds <[email protected]>
Date: Sun, 9 Oct 2016 20:41:17 -0700

> Note that the "correct way" of doing list operations also almost
> inevitably is the shortest way by far, since it gets rid of all the
> special cases. So the patch looks nice. It gets rid of the magic
> "nf_set_hooks_head()" thing too, because once you do list following
> right, the head is no different from any other pointer in the list.

Perhaps we should have some "slist" primitives added to
include/linux/list.h but since the comparison differs for each user I
guess it's hard to abstract in a way that's generic and inlines
properly.

I'll start taking a look at your patch and this stuff as well, thanks
Linus.

2016-10-10 08:24:57

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Subject: Re: slab corruption with current -git

From: David Miller <[email protected]>
Date: Sun, 09 Oct 2016 23:57:45 -0400 (EDT)

> From: Linus Torvalds <[email protected]>
> Date: Sun, 9 Oct 2016 20:41:17 -0700
>
>> Note that the "correct way" of doing list operations also almost
>> inevitably is the shortest way by far, since it gets rid of all the
>> special cases. So the patch looks nice. It gets rid of the magic
>> "nf_set_hooks_head()" thing too, because once you do list following
>> right, the head is no different from any other pointer in the list.
>
> Perhaps we should have some "slist" primitives added to
> include/linux/list.h but since the comparison differs for each user I
> guess it's hard to abstract in a way that's generic and inlines
> properly.
>
> I'll start taking a look at your patch and this stuff as well, thanks
> Linus.

So I've been reviewing this patch and it looks fine, but I also want
to figure out what is actually causing the OOPS and I can't spot it
yet.

One possible way to see that oops is to free the head entry of the
chain without unlinking it. The next unregister will dereference a
POISON pointer.

Actually...

The POISON value comes not from a hook entry, but from the array of
pointers in the per-netns datastructure.

This means that the netns is possibly getting freed up before we
unregister the netfilter hooks.

2016-10-10 13:49:38

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Subject: Re: slab corruption with current -git (was Re: [git pull] vfs pile 1 (splice))

Linus Torvalds <[email protected]> writes:

> On Sun, Oct 9, 2016 at 7:49 PM, Linus Torvalds
> <[email protected]> wrote:
>>
>> There is one *correct* way to remove an entry from a singly linked
>> list, and it looks like this:
>>
>> struct entry **pp, *p;
>>
>> pp = &head;
>> while ((p = *pp) != NULL) {
>> if (right_entry(p)) {
>> *pp = p->next;
>> break;
>> }
>> pp = &p->next;
>> }
>>
>> and that's it. Nothing else.

Sorry, I should have done that.

> This COMPLETELY UNTESTED patch tries to fix the nf_hook_entry code to do this.
>
> I repeat: it's ENTIRELY UNTESTED. I just converted the insertion and
> deletion to the proper pattern, but I could easily have gotten the
> insertion priority test the wrong way around entirely, for example. Or
> it could simply have some other completely broken bug in it. It
> compiles for me, but that's all I actually checked.

Okay, I'm looking it over. Sorry for the mess.

> Note that the "correct way" of doing list operations also almost
> inevitably is the shortest way by far, since it gets rid of all the
> special cases. So the patch looks nice. It gets rid of the magic
> "nf_set_hooks_head()" thing too, because once you do list following
> right, the head is no different from any other pointer in the list.
>
> So the patch stats look good:
>
> net/netfilter/core.c | 108 ++++++++++++++++-----------------------------------
> 1 file changed, 33 insertions(+), 75 deletions(-)
>
> but again, it's entirely *entirely* untested. Please consider this
> just a "this is generally how list insert/delete operations should be
> done, avoiding special cases for the first entry".

I'll review it, and test it. Can you tell me what steps you took to
reproduce the oops? I'll enable slab debugging and try to reproduce
without and with this patch (and I'll also look into David's recent
email as well). Are you simply creating and removing network
namespaces (I did test that, but I should have done a better job)?

> ALSO NOTE! The code assumes that the "nf_hook_mutex" locking only
> protects the actual *lists*, and that the address to the list can be
> looked up without holding the lock. That's generally how things are
> done, and it simplifies error handling (because you can do the "there
> is no such list at all" test before you do anything else. But again, I
> don't actually know the code, and if there is something that actually
> expands the number of lists etc that depends on that mutex, then the
> list head lookup may need to be inside the lock too.

That should be correct, the nf_hook_mutex is only for protecting the
lists.

> Linus

2016-10-10 16:15:14

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Subject: Re: slab corruption with current -git

On Mon, Oct 10, 2016 at 1:24 AM, David Miller <[email protected]> wrote:
>
> So I've been reviewing this patch and it looks fine, but I also want
> to figure out what is actually causing the OOPS and I can't spot it
> yet.

Yeah, I'm not actually sure the old linked list implementation is
buggy - it might just be ugly. I tried to follow the old code, and I
couldn't.

So the patch I sent out was a combination of "that's not how you
should do singly linked lists" and "those special cases make me
worry". In particular, the old code really ended up doing odd things
in the "can't find entry" case, because it would exit the loop with a
non-NULL 'entry' just because the next entry was NULL..

> One possible way to see that oops is to free the head entry of the
> chain without unlinking it. The next unregister will dereference a
> POISON pointer.
>
> Actually...
>
> The POISON value comes not from a hook entry, but from the array of
> pointers in the per-netns datastructure.
>
> This means that the netns is possibly getting freed up before we
> unregister the netfilter hooks.

That is certainly one possible explanation for it, yes. However, I
didn't think that part had changed, had it?

The other thing I find a bit odd in that new single-linked list code is this:

- nf_hook_slow():
...
RCU_INIT_POINTER(state->hook_entries, entry);

which makes me worried.. It copies the head entry of the list, and
maybe it will then (later) end up being used stale. I don't know. But
it looks a bit iffy.

Linus

2016-10-10 16:30:34

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Subject: Re: slab corruption with current -git (was Re: [git pull] vfs pile 1 (splice))

On Mon, Oct 10, 2016 at 6:49 AM, Aaron Conole <[email protected]> wrote:
>
> Okay, I'm looking it over. Sorry for the mess.

So as I already answered to Dave, I'm not actually sure that this was
the buggy code, or that my patch would make any difference at all.

I never got a good reproducer for the bug: I spent much of the weekend
rebooting, because it seems to happen only just after a reboot, as I
log in and start my usual thing.

I initially blamed some off filesystem or block layer issue ("Oh, it
only happens with a cold cache"), partly because the initial
non-poisoned slub oopses happened in filesystem code.

But I now think it's netfilter, and I *think* that what triggers it is
something like the bluetooth subsystem giving up or something. What I
do when I log into a new session tends to be to go to the kernel
subdirectory in one or two terminals, and fire up chrome to read
email. And the problem either happened within half a minute of me
doing that, or it never happens at all.

Which is why I ended up rebooting a *lot*. Just running the kernel
never triggered it.

(It took me some time to figure that out, which is basically why I did
almost no pull requests the whole weekend)

The journal entries for that invalid kernel access is somewhat suggestive:

Oct 09 13:24:03 i7 dbus-daemon[1030]: [system] Failed to activate
service 'org.bluez': timed out

Oct 09 13:24:09 i7 audit[1]: SERVICE_STOP pid=1 uid=0
auid=4294967295 ses=4294967295 subj=system_u:system_r:init_t:s0
msg='unit=systemd-hostnamed comm="systemd"
exe="/usr/lib/systemd/systemd" hostname=? addr=? terminal=?
res=success'

Oct 09 13:24:09 i7 kernel: general protection fault: 0000 [#1] SMP

so it happened just as *some* network setup thing was finishing off (I
don't think it was systemd-hostnamed itself that necessarily matters,
but clearly something was finishing up as the netfilter problem
occurred.

> I'll review it, and test it. Can you tell me what steps you took to
> reproduce the oops?

See above: I can't actually really "reproduce" it. It's probably
highly timing-dependent, and it is not unlikely that it's also very
much about specific setup. I'm running plain Fedora 24, I boot up, log
in, start two or three terminals, fire up chrome, and ...

So far I've seen the problem maybe 5-6 times, but a couple of those
were just silent hangs (I may have rebooted too quickly for things to
hit the disk, or the oops may just have killed the machine too hard).
Two I got the oops inside slub code, and I only have one successful
slub poisoning oops from netfilter.

(Part of the reason I only have one is that once I got that, I stopped
rebooting, and instead started looking at the netfilter code and
started to do some merge window pulls again because I felt that this
is *probably* the core reason, and I cant' afford to not do pulls
during the merge window for _too_ long).

Linus

2016-10-10 19:06:13

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Subject: Re: slab corruption with current -git (was Re: [git pull] vfs pile 1 (splice))

On Mon, Oct 10, 2016 at 9:28 AM, Linus Torvalds
<[email protected]> wrote:
>
> So as I already answered to Dave, I'm not actually sure that this was
> the buggy code, or that my patch would make any difference at all.

My patch does seem to fix things, and in fact the warning about "hook
not found" now triggers.

So I think the bug really was that the singly-linked list handling
code did not correctly handle the case of not finding the entry, and
then freed (incorrectly) the last one that wasn't actually unlinked.

In fact, I get quite a few warnings (56 total) about 30 seconds after
logging in:

[ 54.213170] WARNING: CPU: 1 PID: 111 at net/netfilter/core.c:151
nf_unregister_net_hook+0x8e/0x170
... repeat 54 times ...
[ 54.445520] WARNING: CPU: 7 PID: 111 at net/netfilter/core.c:151
nf_unregister_net_hook+0x8e/0x170

and looking in the journal, the first one is (again) immediately
preceded by that systemd-hostnamed service stopping:

Oct 10 11:45:47 i7 audit[1546]: USER_LOGIN
...
Oct 10 11:46:11 i7 audit[1]: SERVICE_STOP pid=1 uid=0
auid=4294967295 ses=4294967295 subj=system_u:system_r:init_t:s0
msg='unit=fprintd comm="systemd" exe="/usr/lib/systemd/systemd"
hostname=? addr=? terminal=? res=success'
Oct 10 11:46:13 i7 pulseaudio[1697]: [pulseaudio] bluez5-util.c:
GetManagedObjects() failed: org.freedesktop.DBus.Error.NoReply: Did
not receive a reply. Possible causes include: the remote application
did not send a reply, the message bus security policy blocked the
reply, the reply timeout expir
Oct 10 11:46:13 i7 dbus-daemon[1003]: [system] Failed to activate
service 'org.bluez': timed out
Oct 10 11:46:20 i7 audit[1]: SERVICE_STOP pid=1 uid=0
auid=4294967295 ses=4294967295 subj=system_u:system_r:init_t:s0
msg='unit=systemd-hostnamed comm="systemd"
exe="/usr/lib/systemd/systemd" hostname=? addr=? terminal=?
res=success'
Oct 10 11:46:20 i7 kernel: ------------[ cut here ]------------
Oct 10 11:46:20 i7 kernel: WARNING: CPU: 1 PID: 111 at
net/netfilter/core.c:151 nf_unregister_net_hook+0x8e/0x170

so I do think it's something to do with some network startup service
thing (perhaps dhcp, perhaps chrome, who knows) as I do my initial
login.

David - I think that also explains what was wrong with the old code.
In the old code, this loop:

while (hooks_entry && nf_entry_dereference(hooks_entry->next)) {

would exit with "hooks_entry" pointing to the last list entry (because
->next was NULL). Nothing was ever unlinked in the loop itself,
because it never actually found a matching entry, but then after the
loop it would free that last entry because it *thought* that was the
match.

My list rewrite fixes that.

Anyway, I'm assuming it will come to me from the networking tree after
more testing by the maintainers. You can add my

Signed-off-by: Linus Torvalds <[email protected]>

to the patch, though.

David, if you want me to just commit that thing directly, I can
obviously do so, but I do think somebody should look at

(a) that I actually got the priority list ordering right on the insertion side
(b) what it is that makes it try to unregister that hook that isn't
on the list in the first place

but on the whole I consider this issue explained and solved. I'll
continue to run with my patch on my machine (just not committed).

Linus

2016-10-10 19:18:21

[permalink] [raw]

Subject: Re: slab corruption with current -git (was Re: [git pull] vfs pile 1 (splice))

Linus Torvalds <[email protected]> writes:

> On Mon, Oct 10, 2016 at 9:28 AM, Linus Torvalds
> <[email protected]> wrote:
>>
>> So as I already answered to Dave, I'm not actually sure that this was
>> the buggy code, or that my patch would make any difference at all.
>
> My patch does seem to fix things, and in fact the warning about "hook
> not found" now triggers.
>
> So I think the bug really was that the singly-linked list handling
> code did not correctly handle the case of not finding the entry, and
> then freed (incorrectly) the last one that wasn't actually unlinked.
>
> In fact, I get quite a few warnings (56 total) about 30 seconds after
> logging in:
>
> [ 54.213170] WARNING: CPU: 1 PID: 111 at net/netfilter/core.c:151
> nf_unregister_net_hook+0x8e/0x170
> ... repeat 54 times ...
> [ 54.445520] WARNING: CPU: 7 PID: 111 at net/netfilter/core.c:151
> nf_unregister_net_hook+0x8e/0x170
>
> and looking in the journal, the first one is (again) immediately
> preceded by that systemd-hostnamed service stopping:
>
> Oct 10 11:45:47 i7 audit[1546]: USER_LOGIN
> ...
> Oct 10 11:46:11 i7 audit[1]: SERVICE_STOP pid=1 uid=0
> auid=4294967295 ses=4294967295 subj=system_u:system_r:init_t:s0
> msg='unit=fprintd comm="systemd" exe="/usr/lib/systemd/systemd"
> hostname=? addr=? terminal=? res=success'
> Oct 10 11:46:13 i7 pulseaudio[1697]: [pulseaudio] bluez5-util.c:
> GetManagedObjects() failed: org.freedesktop.DBus.Error.NoReply: Did
> not receive a reply. Possible causes include: the remote application
> did not send a reply, the message bus security policy blocked the
> reply, the reply timeout expir
> Oct 10 11:46:13 i7 dbus-daemon[1003]: [system] Failed to activate
> service 'org.bluez': timed out
> Oct 10 11:46:20 i7 audit[1]: SERVICE_STOP pid=1 uid=0
> auid=4294967295 ses=4294967295 subj=system_u:system_r:init_t:s0
> msg='unit=systemd-hostnamed comm="systemd"
> exe="/usr/lib/systemd/systemd" hostname=? addr=? terminal=?
> res=success'
> Oct 10 11:46:20 i7 kernel: ------------[ cut here ]------------
> Oct 10 11:46:20 i7 kernel: WARNING: CPU: 1 PID: 111 at
> net/netfilter/core.c:151 nf_unregister_net_hook+0x8e/0x170
>
> so I do think it's something to do with some network startup service
> thing (perhaps dhcp, perhaps chrome, who knows) as I do my initial
> login.
>
> David - I think that also explains what was wrong with the old code.
> In the old code, this loop:
>
> while (hooks_entry && nf_entry_dereference(hooks_entry->next)) {
>
> would exit with "hooks_entry" pointing to the last list entry (because
> ->next was NULL). Nothing was ever unlinked in the loop itself,
> because it never actually found a matching entry, but then after the
> loop it would free that last entry because it *thought* that was the
> match.
>
> My list rewrite fixes that.
>
> Anyway, I'm assuming it will come to me from the networking tree after
> more testing by the maintainers. You can add my
>
> Signed-off-by: Linus Torvalds <[email protected]>
>
> to the patch, though.
>
> David, if you want me to just commit that thing directly, I can
> obviously do so, but I do think somebody should look at
>
> (a) that I actually got the priority list ordering right on the
> insertion side

It looks correct.

Reviewed-by: Aaron Conole <[email protected]>

> (b) what it is that makes it try to unregister that hook that isn't
> on the list in the first place

This is a still problem, I think. I wasn't able to reproduce the issue
on a fedora-23 VM. My fedora 24 bare-metal system does trigger this,
though. Not sure what changed in userspace/kernel interaction side (not
an excuse, but just an observation).

> but on the whole I consider this issue explained and solved. I'll
> continue to run with my patch on my machine (just not committed).

Okay. Very sorry for this, again.

> Linus

2016-10-11 00:30:34

[permalink] [raw]

Subject: Re: slab corruption with current -git

From: Linus Torvalds <[email protected]>
Date: Mon, 10 Oct 2016 12:05:17 -0700

> David - I think that also explains what was wrong with the old code.
> In the old code, this loop:
>
> while (hooks_entry && nf_entry_dereference(hooks_entry->next)) {
>
> would exit with "hooks_entry" pointing to the last list entry (because
> ->next was NULL). Nothing was ever unlinked in the loop itself,
> because it never actually found a matching entry, but then after the
> loop it would free that last entry because it *thought* that was the
> match.

It only does this when the ops don't match, but yes it can happen.

Linus can you add some extra info to that:

WARN(1, "nf_unregister_net_hook: hook not found!\n");

diagnostic, such as the reg->pf and reg->hooknum values?

That might help track down why this is happening in the first
place.

2016-10-11 00:54:24

[permalink] [raw]

Subject: Re: slab corruption with current -git

On Mon, Oct 10, 2016 at 5:30 PM, David Miller <[email protected]> wrote:
>
> Linus can you add some extra info to that:

Sure. I made it a WARN_ON_ONCE(), but then always just printed the
pf/hooknum. It's all over the map:

reg->pf=2 and reg->hooknum=4
reg->pf=2 and reg->hooknum=2
reg->pf=2 and reg->hooknum=3
reg->pf=10 and reg->hooknum=4
reg->pf=10 and reg->hooknum=2
reg->pf=10 and reg->hooknum=3
reg->pf=7 and reg->hooknum=1
reg->pf=7 and reg->hooknum=2
reg->pf=7 and reg->hooknum=3
reg->pf=2 and reg->hooknum=0
reg->pf=2 and reg->hooknum=3
reg->pf=2 and reg->hooknum=0
reg->pf=2 and reg->hooknum=3
reg->pf=2 and reg->hooknum=4
reg->pf=2 and reg->hooknum=4
reg->pf=2 and reg->hooknum=1
reg->pf=2 and reg->hooknum=1
reg->pf=10 and reg->hooknum=0
reg->pf=10 and reg->hooknum=3
reg->pf=10 and reg->hooknum=0
reg->pf=10 and reg->hooknum=3
reg->pf=10 and reg->hooknum=4
reg->pf=10 and reg->hooknum=4
reg->pf=10 and reg->hooknum=1
reg->pf=10 and reg->hooknum=1
reg->pf=7 and reg->hooknum=3
reg->pf=7 and reg->hooknum=4
reg->pf=7 and reg->hooknum=0
reg->pf=2 and reg->hooknum=4
reg->pf=2 and reg->hooknum=2
reg->pf=2 and reg->hooknum=3
reg->pf=10 and reg->hooknum=4
reg->pf=10 and reg->hooknum=2
reg->pf=10 and reg->hooknum=3
reg->pf=7 and reg->hooknum=1
reg->pf=7 and reg->hooknum=2
reg->pf=7 and reg->hooknum=3
reg->pf=2 and reg->hooknum=0
reg->pf=2 and reg->hooknum=3
reg->pf=2 and reg->hooknum=0
reg->pf=2 and reg->hooknum=3
reg->pf=2 and reg->hooknum=4
reg->pf=2 and reg->hooknum=4
reg->pf=2 and reg->hooknum=1
reg->pf=2 and reg->hooknum=1
reg->pf=10 and reg->hooknum=0
reg->pf=10 and reg->hooknum=3
reg->pf=10 and reg->hooknum=0
reg->pf=10 and reg->hooknum=3
reg->pf=10 and reg->hooknum=4
reg->pf=10 and reg->hooknum=4
reg->pf=10 and reg->hooknum=1
reg->pf=10 and reg->hooknum=1
reg->pf=7 and reg->hooknum=3
reg->pf=7 and reg->hooknum=4
reg->pf=7 and reg->hooknum=0

and putting that through "sort -n" and "uniq -c", I get:

4 reg->pf=10 and reg->hooknum=0
4 reg->pf=10 and reg->hooknum=1
2 reg->pf=10 and reg->hooknum=2
6 reg->pf=10 and reg->hooknum=3
6 reg->pf=10 and reg->hooknum=4
4 reg->pf=2 and reg->hooknum=0
4 reg->pf=2 and reg->hooknum=1
2 reg->pf=2 and reg->hooknum=2
6 reg->pf=2 and reg->hooknum=3
6 reg->pf=2 and reg->hooknum=4
2 reg->pf=7 and reg->hooknum=0
2 reg->pf=7 and reg->hooknum=1
2 reg->pf=7 and reg->hooknum=2
4 reg->pf=7 and reg->hooknum=3

which doesn't look much better. But clearly there's a lot of those
"try to unregister stuff that you can't even find".

Maybe it tells you something.

Linus

2016-10-11 05:39:56

[permalink] [raw]

Subject: Re: slab corruption with current -git (was Re: [git pull] vfs pile 1 (splice))

On Sun, Oct 9, 2016 at 8:41 PM, Linus Torvalds
<[email protected]> wrote:
> This COMPLETELY UNTESTED patch tries to fix the nf_hook_entry code to do this.
>
> I repeat: it's ENTIRELY UNTESTED.

Gaah.

That patch was subtle garbage.

The "add to list" thing did this:

rcu_assign_pointer(entry->next, p);
rcu_assign_pointer(*pp, p);

which is not so subtly broken - that second assignment just assigns
"p" to "*pp", but that was what *pp already contained. Too much
cut-and-paste.

That also explains why I then get the NOT FOUND case, because the add
never actually worked.

It *should* be

rcu_assign_pointer(entry->next, p);
rcu_assign_pointer(*pp, entry);

and then the warnings about "not found" are gone.

Duh.

I guess I will have to double-check that the slub corruption is gone
still with that fixed.

Anyway, new version of the patch (just that one line changed) attached.

Linus

Attachments:

patch.diff (4.89 kB)

2016-10-11 05:47:53

[permalink] [raw]

Subject: Re: slab corruption with current -git (was Re: [git pull] vfs pile 1 (splice))

On Mon, Oct 10, 2016 at 10:39 PM, Linus Torvalds
<[email protected]> wrote:
>
> I guess I will have to double-check that the slub corruption is gone
> still with that fixed.

So I'm not getting any warnings now from SLUB debugging. So the
original bug seems to not have re-surfaced, and the registration bug
is gone, so now the unregistration doesn't warn about anything either.

But I only rebooted three times.

Linus

2016-10-11 08:57:54

[permalink] [raw]

Subject: Re: slab corruption with current -git

From: Linus Torvalds <[email protected]>
Date: Mon, 10 Oct 2016 22:47:50 -0700

> On Mon, Oct 10, 2016 at 10:39 PM, Linus Torvalds
> <[email protected]> wrote:
>>
>> I guess I will have to double-check that the slub corruption is gone
>> still with that fixed.
>
> So I'm not getting any warnings now from SLUB debugging. So the
> original bug seems to not have re-surfaced, and the registration bug
> is gone, so now the unregistration doesn't warn about anything either.
>
> But I only rebooted three times.

Looks good to me, I applied it to my tree with your signoff and will
send you a pull request right now.

Thanks!

2016-10-11 13:17:41

by Michal Kubecek

[permalink] [raw]

Subject: Re: slab corruption with current -git

On Mon, Oct 10, 2016 at 04:24:01AM -0400, David Miller wrote:
> From: David Miller <[email protected]>
> Date: Sun, 09 Oct 2016 23:57:45 -0400 (EDT)
>
> This means that the netns is possibly getting freed up before we
> unregister the netfilter hooks.

Sounds a bit like the issue discussed here:

https://marc.info/?l=netfilter-devel&m=146980917627262&w=2

Could it be (partly) the same race condition?

Michal Kubecek

2016-10-11 14:00:46