[PATCH] audit: grab a reference to context->pwd when it's cached

Mon Oct 8 15:22:05 UTC 2012

On Mon, Oct 8, 2012 at 5:23 AM, Jeff Layton <jlayton at redhat.com> wrote:
> On Fri, 5 Oct 2012 12:41:46 -0700
> Peter Moody <pmoody at google.com> wrote:
>
>> On Fri, Oct 5, 2012 at 10:04 AM, Peter Moody <pmoody at google.com> wrote:
>> > On Fri, Oct 5, 2012 at 9:15 AM, Jeff Layton <jlayton at redhat.com> wrote:
>> >
>> >>>
>> >>>  ------------[ cut here ]------------
>> >>> kernel BUG at fs/buffer.c:1220!
>> >>> invalid opcode: 0000 [#1] SMP
>> >>> CPU 0
>> >>> Pid: 3683, comm: a.out Not tainted 3.5.0 #3
>> >>> RIP: e030:[<ffffffff816a99f4>]  [<ffffffff816a99f4>]
>> >>> check_irqs_on.part.8+0x4/0x6
>> >>> RSP: e02b:ffff8807b156dc28  EFLAGS: 00010046
>> >>> RAX: ffff8807d0dd0000 RBX: ffff8807a7d6df28 RCX: 0000000005883396
>> >>> RDX: 0000000000001000 RSI: 0000000005883396 RDI: ffff8807cfc0c000
>> >>> RBP: ffff8807b156dc28 R08: 0000000000000001 R09: ffff8807a7d6de50
>> >>> R10: f83a2b0a359bf007 R11: 0000000000000000 R12: ffff8807a7d6de54
>> >>> R13: ffff8807a7d6de80 R14: ffff8807cfc1f120 R15: 0000000005883396
>> >>> FS:  00007f97164ec700(0000) GS:ffff8807ffc00000(0063) knlGS:0000000000000000
>> >>> CS:  e033 DS: 002b ES: 002b CR0: 000000008005003b
>> >>> CR2: 00000000f76ca3b0 CR3: 00000007bbb53000 CR4: 0000000000002660
>> >>> DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
>> >>> DR3: 0000000000000000 DR6: 00000000ffff0ff0 DR7: 0000000000000400
>> >>> Process a.out (pid: 3683, threadinfo ffff8807b156c000, task ffff8807bbae8000)
>> >>> Stack:
>> >>>  ffff8807b156dc98 ffffffff8116a099 ffff8807b59f3000 ffff8807b156dd30
>> >>>  ffff8807b156dd60 ffff8807b156de78 ffff8807b156dc78 ffffffff816af231
>> >>>  ffff8807b156dcd8 ffff8807a7d6e538 ffff8807a7d6df28 ffff8807a7d6de54
>> >>> Call Trace:
>> >>>  [<ffffffff8116a099>] __find_get_block+0x1f9/0x200
>> >>>  [<ffffffff816af231>] ? down_read+0x11/0x30
>> >>>  [<ffffffff811d1405>] ext3_clear_blocks+0x75/0x140
>> >>>  [<ffffffff811d15dc>] ext3_free_data+0x10c/0x150
>> >>>  [<ffffffff811e2061>] ? ext3_journal_start_sb+0x31/0x60
>> >>>  [<ffffffff811d1cb5>] ext3_truncate+0x4a5/0x600
>> >>>  [<ffffffff8123d5b8>] ? journal_start+0xb8/0x100
>> >>>  [<ffffffff8106f406>] ? bit_waitqueue+0x16/0xc0
>> >>>  [<ffffffff811d4598>] ext3_evict_inode+0x248/0x2c0
>> >>>  [<ffffffff81153b9a>] evict+0xaa/0x1b0
>> >>>  [<ffffffff81154843>] iput+0x103/0x210
>> >>>  [<ffffffff8114fc88>] dentry_iput+0x88/0xd0
>> >>>  [<ffffffff811505ec>] dput+0x12c/0x250
>> >>>  [<ffffffff81146275>] path_put+0x15/0x30
>> >>>  [<ffffffff810b2f35>] __audit_syscall_exit+0x2e5/0x460
>> >>>  [<ffffffff816b30be>] sysexit_audit+0x29/0x5b
>> >>> Code: 04 00 00 4c 8d 88 c0 02 00 00 31 c0 e8 5f da ff ff 48 85 db 74
>> >>> 0c 80 43 5c 01 48 89 df e8 d5
>> >>> 6a aa ff 5b 41 5c 5d c3 55 48 89 e5 <0f> 0b 55 48 89 e5 0f 0b 55 48 89
>> >>> e5 0f 0b 55 48 89 e5 41 54 53
>> >>> RIP  [<ffffffff816a99f4>] check_irqs_on.part.8+0x4/0x6
>> >>>  RSP <ffff8807b156dc28>
>> >>> ---[ end trace 8d09f8cfbb601c14 ]---
>> >>>
>> >>>
>> >>
>> >> I don't see a function called sysexit_audit in 3.5. I assume that's
>> >> created via some sort of macro goop?
>> >
>> > It looks like it's defined in arch/x86/kernel/entry_32.S
>> >
>> > my asm is non-existent, but it looks like it's calling
>> > __audit_syscall_exit and the disabling interrupts.
>> >
>> >> Could this be a xen specific problem?
>> >
>> > It could be. I asked the xen folks about this and they felt that it
>> > was unlikely that the bug itself was their
>> > (http://lists.xen.org/archives/html/xen-devel/2012-08/msg01052.html,
>> > specifically http://lists.xen.org/archives/html/xen-devel/2012-08/msg01127.html)
>> >
>> > xen could definitely be doing something wonky with irqs though.
>> >
>> >> Might be interesting to add a
>> >> check for irqs being disabled early in __audit_syscall_exit and see
>> >> whether its doing that universally in its syscall exit routine?
>> >
>> > I'll try this, thanks.
>>
>> So, I've added WARN_ON(irqs_disabled()) in various spots in the
>> audit_syscall_exit path all the way up to the path_put in
>> audit_free_names() that ends up triggering this and prior to Oops'ing,
>> I don't see anything.
>>
>> Post oops, I get lots of messages like:
>>
>> ------------[ cut here ]------------
>> WARNING: at kernel/auditsc.c:1021 audit_free_names+0xb0/0xdb()
>> Pid: 3981, comm: python2.6 Tainted: G      D W    3.2.5-at17-ganetixenu #31
>> Call Trace:
>>  [<ffffffff81693483>] ? audit_free_names+0xb0/0xdb
>>  [<ffffffff8105f405>] warn_slowpath_common+0x75/0xb0
>>  [<ffffffff8105f535>] warn_slowpath_null+0x15/0x20
>>  [<ffffffff81693483>] audit_free_names+0xb0/0xdb
>>  [<ffffffff810ac629>] audit_syscall_exit+0x139/0x1e0
>>  [<ffffffff8169fd6a>] sysexit_audit+0x21/0x5f
>> ---[ end trace ec7a15dbe40eaf1d ]---
>> ------------[ cut here ]------------
>> WARNING: at kernel/auditsc.c:1021 audit_free_names+0xb0/0xdb()
>> Pid: 3981, comm: python2.6 Tainted: G      D W    3.2.5-at17-ganetixenu #31
>> Call Trace:
>>  [<ffffffff81693483>] ? audit_free_names+0xb0/0xdb
>>  [<ffffffff8105f405>] warn_slowpath_common+0x75/0xb0
>>  [<ffffffff8105f535>] warn_slowpath_null+0x15/0x20
>>  [<ffffffff81693483>] audit_free_names+0xb0/0xdb
>>  [<ffffffff810ac629>] audit_syscall_exit+0x139/0x1e0
>>  [<ffffffff8169fd6a>] sysexit_audit+0x21/0x5f
>> ---[ end trace ec7a15dbe40eaf1e ]---
>>
>> where that python command is a python par file that may be doing some
>> funny unlinking as well. I haven't tracked it down.
>>
>> thoughts? does this not look like a bug in audit; maybe something
>> somewhere else is disabling interrupts?
>>
>
> I'm not sure whether it's a bug in audit or not. Tracking down what's
> disabling interrupts here is probably the key though...

Yeah. I'm trying to get a reproducer to work on a different version of Xen.

> So I'm guessing the WARN_ON above was in audit_free_names. Was there a
> similar WARN_ON in audit_syscall_exit that did not fire?

Correct. I booted a few kernels with the WARN_ON successively further
down the stack in audit land and it only started firing after BUG_ON()
in fs/buffer.c

Thanks for looking into this.

Cheers,
peter
-- 
Peter Moody      Google    1.650.253.7306
Security Engineer  pgp:0xC3410038