Re: 3.6-rc4 audit_log_d_path oops.

[Eric Paris]

On Thu, Sep 6, 2012 at 11:16 AM, Dave Jones <davej@xxxxxxxxxx> wrote:
> On Thu, Sep 06, 2012 at 09:46:28AM -0400, Dave Jones wrote:
>  > Hit this in overnight fuzz testing..
>  >
>  > BUG: unable to handle kernel NULL pointer dereference at 0000000000000020
>  > IP: [<ffffffff81103365>] audit_log_d_path+0x35/0xf0
>  > PGD 12fded067 PUD 142c06067 PMD 0
>  > Oops: 0000 [#1] SMP
>  > Modules linked in: tun fuse ipt_ULOG binfmt_misc nfnetlink nfc caif_socket caif phonet can llc2 pppoe pppox ppp_generic slhc irda crc_ccitt rds af_key decnet rose x25 atm netrom appletalk ipx p8023 psnap p8022 llc ax25 lockd sunrpc bluetooth rfkill ip6t_REJECT nf_conntrack_ipv6 nf_defrag_ipv6 ip6table_filter ip6_tables nf_conntrack_ipv4 nf_defrag_ipv4 xt_state nf_conntrack kvm_intel kvm crc32c_intel ghash_clmulni_intel microcode pcspkr i2c_i801 e1000e uinput i915 video i2c_algo_bit drm_kms_helper drm i2c_core
>  > CPU 5
>  > Pid: 7007, comm: trinity-child5 Not tainted 3.6.0-rc4+ #36
>  > RIP: 0010:[<ffffffff81103365>]  [<ffffffff81103365>] audit_log_d_path+0x35/0xf0
>  > RSP: 0018:ffff880116b33ec8  EFLAGS: 00010246
>  > RAX: 0000000000000000 RBX: 0000000000000000 RCX: 0000000000000035
>  > RDX: ffffffff819dbf5d RSI: ffffffff819da5bb RDI: 0000000000000000
>  > RBP: ffff880116b33ee8 R08: ffff88001942533e R09: 0000000000000000
>  > R10: 0000000000000001 R11: 0000000000000000 R12: ffff880116b33f38
>  > R13: ffff880116b33f38 R14: ffffffff819fa1eb R15: 0000000000000005
>  > FS:  00007f5742581740(0000) GS:ffff880148600000(0000) knlGS:0000000000000000
>  > CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
>  > CR2: 0000000000000020 CR3: 00000001046d3000 CR4: 00000000001407e0
>  > DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
>  > DR3: 0000000000000000 DR6: 00000000ffff0ff0 DR7: 0000000000000400
>  > Process trinity-child5 (pid: 7007, threadinfo ffff880116b32000, task ffff880019424d00)
>  > Stack:
>  >  ffff880116b33ee8 0000000000000000 ffff880116b33f38 ffff880019424d00
>  >  ffff880116b33f18 ffffffff81103522 00000000000001d0 ffff880119ed6500
>  >  00000000000001d0 0000000000000109 ffff880116b33f78 ffffffff811ebb95
>  > Call Trace:
>  >  [<ffffffff81103522>] audit_log_link_denied+0x92/0x100
>  >  [<ffffffff811ebb95>] sys_linkat+0x195/0x1e0
>  >  [<ffffffff813573de>] ? trace_hardirqs_on_thunk+0x3a/0x3f
>  >  [<ffffffff816a50ed>] system_call_fastpath+0x1a/0x1f
>  > Code: 5d e8 4c 89 65 f0 4c 89 6d f8 0f 1f 44 00 00 48 85 f6 48 89 fb 49 89 d5 74 11 48 89 f2 31 c0 48 c7 c6 bb a5 9d 81 e8 8b e1 ff ff <8b> 73 20 40 f6 c6 01 75 62 48 8b 3d 33 33 93 01 48 85 ff 74 4e
>  > RIP  [<ffffffff81103365>] audit_log_d_path+0x35/0xf0
>  >  RSP <ffff880116b33ec8>
>  > CR2: 0000000000000020
>  > ---[ end trace 85b88c850143bb1c ]---
>  >
>  > That's here in kernel/audit.c
>  >
>  > 1433
>  > 1434         /* We will allow 11 spaces for ' (deleted)' to be appended */
>  > 1435         pathname = kmalloc(PATH_MAX+11, ab->gfp_mask);
>  >
>  > 'ab' is NULL.
>  >
>  > Looks like audit_log_link_denied needs to handle potential failure from
>  > audit_log_start and abort early. (oddly, it looks like every other
>  > function called there checks for !ab.)
>  >
>  > Maybe additional code should be added here to printk the audit message
>  > to dmesg so that we don't lose it entirely, but for now, minimal fix.
>  >
>  > Signed-off-by: Dave Jones <davej@xxxxxxxxxx>
>  >
>  > diff --git a/kernel/audit.c b/kernel/audit.c
>  > index ea3b7b6..c3e85bb 100644
>  > --- a/kernel/audit.c
>  > +++ b/kernel/audit.c
>  > @@ -1466,6 +1466,9 @@ void audit_log_link_denied(const char *operation, struct path *link)
>  >
>  >      ab = audit_log_start(current->audit_context, GFP_KERNEL,
>  >                           AUDIT_ANOM_LINK);
>  > +    if (!ab)
>  > +            return;
>  > +
>  >      audit_log_format(ab, "op=%s action=denied", operation);
>  >      audit_log_format(ab, " pid=%d comm=", current->pid);
>  >      audit_log_untrustedstring(ab, current->comm);
>
> Added Kees, as this was introduced in a51d9eaa41866ab6b4b6ecad7b621f8b66ece0dc
>
> I just realised, the funny thing about this is that the machine running that test
> had selinux/audit disabled. And yet here we are, screwing around with audit buffers.
>
> Should there be a test on audit_enable=0 in audit_log_link_denied() ?
>
> I'm now curious how much more of the audit code is getting run through similar lack of tests

Possible there are other places.  The usual idium is to put a a static
inline audit_[blah] in the header file which just does if
(!audit_dummy_context()) return __audit_[blah]; return 0;

Thus if audit is off or compiled out nothing gets run.
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