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Subject3.6-rc4 audit_log_d_path oops.
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@redhat.com>

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);

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