While running stress tests on adding and deleting ftrace instances I hit
this bug:
BUG: unable to handle kernel NULL pointer dereference at 0000000000000020
IP: selinux_inode_permission+0x85/0x160
PGD 63681067 PUD 7ddbe067 PMD 0
Oops: 0000 [#1] PREEMPT
CPU: 0 PID: 5634 Comm: ftrace-test-mki Not tainted 3.13.0-rc4-test-00033-gd2a6dde-dirty #20
Hardware name: /DG965MQ, BIOS MQ96510J.86A.0372.2006.0605.1717 06/05/2006
task: ffff880078375800 ti: ffff88007ddb0000 task.ti: ffff88007ddb0000
RIP: 0010:[<ffffffff812d8bc5>] [<ffffffff812d8bc5>] selinux_inode_permission+0x85/0x160
RSP: 0018:ffff88007ddb1c48 EFLAGS: 00010246
RAX: 0000000000000000 RBX: 0000000000800000 RCX: ffff88006dd43840
RDX: 0000000000000001 RSI: 0000000000000081 RDI: ffff88006ee46000
RBP: ffff88007ddb1c88 R08: 0000000000000000 R09: ffff88007ddb1c54
R10: 6e6576652f6f6f66 R11: 0000000000000003 R12: 0000000000000000
R13: 0000000000000081 R14: ffff88006ee46000 R15: 0000000000000000
FS: 00007f217b5b6700(0000) GS:ffffffff81e21000(0000) knlGS:0000000000000000
CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033^M
CR2: 0000000000000020 CR3: 000000006a0fe000 CR4: 00000000000007f0
Call Trace:
security_inode_permission+0x1c/0x30
__inode_permission+0x41/0xa0
inode_permission+0x18/0x50
link_path_walk+0x66/0x920
path_openat+0xa6/0x6c0
do_filp_open+0x43/0xa0
do_sys_open+0x146/0x240
SyS_open+0x1e/0x20
system_call_fastpath+0x16/0x1b
Code: 84 a1 00 00 00 81 e3 00 20 00 00 89 d8 83 c8 02 40 f6 c6 04 0f 45 d8 40 f6 c6 08 74 71 80 cf 02 49 8b 46 38 4c 8d 4d cc 45 31 c0 <0f> b7 50 20 8b 70 1c 48 8b 41 70 89 d9 8b 78 04 e8 36 cf ff ff
RIP selinux_inode_permission+0x85/0x160
CR2: 0000000000000020
Investigating, I found that the inode->i_security was NULL, and the
dereference of it caused the oops.
in selinux_inode_permission():
isec = inode->i_security;
rc = avc_has_perm_noaudit(sid, isec->sid, isec->sclass, perms, 0, &avd);
Note, the crash came from stressing the deletion and reading of debugfs
files. I was not able to recreate this via normal files. But I'm not
sure they are safe. It may just be that the race window is much harder
to hit.
What seems to have happened (and what I have traced), is the file is
being opened at the same time the file or directory is being deleted.
As the dentry and inode locks are not held during the path walk, nor is
the inodes ref counts being incremented, there is nothing saving these
structures from being discarded except for an rcu_read_lock().
The rcu_read_lock() protects against freeing of the inode, but it does
not protect freeing of the inode_security_struct. Now if the freeing of
the i_security happens with a call_rcu(), and the i_security field of
the inode is not changed (it gets freed as the inode gets freed) then
there will be no issue here. (Linus Torvalds suggested not setting the
field to NULL such that we do not need to check if it is NULL in the
permission check).
Note, this is a hack, but it fixes the problem at hand. A real fix is
to restructure the destroy_inode() to call all the destructor handlers
from the RCU callback. But that is a major job to do, and requires a
lot of work. For now, we just band-aid this bug with this fix (it
works), and work on a more maintainable solution in the future.
Link: http://lkml.kernel.org/r/20140109101932.0508dec7@gandalf.local.home
Link: http://lkml.kernel.org/r/20140109182756.17abaaa8@gandalf.local.home
Cc: stable@vger.kernel.org
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
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