NFS causing oops when freeing namespace

["Daniel Hokka Zakrisson" <daniel@hozac.com>]

The test-case at the bottom causes the following recursive Oopsing on
2.6.20-rc5:

BUG: unable to handle kernel NULL pointer dereference at virtual address
00000504
 printing eip:
c02292d4
*pde = 00000000
Oops: 0002 [#1]
PREEMPT SMP
Modules linked in:
CPU:    0
EIP:    0060:[<c02292d4>]    Not tainted VLI
EFLAGS: 00010046   (2.6.20-rc5 #11)
EIP is at _raw_spin_trylock+0x4/0x30
eax: 00000000   ebx: 00000213   ecx: 00000001   edx: 00000504
esi: 00000504   edi: c1493da0   ebp: d5b5ff3c   esp: d5b5fee0
ds: 007b   es: 007b   ss: 0068
Process mount (pid: 2618, ti=d5b5e000 task=c17fdaa0 task.ti=d5b5e000)
Stack: c03be6f0 00000000 00000000 c01f6115 c04240dc c17fdaa0 c17fdc2c
00000000
       00000000 d555ebe0 c04734a0 c01d26bd c01d8e9c d553b800 c0161c5d
d707443c
       d54cedc0 c0175e0e d779c9a0 d779cd60 c1493f20 d7f1caa0 c0175e86
d54cee40
Call Trace:
 [<c03be6f0>] _spin_lock_irqsave+0x20/0x90
 [<c01f6115>] lockd_down+0x125/0x190
 [<c01d26bd>] nfs_free_server+0x6d/0xd0
 [<c01d8e9c>] nfs_kill_super+0xc/0x20
 [<c0161c5d>] deactivate_super+0x7d/0xa0
 [<c0175e0e>] release_mounts+0x6e/0x80
 [<c0175e86>] __put_mnt_ns+0x66/0x80
 [<c0132b3e>] free_nsproxy+0x5e/0x60
 [<c011f021>] do_exit+0x791/0x810
 [<c011f0c6>] do_group_exit+0x26/0x70
 [<c0103142>] sysenter_past_esp+0x5f/0x85
 [<c03b0033>] rpc_wake_up+0x3/0x70
 =======================
Code: ff ff c3 8d 74 26 00 c7 00 00 00 00 01 c7 40 08 ed 1e af de c7 40 10
ff ff ff ff c7 40 0c ff ff ff ff c3 8d 74 26 00 89 c2 31 c0 <86> 02 31 c9
84 c0 0f 9f c1 85 c9 74 12 65 a1 04 00 00 00 89 42
EIP: [<c02292d4>] _raw_spin_trylock+0x4/0x30 SS:ESP 0068:d5b5fee0
 <1>BUG: unable to handle kernel NULL pointer dereference at virtual
address 00000024
 printing eip:
c011e8dd
*pde = 00000000
Oops: 0000 [#2]
PREEMPT SMP
Modules linked in:
CPU:    0
EIP:    0060:[<c011e8dd>]    Not tainted VLI
EFLAGS: 00010006   (2.6.20-rc5 #11)
EIP is at do_exit+0x4d/0x810
eax: 00000000   ebx: d5b5fea8   ecx: c17fdaa0   edx: 00000001
esi: c17fdaa0   edi: 00000a3a   ebp: 0000000b   esp: d5b5fde4
ds: 007b   es: 007b   ss: 0068
Process mount (pid: 2618, ti=d5b5e000 task=c17fdaa0 task.ti=d5b5e000)
Stack: 00000a3a d5b5e000 c17fdaa0 d5b5e000 00000003 c046a307 0000000f
d5b5fee0
       00000068 00000013 c011c3cb c042b321 d5b5fe24 d5b5fea8 d5b5fee0
00000068
       00000000 c0104816 c041347f 00000068 d5b5fee0 00000001 d5b5fea8
c0414df5
Call Trace:
 [<c011c3cb>] printk+0x1b/0x20
 [<c0104816>] die+0x246/0x260
 [<c03c09b0>] do_page_fault+0x2e0/0x630
 [<c011c2cf>] vprintk+0x28f/0x370
 [<c03c06d0>] do_page_fault+0x0/0x630
 [<c03beea4>] error_code+0x7c/0x84
 [<c03b007b>] rpc_wake_up+0x4b/0x70
 [<c02292d4>] _raw_spin_trylock+0x4/0x30
 [<c03be6f0>] _spin_lock_irqsave+0x20/0x90
 [<c01f6115>] lockd_down+0x125/0x190
 [<c01d26bd>] nfs_free_server+0x6d/0xd0
 [<c01d8e9c>] nfs_kill_super+0xc/0x20
 [<c0161c5d>] deactivate_super+0x7d/0xa0
 [<c0175e0e>] release_mounts+0x6e/0x80
 [<c0175e86>] __put_mnt_ns+0x66/0x80
 [<c0132b3e>] free_nsproxy+0x5e/0x60
 [<c011f021>] do_exit+0x791/0x810
 [<c011f0c6>] do_group_exit+0x26/0x70
 [<c0103142>] sysenter_past_esp+0x5f/0x85
 [<c03b0033>] rpc_wake_up+0x3/0x70
 =======================
Code: 03 00 00 89 e0 25 00 e0 ff ff f7 40 14 00 ff ff 0f 0f 85 69 03 00 00
8b be a4 00 00 00 85 ff 0f 84 43 03 00 00 8b 86 48 04 00 00 <8b> 50 24 3b
72 10 0f 84 1c 03 00 00 65 a1 08 00 00 00 f6 40 11
EIP: [<c011e8dd>] do_exit+0x4d/0x810 SS:ESP 0068:d5b5fde4

It seems to go on forever. addr2line output follows:
$ addr2line -e vmlinux c02292d4 c03be6f0 c01f6115 c01d26bd c01d8e9c
c0161c5d c0175e0e c0175e86 c0132b3e c011f021 c011e8dd
include/asm/spinlock.h:90
kernel/spinlock.c:280
fs/lockd/svc.c:345
fs/nfs/client.c:782
fs/nfs/super.c:679
fs/super.c:184
include/linux/list.h:299
fs/namespace.c:1879
include/linux/mnt_namespace.h:26
include/linux/nsproxy.h:42
include/linux/pid_namespace.h:42

The problem appears to be that fs/lockd/svc.c:lockd_down tries to lock
current->sighand->siglock, but during the time it's sleeping the process
is reaped and released, setting sighand to NULL.

This also affects 2.6.19.2, but since there are no pid spaces there, it's
"just" a single oops, and doesn't make the machine totally unusable.

Test-case, execute as ./a.out mount -t nfs ...:
#include <stdio.h>
#include <sched.h>
#include <unistd.h>

int main(int argc, char *argv[])
{
	if (unshare(CLONE_NEWNS) == -1) {
		perror("unshare");
		exit(1);
	}
	execv("/bin/mount", argv+1);
	perror("execv(mount)");
	return 1;
}

-- 
Daniel Hokka Zakrisson
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