Re: kernel: signal: NULL ptr deref when killing process

[Sasha Levin]

On 08/21/2014 01:17 PM, Oleg Nesterov wrote:
>> Is there a race between kill() and exit() brought on by the kill path only
>> > using the RCU read lock?  This doesn't prevent ->real_cred from being
>> > modified, but it looks like this should, in combination with
>> > delayed_put_task_struct(), prevent it from being cleared.
> Yes, rcu should protect us from both delayed_put_pid() and delayed_put_task().
> Everything looks correct... And there are a lot of other similar users of
> find_vpid/find_task_by_vpid/pid_task/etc under rcu, I can't recall any bug
> in this area.
> 
> I am puzzled. Note also that ->signal == NULL. Will try to think more,
> but so far I have no any idea.

I've hit something similar earlier today, and it might be related:

[  973.452840] BUG: unable to handle kernel NULL pointer dereference at 00000000000002b0
[  973.455347] IP: flush_sigqueue_mask (include/linux/signal.h:118 kernel/signal.c:715)
[  973.457526] PGD 4dfdc7067 PUD 5f77d9067 PMD 0
[  973.459216] Oops: 0000 [#1] PREEMPT SMP DEBUG_PAGEALLOC
[  973.460086] Dumping ftrace buffer:
[  973.460086]    (ftrace buffer empty)
[  973.460086] Modules linked in:
[  973.460086] CPU: 4 PID: 13145 Comm: trinity-c767 Not tainted 3.17.0-rc2-next-20140826-sasha-00031-gc48c9ac-dirty #1079
[  973.460086] task: ffff880604800000 ti: ffff880586648000 task.ti: ffff880586648000
[  973.460086] RIP: flush_sigqueue_mask (include/linux/signal.h:118 kernel/signal.c:715)
[  973.460086] RSP: 0018:ffff88058664bec8  EFLAGS: 00010046
[  973.460086] RAX: 0000000000000000 RBX: fffffffffffff730 RCX: 0000000000010000
[  973.460086] RDX: 0000000000000000 RSI: 00000000000002a0 RDI: ffff88058664bed8
[  973.460086] RBP: ffff88058664bf10 R08: 0000000000000001 R09: 0000000000000001
[  973.460086] R10: 000000000002d201 R11: 0000000000000254 R12: 0000000000000000
[  973.460086] R13: ffff88058664bf40 R14: ffff880604800000 R15: 0000000000000010
[  973.460086] FS:  00007fe3a3045700(0000) GS:ffff880277c00000(0000) knlGS:0000000000000000
[  973.460086] CS:  0010 DS: 0000 ES: 0000 CR0: 000000008005003b
[  973.460086] CR2: 00000000000002b0 CR3: 00000004e23d5000 CR4: 00000000000006a0
[  973.460086] Stack:
[  973.460086]  ffffffffac183690 01017fffb3247180 0000000000010000 0000000000000000
[  973.460086]  00007fffb3247180 00007fffb3247220 0000000000000011 0000000000000000
[  973.460086]  0000000000000000 ffff88058664bf78 ffffffffac183ef5 0000000000000000
[  973.460086] Call Trace:
[  973.460086] ? do_sigaction (kernel/signal.c:3124 (discriminator 17))
[  973.460086] SyS_rt_sigaction (kernel/signal.c:3360 kernel/signal.c:3341)
[  973.460086] tracesys (arch/x86/kernel/entry_64.S:542)
[ 973.460086] Code: b7 49 09 d5 4d 89 6e 10 48 83 c4 08 5b 41 5c 41 5d 41 5e 41 5f 5d c3 66 2e 0f 1f 84 00 00 00 00 00 66 66 66 66 90 48 8b 0f 31 c0 <48> 8b 56 10 48 85 ca 74 7b 55 48 f7 d1 48 89 e5 41 56 48 21 ca
All code
========
   0:	b7 49                	mov    $0x49,%bh
   2:	09 d5                	or     %edx,%ebp
   4:	4d 89 6e 10          	mov    %r13,0x10(%r14)
   8:	48 83 c4 08          	add    $0x8,%rsp
   c:	5b                   	pop    %rbx
   d:	41 5c                	pop    %r12
   f:	41 5d                	pop    %r13
  11:	41 5e                	pop    %r14
  13:	41 5f                	pop    %r15
  15:	5d                   	pop    %rbp
  16:	c3                   	retq
  17:	66 2e 0f 1f 84 00 00 	nopw   %cs:0x0(%rax,%rax,1)
  1e:	00 00 00
  21:	66 66 66 66 90       	data32 data32 data32 xchg %ax,%ax
  26:	48 8b 0f             	mov    (%rdi),%rcx
  29:	31 c0                	xor    %eax,%eax
  2b:*	48 8b 56 10          	mov    0x10(%rsi),%rdx		<-- trapping instruction
  2f:	48 85 ca             	test   %rcx,%rdx
  32:	74 7b                	je     0xaf
  34:	55                   	push   %rbp
  35:	48 f7 d1             	not    %rcx
  38:	48 89 e5             	mov    %rsp,%rbp
  3b:	41 56                	push   %r14
  3d:	48 21 ca             	and    %rcx,%rdx
	...

Code starting with the faulting instruction
===========================================
   0:	48 8b 56 10          	mov    0x10(%rsi),%rdx
   4:	48 85 ca             	test   %rcx,%rdx
   7:	74 7b                	je     0x84
   9:	55                   	push   %rbp
   a:	48 f7 d1             	not    %rcx
   d:	48 89 e5             	mov    %rsp,%rbp
  10:	41 56                	push   %r14
  12:	48 21 ca             	and    %rcx,%rdx
	...
[  973.460086] RIP flush_sigqueue_mask (include/linux/signal.h:118 kernel/signal.c:715)
[  973.460086]  RSP <ffff88058664bec8>
[  973.460086] CR2: 00000000000002b0


Thanks,
Sasha
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