Re: kernel: signal: NULL ptr deref when killing process
From: Oleg Nesterov
Date: Wed Aug 27 2014 - 10:18:47 EST
On 08/26, Sasha Levin wrote:
>
> 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:
Thanks.
rsi == ¤t->signal->shared_pending == 0x2a0, so current->signal == 544.
Given that this task is current, we can rule out RCU/find_pind/etc problems,
this task_struct and its ->signal must be stable.
Looks like, something was broken recently. I do not remember the bug reports
like this. Say, an unbalanced put_task_struct()... Hmm.
> [ 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
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/