[PATCH -next] fork: silence a false postive warning in __mmdrop

Qian Cai cai at lca.pw
Wed Sep 9 02:50:44 AEST 2020 

On Wed, Jul 22, 2020 at 03:44:06PM +0200, Peter Zijlstra wrote:
> On Wed, Jul 22, 2020 at 09:19:00AM -0400, Qian Cai wrote:
> > On Wed, Jul 22, 2020 at 12:06:37PM +0200, peterz at infradead.org wrote:
> > > On Thu, Jun 04, 2020 at 11:03:44AM -0400, Qian Cai wrote:
> > > > The linux-next commit bf2c59fce407 ("sched/core: Fix illegal RCU from
> > > > offline CPUs") delayed,
> > > > 
> > > > idle->active_mm = &init_mm;
> > > > 
> > > > into finish_cpu() instead of idle_task_exit() which results in a false
> > > > positive warning that was originally designed in the commit 3eda69c92d47
> > > > ("kernel/fork.c: detect early free of a live mm").
> > > > 
> > > >  WARNING: CPU: 127 PID: 72976 at kernel/fork.c:697
> > > >  __mmdrop+0x230/0x2c0
> > > >  do_exit+0x424/0xfa0
> > > >  Call Trace:
> > > >  do_exit+0x424/0xfa0
> > > >  do_group_exit+0x64/0xd0
> > > >  sys_exit_group+0x24/0x30
> > > >  system_call_exception+0x108/0x1d0
> > > >  system_call_common+0xf0/0x278
> > > 
> > > Please explain; because afaict this is a use-after-free.
> > > 
> > > The thing is __mmdrop() is going to actually free the mm, so then what
> > > is finish_cpu()'s mmdrop() going to do?
> > > 
> > > ->active_mm() should have a refcount on the mm.
> > 
> > Well, the refcount issue you mentioned then happens all before bf2c59fce407 was
> > introduced as well, but then it looks harmless because mmdrop() in finish_cpu()
> > will do,
> > 
> > 	if (unlikely(atomic_dec_and_test(&mm->mm_count)))
> > 		__mmdrop(mm);
> 
> That's not harmless, that's a use-after-free. Those can cause memory
> corruption bugs and the like at best. Who knows what's at the location
> of mm->mm_count after we've already freed it.
> 
> > where that atomic_dec_and_test() see the negative refcount and will not involve
> > __mmdrop() again. It is not clear to me that once the CPU is offline if it
> > needs to care about its idle thread mm_count at all. Even if this refcount
> > issue is finally addressed, it could hit this warning in finish_cpu() without
> > this patch.
> > 
> > On the other hand, if you look at the commit 3eda69c92d47, it is clearly that
> > the assumption of,
> > 
> >    WARN_ON_ONCE(mm == current->active_mm);
> > 
> > is totally gone due to bf2c59fce407. Thus, the patch is to fix that discrepancy
> > first and then I'll look at that the imbalance mmdrop()/mmgrab() elsewhere.
> 
> No, you're talking nonsense. We must not free @mm when
> 'current->active_mm == mm', never.

Yes, you are right. It still trigger this below on powerpc with today's
linux-next by fuzzing for a while (saw a few times on recent linux-next before
as well but so far mostly reproducible on powerpc here). Any idea?

[12802.547809][T191552] BUG mm_struct (Tainted: G           O     ): Poison overwritten
[12802.547824][T191552] -----------------------------------------------------------------------------
[12802.547824][T191552] 
[12802.547843][T191552] Disabling lock debugging due to kernel taint
[12802.547867][T191552] INFO: 0x000000000e2a54ec-0x000000000e2a54ec @offset=96464. First byte 0x6a instead of 0x6b
[12802.547889][T191552] INFO: Allocated in dup_mm+0x48/0x6d0 age=955 cpu=108 pid=191552
[12802.547915][T191552] 	__slab_alloc+0xa4/0xf0
[12802.547937][T191552] 	kmem_cache_alloc+0x314/0x4a0
[12802.547959][T191552] 	dup_mm+0x48/0x6d0
dup_mm at kernel/fork.c:1344
[12802.547978][T191552] 	copy_process+0x11bc/0x19a0
[12802.548010][T191552] 	kernel_clone+0x120/0xb80
[12802.548031][T191552] 	__do_sys_clone+0x88/0xd0
[12802.548055][T191552] 	system_call_exception+0xf8/0x1d0
[12802.548083][T191552] 	system_call_common+0xe8/0x218
[12802.548093][T191552] INFO: Freed in __mmdrop+0x144/0x250 age=942 cpu=69 pid=882503
[12802.548140][T191552] 	kmem_cache_free+0x47c/0x500
[12802.548161][T191552] 	__mmdrop+0x144/0x250
__mmdrop at kernel/fork.c:685
[12802.548170][T191552] 	do_exit+0x3f4/0xed0
[12802.548212][T191552] 	do_group_exit+0x5c/0xd0
[12802.548244][T191552] 	sys_exit_group+0x1c/0x20
[12802.548277][T191552] 	system_call_exception+0xf8/0x1d0
[12802.548309][T191552] 	system_call_common+0xe8/0x218
[12802.548342][T191552] INFO: Slab 0x0000000048df84af objects=64 used=64 fp=0x0000000000000000 flags=0x87fff8000010200
[12802.548379][T191552] INFO: Object 0x00000000583c5ba3 @offset=96384 fp=0x00000000681f5d04
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[12802.557351][T191552] Object 000000005edb4355: 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b  kkkkkkkkkkkkkkkk
[12802.557484][T191552] Object 0000000049aaca1e: 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b  kkkkkkkkkkkkkkkk
[12802.557600][T191552] Object 000000000eb0b7f9: 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b  kkkkkkkkkkkkkkkk
[12802.557727][T191552] Object 000000008fdb29be: 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b a5  kkkkkkkkkkkkkkk.
[12802.557831][T191552] Redzone 00000000c5a61231: bb bb bb bb bb bb bb bb                          ........
[12802.557947][T191552] Padding 000000003163b13a: 5a 5a 5a 5a 5a 5a 5a 5a 5a 5a 5a 5a 5a 5a 5a 5a  ZZZZZZZZZZZZZZZZ
[12802.558076][T191552] Padding 0000000092412b1a: 5a 5a 5a 5a 5a 5a 5a 5a 5a 5a 5a 5a 5a 5a 5a 5a  ZZZZZZZZZZZZZZZZ
[12802.558187][T191552] Padding 00000000319fa8cb: 5a 5a 5a 5a 5a 5a 5a 5a 5a 5a 5a 5a 5a 5a 5a 5a  ZZZZZZZZZZZZZZZZ
[12802.558314][T191552] Padding 00000000963c7ce8: 5a 5a 5a 5a 5a 5a 5a 5a 5a 5a 5a 5a 5a 5a 5a 5a  ZZZZZZZZZZZZZZZZ
[12802.558432][T191552] CPU: 71 PID: 191552 Comm: trinity-main Tainted: G    B      O      5.9.0-rc4-next-20200908+ #1
[12802.558551][T191552] Call Trace:
[12802.558590][T191552] [c000201cb3427620] [c000000000701758] dump_stack+0xec/0x144 (unreliable)
[12802.558691][T191552] [c000201cb3427660] [c0000000003cb53c] print_trailer+0x278/0x2a0
[12802.558794][T191552] [c000201cb34276f0] [c0000000003c0d14] check_bytes_and_report+0x184/0x1b0
[12802.558900][T191552] [c000201cb34277a0] [c0000000003c1000] check_object+0x2c0/0x330
[12802.558990][T191552] [c000201cb3427800] [c0000000003c11ec] alloc_debug_processing+0x17c/0x1e0
[12802.559096][T191552] [c000201cb3427880] [c0000000003c5468] ___slab_alloc+0xb78/0xc60
[12802.559190][T191552] [c000201cb3427980] [c0000000003c55f4] __slab_alloc+0xa4/0xf0
[12802.559284][T191552] [c000201cb34279d0] [c0000000003c5954] kmem_cache_alloc+0x314/0x4a0
[12802.559362][T191552] [c000201cb3427a50] [c0000000000c4818] dup_mm+0x48/0x6d0
[12802.559445][T191552] [c000201cb3427b00] [c0000000000c665c] copy_process+0x11bc/0x19a0
[12802.559528][T191552] [c000201cb3427c20] [c0000000000c7210] kernel_clone+0x120/0xb80
[12802.559630][T191552] [c000201cb3427d00] [c0000000000c7cf8] __do_sys_clone+0x88/0xd0
[12802.559714][T191552] [c000201cb3427dc0] [c00000000002c748] system_call_exception+0xf8/0x1d0
[12802.559810][T191552] [c000201cb3427e20] [c00000000000d0a8] system_call_common+0xe8/0x218
[12802.559906][T191552] FIX mm_struct: Restoring 0x000000000e2a54ec-0x000000000e2a54ec=0x6b
[12802.559906][T191552] 
[12802.560030][T191552] FIX mm_struct: Marking all objects used

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