[PATCH] mm: do not rely on mm == current->mm in __get_user_pages_locked

[Peter Xu]

On Mon, Sep 28, 2020 at 12:35:07PM +0200, Jason A. Donenfeld wrote:
> It seems likely this block was pasted from internal_get_user_pages_fast,
> which is not passed an mm struct and therefore uses current's. But
> __get_user_pages_locked is passed an explicit mm, and current->mm is not
> always valid. This was hit when being called from i915, which uses:
> 
>   pin_user_pages_remote->
>     __get_user_pages_remote->
>       __gup_longterm_locked->
>         __get_user_pages_locked

Afaict it's not only an "current->mm can be NULL" issue - because this flag is
used to mark "whether the mm pinned any page", so for remote pinning we
definitely should mark the remote mm rather than the current mm, simply because
it's the target mm page table that we'd want to stablize rather than the
current->mm (even if current->mm always existed).

> 
> Before, this would lead to an OOPS:
> 
>   BUG: kernel NULL pointer dereference, address: 0000000000000064
>   #PF: supervisor write access in kernel mode
>   #PF: error_code(0x0002) - not-present page
>   PGD 0 P4D 0
>   Oops: 0002 [#1] SMP
>   CPU: 10 PID: 1431 Comm: kworker/u33:1 Tainted: P S   U     O      5.9.0-rc7+ #140
>   Hardware name: LENOVO 20QTCTO1WW/20QTCTO1WW, BIOS N2OET47W (1.34 ) 08/06/2020
>   Workqueue: i915-userptr-acquire __i915_gem_userptr_get_pages_worker [i915]
>   RIP: 0010:__get_user_pages_remote+0xd7/0x310
>   Code: f5 01 00 00 83 7d 00 01 0f 85 ed 01 00 00 f7 c1 00 00 04 00 0f 84 58 01 00 00 65 48 8b 04 25 00 6d 01 00 48 8b 80 40 03 00 00 <c7> 40 64 01 00 00 00 65 48 8b 04 25 00 6d 01 00 48 c7 44 24 18 00
>   RSP: 0018:ffff888fdfe47de0 EFLAGS: 00010206
>   RAX: 0000000000000000 RBX: 00007fe188531000 RCX: 0000000000040001
>   RDX: 0000000000000001 RSI: 00007fe188531000 RDI: ffff888ff0748f00
>   RBP: ffff888fdfe47e54 R08: ffff888fedc7d7c8 R09: 0000000000000000
>   R10: 0000000000000018 R11: fefefefefefefeff R12: ffff888ff0748f00
>   R13: ffff888fedc7d7c8 R14: ffff888f81fe3a40 R15: 0000000000042003
>   FS:  0000000000000000(0000) GS:ffff888ffc480000(0000) knlGS:0000000000000000
>   CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
>   CR2: 0000000000000064 CR3: 0000000002009003 CR4: 00000000003706e0
>   DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
>   DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
>   Call Trace:
>    __i915_gem_userptr_get_pages_worker+0xc8/0x260 [i915]
>    process_one_work+0x1ca/0x390
>    worker_thread+0x48/0x3c0
>    ? rescuer_thread+0x3d0/0x3d0
>    kthread+0x114/0x130
>    ? kthread_create_worker_on_cpu+0x40/0x40
>    ret_from_fork+0x1f/0x30
>   CR2: 0000000000000064
> 
> This commit fixes the problem by using the mm pointer passed to the
> function rather than the bogus one in current.
> 
> Fixes: 008cfe4418b3 ("mm: Introduce mm_struct.has_pinned")
> Cc: Jason Gunthorpe <jgg at ziepe.ca>
> Cc: Peter Xu <peterx at redhat.com>
> Signed-off-by: Jason A. Donenfeld <Jason at zx2c4.com>
> ---
>  mm/gup.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/mm/gup.c b/mm/gup.c
> index dfe781d2ad4c..e869c634cc9a 100644
> --- a/mm/gup.c
> +++ b/mm/gup.c
> @@ -1256,7 +1256,7 @@ static __always_inline long __get_user_pages_locked(struct mm_struct *mm,
>  	}
>  
>  	if (flags & FOLL_PIN)
> -		atomic_set(&current->mm->has_pinned, 1);
> +		atomic_set(&mm->has_pinned, 1);
>  
>  	/*
>  	 * FOLL_PIN and FOLL_GET are mutually exclusive. Traditional behavior
> -- 
> 2.28.0
> 

Thanks!  And sorry for this silly mistake.  I even didn't understand how it was
written, because the normal gup change should have come earlier, anyway...

Reviewed-by: Peter Xu <peterx at redhat.com>

-- 
Peter Xu