Quoting Jason A. Donenfeld (2020-09-28 11:35:07)
> 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
>
> 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@xxxxxxxx>
> Cc: Peter Xu <peterx@xxxxxxxxxx>
> Signed-off-by: Jason A. Donenfeld <Jason@xxxxxxxxx>
> ---
> 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(¤t->mm->has_pinned, 1);
> + atomic_set(&mm->has_pinned, 1);
That's literally the same diff as I was just testing :)
I can attest that it fixes the i915 issue, but since that's also your
test case, I'm not adding much information.
-Chris
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