Re: [PATCH] mm: Fix mmap_assert_locked() in follow_pte()

[Andrew Morton <akpm@linux-foundation.org>]

On Wed, 10 Jul 2024 22:13:17 -0700 Pei Li <peili.dev@gmail.com> wrote:

> This patch fixes this warning by acquiring read lock before entering
> untrack_pfn() while write lock is not held.
> 
> syzbot has tested the proposed patch and the reproducer did not
> trigger any issue.
> 

Thanks.

> ---
> Syzbot reported the following warning in follow_pte():
> 
> WARNING: CPU: 3 PID: 5192 at include/linux/rwsem.h:195 rwsem_assert_held include/linux/rwsem.h:195 [inline]
> WARNING: CPU: 3 PID: 5192 at include/linux/rwsem.h:195 mmap_assert_locked include/linux/mmap_lock.h:65 [inline]
> WARNING: CPU: 3 PID: 5192 at include/linux/rwsem.h:195 follow_pte+0x414/0x4c0 mm/memory.c:5980
> 
> This is because we are assuming that mm->mmap_lock should be held when
> entering follow_pte(). This is added in commit c5541ba378e3 (mm:
> follow_pte() improvements).
> 
> However, in the following call stack, we are not acquring the lock:
>  follow_phys arch/x86/mm/pat/memtype.c:957 [inline]
>  get_pat_info+0xf2/0x510 arch/x86/mm/pat/memtype.c:991
>  untrack_pfn+0xf7/0x4d0 arch/x86/mm/pat/memtype.c:1104
>  unmap_single_vma+0x1bd/0x2b0 mm/memory.c:1819
>  zap_page_range_single+0x326/0x560 mm/memory.c:1920
> 
> In zap_page_range_single(), we passed mm_wr_locked as false, as we do
> not expect write lock to be held.
> In the special case where vma->vm_flags is set as VM_PFNMAP, we are
> hitting untrack_pfn() which eventually calls into follow_phys.

I included the above (very relevant) info in the changelog.

And I added 

	Fixes: c5541ba378e3 ("mm: follow_pte() improvements")

and queued the patch for 6.10-rc7.  Hopefully David can review it for us.