Re: [PATCH v3 3/3] btrfs: Avoid live-lock in search_ioctl() on hardware with sub-page faults

[David Sterba]

On Wed, Apr 06, 2022 at 07:09:22PM +0100, Catalin Marinas wrote:
> Commit a48b73eca4ce ("btrfs: fix potential deadlock in the search
> ioctl") addressed a lockdep warning by pre-faulting the user pages and
> attempting the copy_to_user_nofault() in an infinite loop. On
> architectures like arm64 with MTE, an access may fault within a page at
> a location different from what fault_in_writeable() probed. Since the
> sk_offset is rewound to the previous struct btrfs_ioctl_search_header
> boundary, there is no guaranteed forward progress and search_ioctl() may
> live-lock.
> 
> Use fault_in_subpage_writeable() instead of fault_in_writeable() to
> ensure the permission is checked at the right granularity (smaller than
> PAGE_SIZE).
> 
> Signed-off-by: Catalin Marinas <catalin.marinas@xxxxxxx>
> Fixes: a48b73eca4ce ("btrfs: fix potential deadlock in the search ioctl")
> Reported-by: Al Viro <viro@xxxxxxxxxxxxxxxxxx>

Acked-by: David Sterba <dsterba@xxxxxxxx>