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

[David Sterba]

On Wed, Nov 24, 2021 at 07:20:24PM +0000, 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_exact_writeable() instead which probes the entire user
> buffer for faults at sub-page granularity.
> 
> Signed-off-by: Catalin Marinas <catalin.marinas at arm.com>
> Reported-by: Al Viro <viro at zeniv.linux.org.uk>

Acked-by: David Sterba <dsterba at suse.com>