[PATCH v2 4/4] btrfs: Avoid live-lock in search_ioctl() on hardware with sub-page faults

[Catalin Marinas <catalin.marinas@arm.com>]

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.

Request a 'min_size' of (*buf_size - sk_offset) from
fault_in_writeable() to check this range for sub-page faults.

Fixes: a48b73eca4ce ("btrfs: fix potential deadlock in the search ioctl")
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
Reported-by: Al Viro <viro@zeniv.linux.org.uk>
---
 fs/btrfs/ioctl.c | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/fs/btrfs/ioctl.c b/fs/btrfs/ioctl.c
index c7d74c8776a1..439cf38f320a 100644
--- a/fs/btrfs/ioctl.c
+++ b/fs/btrfs/ioctl.c
@@ -2222,8 +2222,13 @@ static noinline int search_ioctl(struct inode *inode,
 	key.offset = sk->min_offset;
 
 	while (1) {
+		size_t len = *buf_size - sk_offset;
 		ret = -EFAULT;
-		if (fault_in_writeable(ubuf + sk_offset, *buf_size - sk_offset, 0))
+		/*
+		 * Ensure that the whole user buffer is faulted in at sub-page
+		 * granularity, otherwise the loop may live-lock.
+		 */
+		if (fault_in_writeable(ubuf + sk_offset, len, len))
 			break;
 
 		ret = btrfs_search_forward(root, &key, path, sk->min_transid);