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Subject[PATCH v4 3/3] btrfs: Avoid live-lock in search_ioctl() on hardware with sub-page faults
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

Use fault_in_subpage_writeable() instead of fault_in_writeable() to
ensure the permission is checked at the right granularity (smaller than

Signed-off-by: Catalin Marinas <>
Fixes: a48b73eca4ce ("btrfs: fix potential deadlock in the search ioctl")
Reported-by: Al Viro <>
Acked-by: David Sterba <>
Cc: Chris Mason <>
Cc: Josef Bacik <>
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 be6c24577dbe..9bf0616a3069 100644
--- a/fs/btrfs/ioctl.c
+++ b/fs/btrfs/ioctl.c
@@ -2565,7 +2565,12 @@ static noinline int search_ioctl(struct inode *inode,

while (1) {
ret = -EFAULT;
- if (fault_in_writeable(ubuf + sk_offset, *buf_size - sk_offset))
+ /*
+ * Ensure that the whole user buffer is faulted in at sub-page
+ * granularity, otherwise the loop may live-lock.
+ */
+ if (fault_in_subpage_writeable(ubuf + sk_offset,
+ *buf_size - sk_offset))

ret = btrfs_search_forward(root, &key, path, sk->min_transid);
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