[PATCH] cifs: fix loff_t underflow in cifs_remap_file_range() when len == 0

[Frank Sorenson <sorenson@redhat.com>]

With len == 0 (clone to EOF), the effective length is computed as:

    len = src_inode->i_size - off;

If off > i_size, this is a negative loff_t, corrupting the ByteCount
in the FSCTL_DUPLICATE_EXTENTS_TO_FILE request and inverting the range
in filemap_write_and_wait_range().  The existing off >= i_size check
fires only after the ioctl has already been sent.

Snapshot i_size_read() once for both the bounds check and the length
calculation, eliminating the TOCTOU and 32-bit torn-read risk.  Reject
off > src_size with -EINVAL.  Treat off == src_size as a no-op,
consistent with __generic_remap_file_range_prep().

Fixes: 04b38d601239 ("vfs: pull btrfs clone API to vfs layer")
Cc: stable@vger.kernel.org
Signed-off-by: Frank Sorenson <sorenson@redhat.com>
---
 fs/smb/client/cifsfs.c | 15 +++++++++++++--
 1 file changed, 13 insertions(+), 2 deletions(-)

diff --git a/fs/smb/client/cifsfs.c b/fs/smb/client/cifsfs.c
index e9e5ba4fa865..d8be6c707b62 100644
--- a/fs/smb/client/cifsfs.c
+++ b/fs/smb/client/cifsfs.c
@@ -1415,8 +1415,19 @@ static loff_t cifs_remap_file_range(struct file *src_file, loff_t off,
 	 */
 	lock_two_nondirectories(target_inode, src_inode);
 
-	if (len == 0)
-		len = src_inode->i_size - off;
+	if (len == 0) {
+		loff_t src_size = i_size_read(src_inode);
+
+		if (off > src_size) {
+			rc = -EINVAL;
+			goto unlock;
+		}
+		len = src_size - off;
+		if (!len) {
+			rc = 0;
+			goto unlock;
+		}
+	}
 
 	cifs_dbg(FYI, "clone range\n");
 
-- 
2.55.0