Linux: smb/client: invalidate fscache for fallocate range operations

In the Linux kernel, the following vulnerability has been resolved:

smb/client: invalidate fscache for fallocate range operations

smb3_zero_range(), smb3_punch_hole(), smb3_insert_range(), and smb3_collapse_range() modify file contents through server-side range operations. These operations discard the affected page cache, but leave the FS-Cache cookie valid, so a later read may return data cached before the range operation.

Fix this by invalidating FS-Cache after outstanding I/O has completed and before modifying the file on the server.

Run the following as root on a CIFS mount with fsc enabled and an active CacheFiles backend:

bash -c ' MNT=/mnt/cifs FILE="$MNT/repro"

Generate four 1 MiB random blocks: [A][B][C][D].

dd if=/dev/urandom of=/tmp/src bs=1M count=4 status=none

Expected contents after zeroing B: [A][zero][C][D].

cp /tmp/src /tmp/expected dd if=/dev/zero of=/tmp/expected bs=1M seek=1 count=1 \ conv=notrunc status=none cp /tmp/src "$FILE"

Populate FS-Cache, then discard the page cache.

sync echo 1 > /proc/sys/vm/drop_caches cat "$FILE" > /dev/null sync echo 1 > /proc/sys/vm/drop_caches

fallocate --zero-range -o 1M -l 1M "$FILE"

if cmp -s /tmp/expected "$FILE"; then echo "readback: OK" else echo "readback: STALE DATA" fi '

Before this change, the readback differs from /tmp/expected:

readback: STALE DATA

After this change, it matches:

readback: OK