[PATCH v2 14/16] block, loop: Increment diskseq when releasing a loop device

[Demi Marie Obenour <demi@xxxxxxxxxxxxxxxxxxxxxx>]

The previous patch for checking diskseq in blkback is not enough to
prevent the following race:

1. Program X opens a loop device
2. Program X gets the diskseq of the loop device.
3. Program X associates a file with the loop device.
4. Program X passes the loop device major, minor, and diskseq to
   something, such as Xen blkback.
5. Program X exits.
6. Program Y detaches the file from the loop device.
7. Program Y attaches a different file to the loop device.
8. Xen blkback finally gets around to opening the loop device and checks
   that the diskseq is what it expects it to be.  Even though the
   diskseq is the expected value, the result is that blkback is
   accessing the wrong file.

To prevent this race condition, increment the diskseq of a loop device
when it is detached from its file descriptor.  This causes blkback (or
any other program, for that matter) to fail at step 8.  Export the
inc_diskseq() function to make this possible.

Signed-off-by: Demi Marie Obenour <demi@xxxxxxxxxxxxxxxxxxxxxx>
---
I considered destroying the loop device altogether instead of bumping
its diskseq, but was not able to accomplish that.  Suggestions welcome.
---
 block/genhd.c        | 1 +
 drivers/block/loop.c | 6 ++++++
 2 files changed, 7 insertions(+)

diff --git a/block/genhd.c b/block/genhd.c
index 
1cb489b927d50ab06a84a4bfd6913ca8ba7318d4..c0ca2c387732171321555cd57565fbc606768505
 100644
--- a/block/genhd.c
+++ b/block/genhd.c
@@ -1502,3 +1502,4 @@ void inc_diskseq(struct gendisk *disk)
 {
        disk->diskseq = atomic64_inc_return(&diskseq);
 }
+EXPORT_SYMBOL(inc_diskseq);
diff --git a/drivers/block/loop.c b/drivers/block/loop.c
index 
bc31bb7072a2cb7294d32066f5d0aa14130349b4..05ea5fb41508b4106f184dd6b4c37942716bdcac
 100644
--- a/drivers/block/loop.c
+++ b/drivers/block/loop.c
@@ -1205,6 +1205,12 @@ static void __loop_clr_fd(struct loop_device *lo, bool 
release)
        if (!part_shift)
                set_bit(GD_SUPPRESS_PART_SCAN, &lo->lo_disk->state);
        mutex_lock(&lo->lo_mutex);
+
+       /*
+        * Increment the disk sequence number, so that userspace knows this
+        * device now points to something else.
+        */
+       inc_diskseq(lo->lo_disk);
        lo->lo_state = Lo_unbound;
        mutex_unlock(&lo->lo_mutex);
 
-- 
Sincerely,
Demi Marie Obenour (she/her/hers)
Invisible Things Lab