__legitimize_mnt(): check for MNT_SYNC_UMOUNT should be under mount_lock

Vulnerability Description

In the Linux kernel, the following vulnerability has been resolved: __legitimize_mnt(): check for MNT_SYNC_UMOUNT should be under mount_lock ... or we risk stealing final mntput from sync umount - raising mnt_count after umount(2) has verified that victim is not busy, but before it has set MNT_SYNC_UMOUNT; in that case __legitimize_mnt() doesn't see that it's safe to quietly undo mnt_count increment and leaves dropping the reference to caller, where it'll be a full-blown mntput(). Check under mount_lock is needed; leaving the current one done before taking that makes no sense - it's nowhere near common enough to bother with.

References

• https://git.kernel.org/stable/c/628fb00195ce21a90cf9e4e3d105cd9e58f77b40
• https://git.kernel.org/stable/c/b89eb56a378b7b2c1176787fc228d0a57172bdd5
• https://git.kernel.org/stable/c/f6d45fd92f62845cbd1eb5128fd8f0ed7d0c5a42
• https://git.kernel.org/stable/c/9b0915e72b3cf52474dcee0b24a2f99d93e604a3
• https://git.kernel.org/stable/c/d8ece4ced3b051e656c77180df2e69e19e24edc1
• https://git.kernel.org/stable/c/8cafd7266fa02e0863bacbf872fe635c0b9725eb
• https://git.kernel.org/stable/c/b55996939c71a3e1a38f3cdc6a8859797efc9083
• https://git.kernel.org/stable/c/250cf3693060a5f803c5f1ddc082bb06b16112a9