Pull mount_setattr fix from Christian Brauner:
"The recent cleanup in
e257039f0fc7 ("mount_setattr(): clean the
control flow and calling conventions") switched the mount attribute
codepaths from do-while to for loops as they are more idiomatic when
walking mounts.
However, we did originally choose do-while constructs because if we
request a mount or mount tree to be made read-only we need to hold
writers in the following way: The mount attribute code will grab
lock_mount_hash() and then call mnt_hold_writers() which will
_unconditionally_ set MNT_WRITE_HOLD on the mount.
Any callers that need write access have to call mnt_want_write(). They
will immediately see that MNT_WRITE_HOLD is set on the mount and the
caller will then either spin (on non-preempt-rt) or wait on
lock_mount_hash() (on preempt-rt).
The fact that MNT_WRITE_HOLD is set unconditionally means that once
mnt_hold_writers() returns we need to _always_ pair it with
mnt_unhold_writers() in both the failure and success paths.
The do-while constructs did take care of this. But Al's change to a
for loop in the failure path stops on the first mount we failed to
change mount attributes _without_ going into the loop to call
mnt_unhold_writers().
This in turn means that once we failed to make a mount read-only via
mount_setattr() - i.e. there are already writers on that mount - we
will block any writers indefinitely. Fix this by ensuring that the for
loop always unsets MNT_WRITE_HOLD including the first mount we failed
to change to read-only. Also sprinkle a few comments into the cleanup
code to remind people about what is happening including myself. After
all, I didn't catch it during review.
This is only relevant on mainline and was reported by syzbot. Details
about the syzbot reports are all in the commit message"
* tag 'fs.fixes.v5.18-rc4' of git://git.kernel.org/pub/scm/linux/kernel/git/brauner/linux:
fs: unset MNT_WRITE_HOLD on failure