autofs: fix memory leak of waitqueues in autofs_catatonic_mode

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

autofs: fix memory leak of waitqueues in autofs_catatonic_mode

Syzkaller reports a memory leak:

BUG: memory leak

unreferenced object 0xffff88810b279e00 (size 96):

comm "syz-executor399", pid 3631, jiffies 4294964921 (age 23.870s)

hex dump (first 32 bytes):

00 00 00 00 00 00 00 00 08 9e 27 0b 81 88 ff ff ..........'.....

08 9e 27 0b 81 88 ff ff 00 00 00 00 00 00 00 00 ..'.............

backtrace:

[<ffffffff814cfc90>] kmalloc_trace+0x20/0x90 mm/slab_common.c:1046

[<ffffffff81bb75ca>] kmalloc include/linux/slab.h:576 [inline]

[<ffffffff81bb75ca>] autofs_wait+0x3fa/0x9a0 fs/autofs/waitq.c:378

[<ffffffff81bb88a7>] autofs_do_expire_multi+0xa7/0x3e0 fs/autofs/expire.c:593

[<ffffffff81bb8c33>] autofs_expire_multi+0x53/0x80 fs/autofs/expire.c:619

[<ffffffff81bb6972>] autofs_root_ioctl_unlocked+0x322/0x3b0 fs/autofs/root.c:897

[<ffffffff81bb6a95>] autofs_root_ioctl+0x25/0x30 fs/autofs/root.c:910

[<ffffffff81602a9c>] vfs_ioctl fs/ioctl.c:51 [inline]

[<ffffffff81602a9c>] __do_sys_ioctl fs/ioctl.c:870 [inline]

[<ffffffff81602a9c>] __se_sys_ioctl fs/ioctl.c:856 [inline]

[<ffffffff81602a9c>] __x64_sys_ioctl+0xfc/0x140 fs/ioctl.c:856

[<ffffffff84608225>] do_syscall_x64 arch/x86/entry/common.c:50 [inline]

[<ffffffff84608225>] do_syscall_64+0x35/0xb0 arch/x86/entry/common.c:80

[<ffffffff84800087>] entry_SYSCALL_64_after_hwframe+0x63/0xcd

autofs_wait_queue structs should be freed if their wait_ctr becomes zero.

Otherwise they will be lost.

In this case an AUTOFS_IOC_EXPIRE_MULTI ioctl is done, then a new

waitqueue struct is allocated in autofs_wait(), its initial wait_ctr

equals 2. After that wait_event_killable() is interrupted (it returns

-ERESTARTSYS), so that 'wq->name.name == NULL' condition may be not

satisfied. Actually, this condition can be satisfied when

autofs_wait_release() or autofs_catatonic_mode() is called and, what is

also important, wait_ctr is decremented in those places. Upon the exit of

autofs_wait(), wait_ctr is decremented to 1. Then the unmounting process

begins: kill_sb calls autofs_catatonic_mode(), which should have freed the

waitqueues, but it only decrements its usage counter to zero which is not

a correct behaviour.

edit:imk

This description is of course not correct. The umount performed as a result

of an expire is a umount of a mount that has been automounted, it's not the

autofs mount itself. They happen independently, usually after everything

mounted within the autofs file system has been expired away. If everything

hasn't been expired away the automount daemon can still exit leaving mounts

in place. But expires done in both cases will result in a notification that

calls autofs_wait_release() with a result status. The problem case is the

summary execution of of the automount daemon. In this case any waiting

processes won't be woken up until either they are terminated or the mount

is umounted.

end edit: imk

So in catatonic mode we should free waitqueues which counter becomes zero.

edit: imk

Initially I was concerned that the calling of autofs_wait_release() and

autofs_catatonic_mode() was not mutually exclusive but that can't be the

case (obviously) because the queue entry (or entries) is removed from the

list when either of these two functions are called. Consequently the wait

entry will be freed by only one of these functions or by the woken process

in autofs_wait() depending on the order of the calls.

end edit: imk