Re: [PATCH] mm: Downgrade mmap_sem before locking or populating onmmap

[Al Viro]

On Sun, Dec 16, 2012 at 05:04:03PM +0000, Al Viro wrote:

> That's just from a couple of days of RTFS.  The locking in there is far too
> convoluted as it is; worse, it's not localized code-wise, so rechecking
> correctness is going to remain a big time-sink ;-/
> 
> Making it *more* complex doesn't look like a good idea, TBH...

... and another fun place: kvm_setup_async_pf() grabs a _passive_ reference
to current->mm (->mm_count, not ->mm_users), sticks it into work->mm and
schedules execution of async_pf_execute().  Which does use_mm() (still no
active refs acquired), grabs work->mm->mmap_sem shared and proceeds to call
get_user_pages().  What's going to happen if somebody does kill -9 to
the process that had started that?

get_user_pages() in parallel with exit_mmap() is a Bad Thing(tm) and I don't
see anything on the exit path that would've waited for that work to finish.
I might've missed something here, but...  Note that aio (another place
playing with use_mm(), also without an active ref) has an explicit hook
for mmput() to call before proceeding to exit_mmap(); I don't see anything
similar here.

Not that aio.c approach had been all that safe - get_task_mm() will refuse
to pick use_mm'ed one, but there are places open-coding it without the
check for PF_KTHREAD.  Few of them, fortunately, but...
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