On Sun, Feb 5, 2017 at 2:51 AM, Al Viro [off-list ref] wrote:

On Sat, Feb 04, 2017 at 11:11:27PM +0100, Miklos Szeredi wrote:

quoted

Well, it's not historical; at least not yet.  The deadlock is there
alright: mmap fuse file to addr; read byte from mapped page -> page
locked; this triggeres read request served in same process but
separate thread; write addr-headerlen to fuse dev; trying to lock same
page -> deadlock.

Let me see if I got it straight - you have the same fuse file mmapped
in two processes, one of them being fuse server (either sharing the
entire address space, or the same area mapped in both).  Another process
faults the sucker in; filemap_fault() locks the page and goes
fuse_readpage() -> fuse_do_readpage() -> fuse_send_read() ->
-> fuse_request_send() -> __fuse_request_send() which puts request into
queue and goes to sleep in request_wait_answer().  Eventually, read()
on /dev/fuse (or splice(), whatever) by server picks that request and reply
is formed and fed back into /dev/fuse.  There we (in fuse_do_dev_write())
call copy_out_args(), which tries to copy into our (still locked) page
a piece of data coming from server-supplied iovec.  As it is, you
are calling get_user_pages_fast(), triggering handle_mm_fault().  Since that
malicous FPOS of a server tried to feed you the _same_ mmapped file, you
hit a deadlock.  In server's context.  Correct?

Yes.

Convoluted, but possible.  But.  Why the hell do we care whether that deadlock
hits in get_user_pages_fast() or in copy_from_user()?  Put it another way,
what difference does it make whether we take that fault with or without
FR_LOCKED in req->flags?

The difference is that if the page fault happens without FR_LOCKED,
then we can abort the request then and there (done by moving the
request to to_end1 and calling request_end() on it).

If abort happens with FR_LOCKED, we can't end the request now, because
data is possibly being copied to/from the request args.  But we are
guaranteed that the request will end shortly, because no sleeping
under FR_LOCKED is allowed.

But with copy_from_user() page fault and copy aren't separated and so
we don't know whether it's safe to abort or not.

Maybe I'm missing something obvious, but I don't see a simple way out of this.

quoted

The deadlock can be broken by aborting or force unmounting: return
error for original read request; page unlocked; device write can get
page lock and return.

The reason we need to prohibit pagefault while copying is that when
request is aborted and the caller returns the memory in the request
may become invalid (e.g. data from stack).

???

IDGI.  Your request is marked aborted and should presumably fail, so
that when request_wait_answer() wakes up and finds it screwed, fuse_readpage()
would just return an error and filemap_fault() will return VM_FAULT_SIGBUS,
with page left not uptodate and _not_ inserted into page tables.  What's
leaking where?

That case is fine.   But nothing guarantees that fuse_abort_conn()
won't be called (in the non-deadlock case) when data is being copied
to the request args.  Ending the request at such a point could easily
lead to use after free,

Thanks,
Miklos