On Wed, Nov 27, 2024 at 09:44:21PM +0100, Jann Horn wrote: > On Wed, Nov 27, 2024 at 9:25 PM Kent Overstreet > <kent.overstreet@xxxxxxxxx> wrote: > > On Wed, Nov 27, 2024 at 11:09:14AM -0700, Jens Axboe wrote: > > > On 11/27/24 9:57 AM, Jann Horn wrote: > > > > Hi! > > > > > > > > In fs/bcachefs/fs-io-direct.c, "struct dio_write" contains a pointer > > > > to an mm_struct. This pointer is grabbed in bch2_direct_write() > > > > (without any kind of refcount increment), and used in > > > > bch2_dio_write_continue() for kthread_use_mm()/kthread_unuse_mm() > > > > which are used to enable userspace memory access from kthread context. > > > > I believe kthread_use_mm()/kthread_unuse_mm() require that the caller > > > > guarantees that the MM hasn't gone through exit_mmap() yet (normally > > > > by holding an mmget() reference). > > > > > > > > If we reach this codepath via io_uring, do we have a guarantee that > > > > the mm_struct that called bch2_direct_write() is still alive and > > > > hasn't yet gone through exit_mmap() when it is accessed from > > > > bch2_dio_write_continue()? > > > > > > > > I don't know the async direct I/O codepath particularly well, so I > > > > cc'ed the uring maintainers, who probably know this better than me. > > > > > > I _think_ this is fine as-is, even if it does look dubious and bcachefs > > > arguably should grab an mm ref for this just for safety to avoid future > > > problems. The reason is that bcachefs doesn't set FMODE_NOWAIT, which > > > means that on the io_uring side it cannot do non-blocking issue of > > > requests. This is slower as it always punts to an io-wq thread, which > > > shares the same mm. Hence if the request is alive, there's always a > > > thread with the same mm alive as well. > > > > > > Now if FMODE_NOWAIT was set, then the original task could exit. I'd need > > > to dig a bit deeper to verify that would always be safe and there's not > > > a of time today with a few days off in the US looming, so I'll defer > > > that to next week. It certainly would be fine with an mm ref grabbed. > > > > Wouldn't delivery of completions be tied to an address space (not a > > process) like it is for aio? > > An io_uring instance is primarily exposed to userspace as a file > descriptor, so AFAIK it is possible for the io_uring instance to live > beyond when the last mmput() happens. io_uring initially only holds an > mmgrab() reference on the MM (a comment above that explains: "This is > just grabbed for accounting purposes"), which I think is not enough to > make it stable enough for kthread_use_mm(); I think in io_uring, only > the worker threads actually keep the MM fully alive (and AFAIK the > uring worker threads can exit before the uring instance itself is torn > down). > > To receive io_uring completions, there are multiple ways, but they > don't use a pointer from the io_uring instance to the MM to access > userspace memory. Instead, you can have a VMA that points to the > io_uring instance, created by calling mmap() on the io_uring fd; or > alternatively, with IORING_SETUP_NO_MMAP, you can have io_uring grab > references to userspace-provided pages. > > On top of that, I think it might currently be possible to use the > io_uring file descriptor from another task to submit work. (That would > probably be fairly nonsensical, but I think the kernel does not > currently prevent it.) Ok, that's a wrinkle. Jens, is it really FMODE_NOWAIT that controls whether we can hit this? A very cursory glance leads me to suspect "no", it seems like this is a bug if io_uring is allowed on bcachefs at all.
