io_uring: protect mem region deregistration

io_create_region_mmap_safe() protects publishing of a region against
concurrent mmap calls, however we should also protect against it when
removing a region. There is a gap io_register_mem_region() where it
safely publishes a region, but then copy_to_user goes wrong and it
unsafely frees the region.

Cc: stable@vger.kernel.org
Fixes: 087f997 ("io_uring/memmap: implement mmap for regions")
Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

Author: isilence

io_uring/register.c

@@ -613,6 +613,7 @@ static int io_register_mem_region(struct io_ring_ctx *ctx, void __user *uarg)
 	if (ret)
 		return ret;
 	if (copy_to_user(rd_uptr, &rd, sizeof(rd))) {
+		guard(mutex)(&ctx->mmap_lock);
 		io_free_region(ctx, &ctx->param_region);
 		return -EFAULT;
 	}