fuse: Don't use request timeouts on virtiofs

virtiofs cannot abort a request once it has been added to a virtqueue, because the device owns its buffers until it returns them on the used ring. This is why virtiofs sets no_control and no_force_umount; see commit 15c8e72e88e0 ("fuse: allow skipping control interface and forced unmount"), which says that "virtio-fs does not support aborting requests which are being processed".

The request timeout added by commit 0f6439f61a6e ("fuse: add kernel-enforced timeout option for requests") aborts them anyway. When a request takes longer than the timeout, fuse_check_timeout() calls fuse_chan_abort(), which moves the requests on each virtqueue's fpq->processing list to its end list and ends them, so they are freed. The device can then still write the reply into the freed request and its pages, and virtio_fs_requests_done_work() later gets the freed request back from virtqueue_get_buf().

The timeout is armed when the server sets FUSE_REQUEST_TIMEOUT in its INIT reply, or when the fs.fuse.default_request_timeout or fs.fuse.max_request_timeout sysctl is set, for example for other FUSE mounts on the same system. A slow host is then enough to hit it.

Don't arm the request timeout, and don't offer FUSE_REQUEST_TIMEOUT, on connections that set no_force_umount.

---
I used no_force_umount to tell these connections apart, since virtiofs sets it for the same reason. A separate flag would work as well if you prefer that.

Tested on x86_64 under KVM, on top of v7.4-rc1 (6628e7d4c6b2) with KASAN enabled, using QEMU's vhost-user-fs-pci and the C virtiofsd from QEMU 6.2:

1. In the guest, set fs.fuse.default_request_timeout=15 and mount virtiofs.
2. On the host, stop virtiofsd with SIGSTOP.
3. In the guest, cat a file on the mount.
4. After 50 seconds, resume virtiofsd with SIGCONT.

Without this patch, the cat fails with ECONNABORTED after about 26 seconds. Once virtiofsd resumes, the guest reports:

  BUG: KASAN: slab-use-after-free in virtio_fs_requests_done_work+0x7be/0xbc0
  Read of size 8 at addr ffff888005176020 by task kworker/1:1/29
  Workqueue: events virtio_fs_requests_done_work
  ...
  Allocated by task 81:
   fuse_request_alloc+0x19/0x200
   fuse_get_req+0x153/0x2d0
   fuse_chan_send+0x4cf/0x840
   fuse_do_getattr+0x23a/0x4b0
  ...
  Freed by task 81:
   kmem_cache_free+0xbc/0x3e0
   fuse_chan_send+0x4af/0x840
   fuse_do_getattr+0x23a/0x4b0
  ...
  Oops: general protection fault, probably for non-canonical address 0xe012fc11a000000a
  RIP: 0010:virtio_fs_requests_done_work+0x25a/0xbc0

Setting fs.fuse.max_request_timeout=15 instead gives the same result. With this patch, under either sysctl, the cat waits until virtiofsd resumes and then returns the file contents, and the mount keeps working.

I could not test the FUSE_REQUEST_TIMEOUT part, because this virtiofsd does not know about flags2.

The tests above were run with help from an AI agent.

 fs/fuse/inode.c | 15 ++++++++++++---
 1 file changed, 12 insertions(+), 3 deletions(-)

index 1341b681d0ed..f1aae9b5475c 100644
@@ -1451,7 +1451,14 @@ static void process_init_reply(struct fuse_args *args, int error)
 			fc->no_flock = 1;
 		}
 
-		fuse_init_server_timeout(fc->chan, timeout);
+		/*
+		 * A request timeout aborts the connection, which ends the
+		 * requests that are still in flight. A transport that cannot
+		 * take back requests it has handed to the device, such as
+		 * virtiofs, sets no_force_umount for the same reason.
+		 */
+		if (!fc->no_force_umount)
+			fuse_init_server_timeout(fc->chan, timeout);
 
 		fm->sb->s_bdi->ra_pages =
 				min(fm->sb->s_bdi->ra_pages, ra_pages);

@@ -1502,8 +1509,10 @@ static struct fuse_init_args *fuse_new_init(struct fuse_mount *fm)
 		FUSE_HANDLE_KILLPRIV_V2 | FUSE_SETXATTR_EXT | FUSE_INIT_EXT |
 		FUSE_SECURITY_CTX | FUSE_CREATE_SUPP_GROUP |
 		FUSE_HAS_EXPIRE_ONLY | FUSE_DIRECT_IO_ALLOW_MMAP |
-		FUSE_NO_EXPORT_SUPPORT | FUSE_HAS_RESEND | FUSE_ALLOW_IDMAP |
-		FUSE_REQUEST_TIMEOUT;
+		FUSE_NO_EXPORT_SUPPORT | FUSE_HAS_RESEND | FUSE_ALLOW_IDMAP;
+	/* See process_init_reply() */
+	if (!fm->fc->no_force_umount)
+		flags |= FUSE_REQUEST_TIMEOUT;
 #ifdef CONFIG_FUSE_VDAX
 	if (fm->fc->vdax)
 		flags |= FUSE_MAP_ALIGNMENT;

base-commit: 6628e7d4c6b25c8d18f1345e3a20d21829aae7e4

2.43.0

Hi Yuho,

    kylinos.cn/T/#m34d62acd7887a86ec32e9091145e4b19fdbb3fc3