When a submount (e.g. virtiofs with fc->auto_submounts=true) is umounted while an async RELEASE request is still pending, an illegal inode address accessing occurs in fuse_release_end()->iput(). Trigger process: 1. virtiofs has a submount; user opens and closes a file under it 2. Close calls fuse_file_put() with sync=false, sending RELEASE asynchronously 3. fuse_release_end() is scheduled to run later via igrab() holding inode ref 4. File is freed, mount/dentry refcounts are released 5. User umounts the submount; fuse connection detects remaining superblock and does NOT flush the connection's requests 6. generic_shutdown_super() destroys the superblock and poisons busy inodes' inode->i_sb = VFS_PTR_POISON 7. Later, fuse_request_end() calls fuse_release_end() which does iput(inode) 8. iput() accesses inode->i_sb->s_op at the poisoned address, crash! There are two solutions to fix it: Solution A: Hold path reference in fuse_file_put, and put path synchronously, which could reintroduce the issue fixed by commit 5a18ec176c934 ("fuse: fix hang of single threaded fuseblk filesystem"). Solution B (chosen): Use synchronous RELEASE for auto_submounts (which is only supported by virtiofs). The virtiofsd(fuse daemon) and virtiofs won't work together under one same kernel instance, so the problem fixed by commit 26e5c67deb2e ("fuse: fix livelock in synchronous file put from fuseblk workers") won't be brought back in virtiofs. Fetch a reproducer in https://bugzilla.kernel.org/show_bug.cgi?id=221519. Fixes: 26e5c67deb2e ("fuse: fix livelock in synchronous file put from fuseblk workers") Signed-off-by: Zhihao Cheng --- fs/fuse/file.c | 19 +++++++++++++------ 1 file changed, 13 insertions(+), 6 deletions(-) diff --git a/fs/fuse/file.c b/fs/fuse/file.c index c59452d60b8d..a6192b96d861 100644 --- a/fs/fuse/file.c +++ b/fs/fuse/file.c @@ -375,13 +375,20 @@ void fuse_file_release(struct inode *inode, struct fuse_file *ff, * synchronous RELEASE is allowed (and desirable) in this case * because the server can be trusted not to screw up. * - * Always use the asynchronous file put because the current thread - * might be the fuse server. This can happen if a process starts some - * aio and closes the fd before the aio completes. Since aio takes its - * own ref to the file, the IO completion has to drop the ref, which is - * how the fuse server can end up closing its clients' files. + * For auto_submounts (e.g. virtiofs), always use synchronous + * release to avoid illegal inode address access when umount + * happens before async release completes. The async release + * holds inode reference via igrab(), but umount can shutdown + * superblock and poison inode->i_sb before release ends, + * causing crash in fuse_release_end()->iput(). Otherwise, + * always use the asynchronous file put because the current + * thread might be the fuse server. This can happen if a + * process starts some aio and closes the fd before the aio + * completes. Since aio takes its own ref to the file, the IO + * completion has to drop the ref, which is how the fuse server + * can end up closing its clients' files. */ - fuse_file_put(ff, false); + fuse_file_put(ff, ff->fm->fc->auto_submounts); } void fuse_release_common(struct file *file, bool isdir) -- 2.52.0