[PATCH v2] fuse: reject fuse_notify() pagecache ops on directories

[PATCH v2] fuse: reject fuse_notify() pagecache ops on directories

[Jann Horn]

The operations FUSE_NOTIFY_STORE and FUSE_NOTIFY_RETRIEVE allow the
FUSE daemon to actively write/read pagecache contents.

For directories with FOPEN_CACHE_DIR, the pagecache is used as
kernel-internal cache storage, and userspace is not supposed to have
direct access to this cache - in particular, fuse_parse_cache() will hit
WARN_ON() if the cache contains bogus data.

Reject FUSE_NOTIFY_STORE and FUSE_NOTIFY_RETRIEVE on anything other than
regular files with -EINVAL.

Fixes: 5d7bc7e8680c ("fuse: allow using readdir cache")
Cc: stable@vger.kernel.org
Signed-off-by: Jann Horn <jannh@google.com>
---
Changes in v2:
- reject anything other than regular files (Miklos)
- Link to v1: https://lore.kernel.org/r/20260519-fuse-dir-pagecache-v1-1-1f060c65930d@google.com
---
 fs/fuse/dev.c | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/fs/fuse/dev.c b/fs/fuse/dev.c
index 5dda7080f4a9..f07c97358b36 100644
--- a/fs/fuse/dev.c
+++ b/fs/fuse/dev.c
@@ -1793,6 +1793,10 @@ static int fuse_notify_store(struct fuse_conn *fc, unsigned int size,
 	inode = fuse_ilookup(fc, nodeid,  NULL);
 	if (!inode)
 		goto out_up_killsb;
+	if (!S_ISREG(inode->i_mode)) {
+		err = -EINVAL;
+		goto out_iput;
+	}
 
 	mapping = inode->i_mapping;
 	file_size = i_size_read(inode);
@@ -1966,7 +1970,10 @@ static int fuse_notify_retrieve(struct fuse_conn *fc, unsigned int size,
 
 	inode = fuse_ilookup(fc, nodeid, &fm);
 	if (inode) {
-		err = fuse_retrieve(fm, inode, &outarg);
+		if (!S_ISREG(inode->i_mode))
+			err = -EINVAL;
+		else
+			err = fuse_retrieve(fm, inode, &outarg);
 		iput(inode);
 	}
 	up_read(&fc->killsb);

---
base-commit: ab5fce87a778cb780a05984a2ca448f2b41aafbf
change-id: 20260519-fuse-dir-pagecache-382a54146826

--  
Jann Horn <jannh@google.com>

Re: [PATCH v2] fuse: reject fuse_notify() pagecache ops on directories

[Miklos Szeredi]

On Tue, 19 May 2026 at 16:29, Jann Horn <jannh@google.com> wrote:
>
> The operations FUSE_NOTIFY_STORE and FUSE_NOTIFY_RETRIEVE allow the
> FUSE daemon to actively write/read pagecache contents.
>
> For directories with FOPEN_CACHE_DIR, the pagecache is used as
> kernel-internal cache storage, and userspace is not supposed to have
> direct access to this cache - in particular, fuse_parse_cache() will hit
> WARN_ON() if the cache contains bogus data.
>
> Reject FUSE_NOTIFY_STORE and FUSE_NOTIFY_RETRIEVE on anything other than
> regular files with -EINVAL.
>
> Fixes: 5d7bc7e8680c ("fuse: allow using readdir cache")
> Cc: stable@vger.kernel.org
> Signed-off-by: Jann Horn <jannh@google.com>

Acked-by: Miklos Szeredi <mszeredi@redhat.com>

Christian, can you please apply this to vfs.fixes?

Thanks,
Miklos

Re: [PATCH v2] fuse: reject fuse_notify() pagecache ops on directories

[Christian Brauner]

On Tue, 19 May 2026 16:29:38 +0200, Jann Horn wrote:
> The operations FUSE_NOTIFY_STORE and FUSE_NOTIFY_RETRIEVE allow the
> FUSE daemon to actively write/read pagecache contents.
> 
> For directories with FOPEN_CACHE_DIR, the pagecache is used as
> kernel-internal cache storage, and userspace is not supposed to have
> direct access to this cache - in particular, fuse_parse_cache() will hit
> WARN_ON() if the cache contains bogus data.
> 
> [...]

Applied to the vfs.fixes branch of the vfs/vfs.git tree.
Patches in the vfs.fixes branch should appear in linux-next soon.

Please report any outstanding bugs that were missed during review in a
new review to the original patch series allowing us to drop it.

It's encouraged to provide Acked-bys and Reviewed-bys even though the
patch has now been applied. If possible patch trailers will be updated.

Note that commit hashes shown below are subject to change due to rebase,
trailer updates or similar. If in doubt, please check the listed branch.

tree:   https://git.kernel.org/pub/scm/linux/kernel/git/vfs/vfs.git
branch: vfs.fixes

[1/1] fuse: reject fuse_notify() pagecache ops on directories
      https://git.kernel.org/vfs/vfs/c/ef5728148f16