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

[Jann Horn <jannh@google.com>]

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>