[PATCH] fuse: reject oversized dirents in page cache

[PATCH] fuse: reject oversized dirents in page cache

[Miklos Szeredi]

From: Samuel Page <sam@bynar.io>

fuse_add_dirent_to_cache() computes a serialized dirent size from the
server-controlled namelen field and copies the dirent into a single
page-cache page. The existing logic only checks whether the dirent fits
in the remaining space of the current page and advances to a fresh page
if not. It never checks whether the dirent itself exceeds PAGE_SIZE.

As a result, a malicious FUSE server can return a dirent with
namelen=4095, producing a serialized record size of 4120 bytes. On 4 KiB
page systems this causes memcpy() to overflow the cache page by 24 bytes
into the following kernel page.

Reject dirents that cannot fit in a single page before copying them into
the readdir cache.

Fixes: 69e34551152a ("fuse: allow caching readdir")
Cc: stable@vger.kernel.org # v6.16+
Assisted-by: Bynario AI
Signed-off-by: Samuel Page <sam@bynar.io>
Reported-by: Qi Tang <tpluszz77@gmail.com>
Reported-by: Zijun Hu <nightu@northwestern.edu>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>
---
 fs/fuse/readdir.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/fs/fuse/readdir.c b/fs/fuse/readdir.c
index c88194e52d18..db5ae8ec1030 100644
--- a/fs/fuse/readdir.c
+++ b/fs/fuse/readdir.c
@@ -41,6 +41,10 @@ static void fuse_add_dirent_to_cache(struct file *file,
 	unsigned int offset;
 	void *addr;
 
+	/* Dirent doesn't fit in readdir cache page?  Skip caching. */
+	if (reclen > PAGE_SIZE)
+		return;
+
 	spin_lock(&fi->rdc.lock);
 	/*
 	 * Is cache already completed?  Or this entry does not go at the end of
-- 
2.53.0

Re: [PATCH] fuse: reject oversized dirents in page cache

[Christian Brauner]

On Mon, 20 Apr 2026 11:01:37 +0200, Miklos Szeredi wrote:
> fuse_add_dirent_to_cache() computes a serialized dirent size from the
> server-controlled namelen field and copies the dirent into a single
> page-cache page. The existing logic only checks whether the dirent fits
> in the remaining space of the current page and advances to a fresh page
> if not. It never checks whether the dirent itself exceeds PAGE_SIZE.
> 
> As a result, a malicious FUSE server can return a dirent with
> namelen=4095, producing a serialized record size of 4120 bytes. On 4 KiB
> page systems this causes memcpy() to overflow the cache page by 24 bytes
> into the following kernel page.
> 
> [...]

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 oversized dirents in page cache
      https://git.kernel.org/vfs/vfs/c/5c3b2e242190