Re: [PATCH] 9p: fix access mode flags being ORed instead of replaced

Pierre Barre
2026-04-02T10:10:29Z

To reproduce: mount a 9P2000.L filesystem with access=user on kernel 6.19+:

# mount -t 9p -o trans=tcp,port=5564,version=9p2000.L,access=user 127.0.0.1 /mnt/9p

Then as root:

touch /mnt/9p/test
chown root:root /mnt/9p/test

# chown: changing ownership of '/mnt/9p/test': Operation not permitted

Tracing the server side confirms the attach arrives with uid=65534 (nobody) instead of 0 (root).

On Thu, Apr 2, 2026, at 12:03, Pierre Barre wrote:
> Since commit 1f3e4142c0eb ("9p: convert to the new mount API"),
> v9fs_apply_options() applies parsed mount flags with |= onto flags
> already set by v9fs_session_init(). For 9P2000.L, session_init sets
> V9FS_ACCESS_CLIENT as the default, so when the user mounts with
> "access=user", both bits end up set. Access mode checks compare
> against exact values, so having both bits set matches neither mode.
>
> This causes v9fs_fid_lookup() to fall through to the default switch
> case, using INVALID_UID (nobody/65534) instead of current_fsuid()
> for all fid lookups. Root is then unable to chown or perform other
> privileged operations.
>
> Fix by clearing the access mask before applying the user's choice.
>
> Fixes: 1f3e4142c0eb ("9p: convert to the new mount API")
> Signed-off-by: Pierre Barre <pierre@barre.sh>
> ---
> fs/9p/v9fs.c | 4 ++++
> 1 file changed, 4 insertions(+)
>
> diff --git a/fs/9p/v9fs.c b/fs/9p/v9fs.c
> index 057487efaaeb..05a5e1c4df35 100644
> --- a/fs/9p/v9fs.c
> +++ b/fs/9p/v9fs.c
> @@ -413,7 +413,11 @@ static void v9fs_apply_options(struct
> v9fs_session_info *v9ses,
> /*