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

[Pierre Barre]

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@xxxxxxxx>
---
 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,
        /*
         * Note that we must |= flags here as session_init already
         * set basic flags. This adds in flags from parsed options.
+        * Access flags are mutually exclusive, so clear any access
+        * bits set by session_init before applying the user's choice.
         */
+       if (ctx->session_opts.flags & V9FS_ACCESS_MASK)
+               v9ses->flags &= ~V9FS_ACCESS_MASK;
        v9ses->flags |= ctx->session_opts.flags;
 #ifdef CONFIG_9P_FSCACHE
        v9ses->cachetag = ctx->session_opts.cachetag;
--
2.51.0