linux-kernel - [PATCH v4 13/15] fs/fuse: warn if fuse_access is called when idmapped mounts are allowed

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Message-Id: <20240903151626.264609-14-aleksandr.mikhalitsyn@canonical.com>
Date: Tue,  3 Sep 2024 17:16:24 +0200
From: Alexander Mikhalitsyn <aleksandr.mikhalitsyn@...onical.com>
To: mszeredi@...hat.com
Cc: brauner@...nel.org,
	stgraber@...raber.org,
	linux-fsdevel@...r.kernel.org,
	Seth Forshee <sforshee@...nel.org>,
	Miklos Szeredi <miklos@...redi.hu>,
	Amir Goldstein <amir73il@...il.com>,
	Bernd Schubert <bschubert@....com>,
	Alexander Mikhalitsyn <aleksandr.mikhalitsyn@...onical.com>,
	linux-kernel@...r.kernel.org
Subject: [PATCH v4 13/15] fs/fuse: warn if fuse_access is called when idmapped mounts are allowed

It is not possible with the current fuse code, but let's protect ourselves
from regressions in the future.

Cc: Christian Brauner <brauner@...nel.org>
Cc: Seth Forshee <sforshee@...nel.org>
Cc: Miklos Szeredi <miklos@...redi.hu>
Cc: Amir Goldstein <amir73il@...il.com>
Cc: Bernd Schubert <bschubert@....com>
Cc: <linux-fsdevel@...r.kernel.org>
Signed-off-by: Alexander Mikhalitsyn <aleksandr.mikhalitsyn@...onical.com>
---
v4:
	- this commit added
---
 fs/fuse/dir.c | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/fs/fuse/dir.c b/fs/fuse/dir.c
index d316223bd00b..dd967402bf12 100644
--- a/fs/fuse/dir.c
+++ b/fs/fuse/dir.c
@@ -1473,6 +1473,14 @@ static int fuse_access(struct inode *inode, int mask)
 
 	BUG_ON(mask & MAY_NOT_BLOCK);
 
+	/*
+	 * We should not send FUSE_ACCESS to the userspace
+	 * when idmapped mounts are enabled as for this case
+	 * we have fc->default_permissions = 1 and access
+	 * permission checks are done on the kernel side.
+	 */
+	WARN_ON_ONCE(!(fm->sb->s_iflags & SB_I_NOIDMAP));
+
 	if (fm->fc->no_access)
 		return 0;
 
-- 
2.34.1