linux-kernel - [PATCH] filelocks: use mount idmapping for setlease permission check

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Message-Id: <20230309-generic_setlease-use-idmapping-v1-1-6c970395ac4d@kernel.org>
Date:   Thu, 09 Mar 2023 14:39:09 -0600
From:   "Seth Forshee (DigitalOcean)" <sforshee@...nel.org>
To:     Jeff Layton <jlayton@...nel.org>,
        Chuck Lever <chuck.lever@...cle.com>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Christian Brauner <brauner@...nel.org>
Cc:     linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        "Seth Forshee (DigitalOcean)" <sforshee@...nel.org>
Subject: [PATCH] filelocks: use mount idmapping for setlease permission
 check

A user should be allowed to take out a lease via an idmapped mount if
the fsuid matches the mapped uid of the inode. generic_setlease() is
checking the unmapped inode uid, causing these operations to be denied.

Fix this by comparing against the mapped inode uid instead of the
unmapped uid.

Fixes: 9caccd41541a ("fs: introduce MOUNT_ATTR_IDMAP")
Signed-off-by: Seth Forshee (DigitalOcean) <sforshee@...nel.org>
---
 fs/locks.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/fs/locks.c b/fs/locks.c
index 66b4eef09db5..cf07f72eaf45 100644
--- a/fs/locks.c
+++ b/fs/locks.c
@@ -1863,9 +1863,10 @@ int generic_setlease(struct file *filp, long arg, struct file_lock **flp,
 			void **priv)
 {
 	struct inode *inode = file_inode(filp);
+	vfsuid_t vfsuid = i_uid_into_vfsuid(file_mnt_idmap(filp), inode);
 	int error;
 
-	if ((!uid_eq(current_fsuid(), inode->i_uid)) && !capable(CAP_LEASE))
+	if ((!vfsuid_eq_kuid(vfsuid, current_fsuid())) && !capable(CAP_LEASE))
 		return -EACCES;
 	if (!S_ISREG(inode->i_mode))
 		return -EINVAL;

---
base-commit: fe15c26ee26efa11741a7b632e9f23b01aca4cc6
change-id: 20230309-generic_setlease-use-idmapping-34e099aa2206

Best regards,
-- 
Seth Forshee (DigitalOcean) <sforshee@...nel.org>