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From:   Christian Brauner <christian.brauner@ubuntu.com>
To:     Alexander Viro <viro@zeniv.linux.org.uk>,
        Christoph Hellwig <hch@infradead.org>,
        linux-fsdevel@vger.kernel.org
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        Christian Brauner <christian.brauner@ubuntu.com>,
        Christoph Hellwig <hch@lst.de>
Subject: [PATCH v2 28/39] exec: handle idmapped mounts
Date:   Sun, 15 Nov 2020 11:37:07 +0100
Message-Id: <20201115103718.298186-29-christian.brauner@ubuntu.com>
In-Reply-To: <20201115103718.298186-1-christian.brauner@ubuntu.com>
References: <20201115103718.298186-1-christian.brauner@ubuntu.com>
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When executing a setuid binary the kernel will verify in bprm_fill_uid() that
the inode has a mapping in the caller's user namespace before setting the
callers uid and gid. Let bprm_fill_uid() handle idmapped mounts. If the inode
is accessed through an idmapped mount it is mapped according to the mount's
user namespace. Afterwards the checks are identical to non-idmapped mounts.On
regular mounts this is a nop.

Cc: Christoph Hellwig <hch@lst.de>
Cc: David Howells <dhowells@redhat.com>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: linux-fsdevel@vger.kernel.org
Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>
---
/* v2 */
unchanged
---
 fs/exec.c | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/fs/exec.c b/fs/exec.c
index 10c06fdf78a7..7d6d3dd17e84 100644
--- a/fs/exec.c
+++ b/fs/exec.c
@@ -1567,6 +1567,7 @@ static void check_unsafe_exec(struct linux_binprm *bprm)
 static void bprm_fill_uid(struct linux_binprm *bprm, struct file *file)
 {
 	/* Handle suid and sgid on files */
+	struct user_namespace *user_ns;
 	struct inode *inode;
 	unsigned int mode;
 	kuid_t uid;
@@ -1583,13 +1584,15 @@ static void bprm_fill_uid(struct linux_binprm *bprm, struct file *file)
 	if (!(mode & (S_ISUID|S_ISGID)))
 		return;
 
+	user_ns = mnt_user_ns(file->f_path.mnt);
+
 	/* Be careful if suid/sgid is set */
 	inode_lock(inode);
 
 	/* reload atomically mode/uid/gid now that lock held */
 	mode = inode->i_mode;
-	uid = inode->i_uid;
-	gid = inode->i_gid;
+	uid = i_uid_into_mnt(user_ns, inode);
+	gid = i_gid_into_mnt(user_ns, inode);
 	inode_unlock(inode);
 
 	/* We ignore suid/sgid if there are no mappings for them in the ns */
-- 
2.29.2