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Date:   Mon, 25 Jan 2021 10:39:01 -0600
From:   ebiederm@...ssion.com (Eric W. Biederman)
To:     Christian Brauner <christian.brauner@...ntu.com>
Cc:     Alexander Viro <viro@...iv.linux.org.uk>,
        Christoph Hellwig <hch@....de>, linux-fsdevel@...r.kernel.org,
        John Johansen <john.johansen@...onical.com>,
        James Morris <jmorris@...ei.org>,
        Mimi Zohar <zohar@...ux.ibm.com>,
        Dmitry Kasatkin <dmitry.kasatkin@...il.com>,
        Stephen Smalley <stephen.smalley.work@...il.com>,
        Casey Schaufler <casey@...aufler-ca.com>,
        Arnd Bergmann <arnd@...db.de>,
        Andreas Dilger <adilger.kernel@...ger.ca>,
        OGAWA Hirofumi <hirofumi@...l.parknet.co.jp>,
        Geoffrey Thomas <geofft@...reload.com>,
        Mrunal Patel <mpatel@...hat.com>,
        Josh Triplett <josh@...htriplett.org>,
        Andy Lutomirski <luto@...nel.org>,
        Theodore Tso <tytso@....edu>, Alban Crequy <alban@...volk.io>,
        Tycho Andersen <tycho@...ho.ws>,
        David Howells <dhowells@...hat.com>,
        James Bottomley <James.Bottomley@...senpartnership.com>,
        Seth Forshee <seth.forshee@...onical.com>,
        St├ęphane Graber <stgraber@...ntu.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Aleksa Sarai <cyphar@...har.com>,
        Lennart Poettering <lennart@...ttering.net>,
        smbarber@...omium.org, Phil Estes <estesp@...il.com>,
        Serge Hallyn <serge@...lyn.com>,
        Kees Cook <keescook@...omium.org>,
        Todd Kjos <tkjos@...gle.com>, Paul Moore <paul@...l-moore.com>,
        Jonathan Corbet <corbet@....net>,
        containers@...ts.linux-foundation.org,
        linux-security-module@...r.kernel.org, linux-api@...r.kernel.org,
        linux-ext4@...r.kernel.org, linux-xfs@...r.kernel.org,
        linux-integrity@...r.kernel.org, selinux@...r.kernel.org
Subject: Re: [PATCH v6 23/40] exec: handle idmapped mounts

Christian Brauner <christian.brauner@...ntu.com> writes:

> 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. If the initial user namespace is
> passed nothing changes so non-idmapped mounts will see identical
> behavior as before.

This does not handle the v3 capabilites xattr with embeds a uid.
So at least at that level you are missing some critical conversions.

Eric

> Link: https://lore.kernel.org/r/20210112220124.837960-32-christian.brauner@ubuntu.com
> Cc: Christoph Hellwig <hch@....de>
> Cc: David Howells <dhowells@...hat.com>
> Cc: Al Viro <viro@...iv.linux.org.uk>
> Cc: linux-fsdevel@...r.kernel.org
> Reviewed-by: Christoph Hellwig <hch@....de>
> Signed-off-by: Christian Brauner <christian.brauner@...ntu.com>
> ---
> /* v2 */
> unchanged
>
> /* v3 */
> unchanged
>
> /* v4 */
> - Serge Hallyn <serge@...lyn.com>:
>   - Use "mnt_userns" to refer to a vfsmount's userns everywhere to make
>     terminology consistent.
>
> /* v5 */
> unchanged
> base-commit: 7c53f6b671f4aba70ff15e1b05148b10d58c2837
>
> /* v6 */
> base-commit: 19c329f6808995b142b3966301f217c831e7cf31
>
> - Christoph Hellwig <hch@....de>:
>   - Use new file_mnt_user_ns() helper.
> ---
>  fs/exec.c | 7 +++++--
>  1 file changed, 5 insertions(+), 2 deletions(-)
>
> diff --git a/fs/exec.c b/fs/exec.c
> index d803227805f6..48d1e8b1610b 100644
> --- a/fs/exec.c
> +++ b/fs/exec.c
> @@ -1580,6 +1580,7 @@ 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 *mnt_userns;
>  	struct inode *inode;
>  	unsigned int mode;
>  	kuid_t uid;
> @@ -1596,13 +1597,15 @@ static void bprm_fill_uid(struct linux_binprm *bprm, struct file *file)
>  	if (!(mode & (S_ISUID|S_ISGID)))
>  		return;
>  
> +	mnt_userns = file_mnt_user_ns(file);
> +
>  	/* 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(mnt_userns, inode);
> +	gid = i_gid_into_mnt(mnt_userns, inode);
>  	inode_unlock(inode);
>  
>  	/* We ignore suid/sgid if there are no mappings for them in the ns */

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