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        Tue, 14 Feb 2017 02:57:10 -0500
From: ebiederm@xmission.com (Eric W. Biederman)
To: Vivek Goyal <vgoyal@redhat.com>
Cc: linux-fsdevel <linux-fsdevel@vger.kernel.org>,
        Amir Goldstein <amir73il@gmail.com>,
        Al Viro <viro@zeniv.linux.org.uk>,
        James Bottomley <James.Bottomley@HansenPartnership.com>,
        Miklos Szeredi <miklos@szeredi.hu>
References: <20170213204526.GG2065@redhat.com>
Date: Tue, 14 Feb 2017 20:52:40 +1300
In-Reply-To: <20170213204526.GG2065@redhat.com> (Vivek Goyal's message of
        "Mon, 13 Feb 2017 15:45:26 -0500")
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Subject: Re: [PATCH][V2] vfs: Use upper filesystem inode in bprm_fill_uid()
Sender: linux-fsdevel-owner@vger.kernel.org
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Vivek Goyal <vgoyal@redhat.com> writes:

> Right now bprm_fill_uid() uses inode fetched from file_inode(bprm->file).
> This in turn returns inode of lower filesystem (in a stacked filesystem
> setup).
>
> I was playing with modified patches of shiftfs posted by james bottomley
> and realized that through shiftfs setuid bit does not take effect. And
> reason being that we fetch uid/gid from inode of lower fs (and not from
> shiftfs inode). And that results in following checks failing.
>
> /* We ignore suid/sgid if there are no mappings for them in the ns */
> if (!kuid_has_mapping(bprm->cred->user_ns, uid) ||
>     !kgid_has_mapping(bprm->cred->user_ns, gid))
> 	return;
>
> uid/gid fetched from lower fs inode might not be mapped inside the user
> namespace of container. So we need to look at uid/gid fetched from
> upper filesystem (shiftfs in this particular case) and these should be
> mapped and setuid bit can take affect.
>  
> Signed-off-by: Vivek Goyal <vgoyal@redhat.com>

Applied thanks,

Eric

> ---
>  fs/exec.c |    2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> Index: rhvgoyal-linux/fs/exec.c
> ===================================================================
> --- rhvgoyal-linux.orig/fs/exec.c	2017-02-13 15:22:57.107114781 -0500
> +++ rhvgoyal-linux/fs/exec.c	2017-02-13 15:24:19.568114781 -0500
> @@ -1479,7 +1479,7 @@ static void bprm_fill_uid(struct linux_b
>  	if (task_no_new_privs(current))
>  		return;
>  
> -	inode = file_inode(bprm->file);
> +	inode = bprm->file->f_path.dentry->d_inode;
>  	mode = READ_ONCE(inode->i_mode);
>  	if (!(mode & (S_ISUID|S_ISGID)))
>  		return;