linux-kernel - Re: [PATCH v2 1/3] vfs: Check for invalid i_uid in may_follow

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Message-ID: <20140905190053.GB84919@ubuntu-hedt>
Date:	Fri, 5 Sep 2014 14:00:53 -0500
From:	Seth Forshee <seth.forshee@...onical.com>
To:	Serge Hallyn <serge.hallyn@...ntu.com>
Cc:	Miklos Szeredi <miklos@...redi.hu>,
	Alexander Viro <viro@...iv.linux.org.uk>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	fuse-devel@...ts.sourceforge.net, linux-kernel@...r.kernel.org,
	linux-fsdevel@...r.kernel.org, seth.forshee@...onical.com
Subject: Re: [PATCH v2 1/3] vfs: Check for invalid i_uid in may_follow_link()

On Fri, Sep 05, 2014 at 05:05:09PM +0000, Serge Hallyn wrote:
> Quoting Seth Forshee (seth.forshee@...onical.com):
> > Filesystem uids which don't map into a user namespace may result
> > in inode->i_uid being INVALID_UID. A symlink and its parent
> > could have different owners in the filesystem can both get
> > mapped to INVALID_UID, which may result in following a symlink
> > when this would not have otherwise been permitted. Prevent this
> > by adding a check that the uid is valid before the comparison.
> > 
> > Signed-off-by: Seth Forshee <seth.forshee@...onical.com>
> 
> I'm a bit uncomfortable about this, but I can't put my finger
> on why.  Wonder if it could mess up root looking into
> a malicious user's task by looking under /proc/self/root.
> I suppose not, as this should only be the case (with root in
> init_user_ns) for fuse?
> 
> Anyway it seems needed for keeping root from falling into a trap.

This shouldn't mess up looking into /proc/self/root or anything else
where the symlink isn't under control of the malicious user. Plus this
restriction only applies to world-writable directories with the sticky
bit set.

> Acked-by: Serge E. Hallyn <serge.hallyn@...ntu.com>
> 
> > ---
> >  fs/namei.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> > 
> > diff --git a/fs/namei.c b/fs/namei.c
> > index a996bb48dfab..193da09e903e 100644
> > --- a/fs/namei.c
> > +++ b/fs/namei.c
> > @@ -741,7 +741,7 @@ static inline int may_follow_link(struct path *link, struct nameidata *nd)
> >  		return 0;
> >  
> >  	/* Allowed if parent directory and link owner match. */
> > -	if (uid_eq(parent->i_uid, inode->i_uid))
> > +	if (uid_valid(inode->i_uid) && uid_eq(parent->i_uid, inode->i_uid))
> >  		return 0;
> >  
> >  	audit_log_link_denied("follow_link", link);
> > -- 
> > 1.9.1
> > 
> --
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