Re: [PATCH v2 1/3] vfs: Check for invalid i_uid in may_follow_link()

[Serge Hallyn <serge.hallyn@ubuntu.com>]

Quoting Seth Forshee (seth.forshee@canonical.com):
> On Fri, Sep 05, 2014 at 05:05:09PM +0000, Serge Hallyn wrote:
> > Quoting Seth Forshee (seth.forshee@canonical.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@canonical.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.

Oh - yeah, i saw that and glossed over it for some reason.  Thanks.

> > Acked-by: Serge E. Hallyn <serge.hallyn@ubuntu.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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