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Date: Sat, 7 Feb 2015 07:32:43 -0500
From: Rich Felker <>
Subject: Re: realpath() and setfsuid programs

On Sat, Feb 07, 2015 at 07:26:03AM -0500, Rich Felker wrote:
> On Sat, Feb 07, 2015 at 09:53:54AM +0200, Timo Teras wrote:
> > Hi,
> > 
> > It seems realpath() does not work in binaries using setfsuid(). (At
> > least on grsec kernels, vanilla kernel might be affected too.)
> > 
> > The problem is that realpath() opens the file, and then
> > uses just readlink on /proc/self/fd/<fd> to read the canonicalized
> > path.
> > 
> > However, /proc/self/fd is not accessible if setfsuid() has been used to
> > drop privileges.
> > 
> > The problem I'm looking at in this case is fuse. fusermount, the
> > suid wrapper to do user fuse mounts, seems to basically do:
> >  oldfsuid = setfsuid(getuid())
> >  oldfsgid = setfsgid(getgid())
> >  take realpath of mountpoint
> >  chdir("/")
> >  setfsuid(oldfsuid)
> >  setfsgid(oldfsgid)
> > 
> > I believe they want to drop privileges so it works as also access check
> > to the mount point directory. As realpath() in practice checks that the
> > user has access to the entry too.
> Could you clarify what you think the security intent of this code is?
> As far as I can tell it's nonsense. realpath is not usable for much of
> anything security-related; in particular, it's non-atomic and subject
> to all sorts of trickery involving renaming/moving directories during
> its operation, even moreso when it's done component-by-component in
> userspace.
> Why is the check not simply an ownership check for the mount point? I
> suspect it has to do with the need to pass a pathname rather than fd
> to mount, which is subject to renaming/moving races, but the realpath
> call would be subject to the same and worse. Presumably the correct
> way to do this is to open a fd to the mountpoint then pass
> /proc/self/fd/%d to the mount function after checking ownership.

Or of course just using chdir and checking ownership of ".".


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