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Date: Sat, 7 Feb 2015 07:26:03 -0500
From: Rich Felker <dalias@...c.org>
To: musl@...ts.openwall.com
Subject: Re: realpath() and setfsuid programs
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.
> This works glibc, as realpath() canonicalizes the path
> component-by-component in userland. But musl breaks due to the /proc
> not being accessible while privileges dropped.
>
> Any suggestions?
I think first we should figure out if the code even makes sense. I
suspect it's a bug.
Rich
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