Re: AS-safe change for multithreaded setxid breaks chroot() along with set*id()

[Rich Felker <dalias@libc.org>]

On Fri, Apr 01, 2016 at 08:28:48PM +0000, Josiah Worcester wrote:
> On Fri, Apr 1, 2016 at 1:23 PM Lance Chen <cyen0312@gmail.com> wrote:
> 
> > Hello there,
> >
> > I have been trying to run powerdns on Alpine Linux, which features musl,
> > without success. It turns out that the AS-safe strategy introduced in
> > the patch
> >
> > http://git.musl-libc.org/cgit/musl/commit/?id=78a8ef47c4d92b7680c52a85f80a81e29da86bb9
> > ,
> > especially the use of reading /proc/self/task, causes the failure. The
> > failure condition is a combination of three features, pthread, chroot,
> > and set*id. After chroot()'ing to some directory, calling set*id() will
> > try to access /proc/self/task, and fail to find the directory in the new
> > root. I've created a minimal POC at
> > https://gist.github.com/Lance0312/c7a82793e35b322bfbdfe96b79dd3152.
> 
> A fairly simple fix for this would be to create and mount /proc in the
> chroot. This is likely to be needed for some other interfaces in libc as
> well as other libraries and utilities, because there's functionality of
> Linux which is only really exposed in that filesystem. I don't know of
> anything in particular that would fail on glibc, but I am confident that
> you'll have issues, just like if you don't have /dev in the chroot.

Indeed. Due to Linux limitations, it's impossible to provide complete
behavior for some functions without /proc. In the case of set*id, the
right solution is to fix the kernel so that it can atomically change
the ids of a whole process, but I looked at doing that and it's hard
(because of a lot of architectural flaws on the kernel's part, like
using an allocated struct cred and RCU mess to represent ids rather
than just putting them inline in the task struct).

Rich