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

[Josiah Worcester <josiahw@gmail.com>]

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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.

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