Part 2! Request for help / ideas to debug issue

[Gustavo Niemeyer]

Thanks for finding and debugging this John.

I can't see how this one would be in Go. I can't think of any related
action that would disable the setuid bit on the exec syscall.

Perhaps someone from the kernel team can help here?



On Mon, Mar 13, 2017 at 9:59 AM, John Lenton <john.lenton at canonical.com>
wrote:

> This one is slightly more interesting.
>
> You need 1.8 (or patched <1.8 as per the previous thread) for this one
> to make sense; without it you're just going to get drowned in warning
> messages and not see the real issue.
>
> This one is the real issue :-)
>
> In go, when calling syscall.Exec to a setuid root binary, sometimes
> (about 4% of the times, on my machine, but it's hardware- and
> load-dependent), the exec'ed process will find itself running with
> effective uid different to zero. That is, a setuid root binary will
> find itself running as non-root. As the process that sets up
> confinement is setuid root (in distros where setuid is favoured over
> capabilities), this means the snap app falls on its face.
>
>     TODO: check if something similar happens when using caps
>
> This is *probably* a bug in Go, but it only seems to arise when using
> syscall.Exec, which as far as I can tell is unsupported (the whole
> syscall package is unsupported -- not covered by the go1 compatibility
> promise -- and its replacement, golang.org/x/sys/unix, is ominously
> missing Exec).
>
> Having said that, it might be a bug in the kernel ;-)
> And I say this because if you pin the process to a single cpu, the
> issue doesn't arise.
>
> Anyway, code to repro this is at
> https://gist.github.com/chipaca/806c90d96c437444f27f45a83d00a813
>
> on my machine,
>
> $ for i in `seq -w 9999`; do ./a_c; done | wc -l
> 0
> $ for i in `seq -w 9999`; do ./a_go; done | wc -l
> 394
>
> And,
>
> $ for i in `seq -w 9999`; do taskset 2 ./a_go; done | wc -l
> 0
>
> Gnarly!
>
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-- 
gustavo @ http://niemeyer.net