linux-kernel - gdbserver + fsgsbase kaputt

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Message-ID: <20210111181520.GE25645@zn.tnic>
Date:   Mon, 11 Jan 2021 19:15:20 +0100
From:   Borislav Petkov <bp@...en8.de>
To:     "Chang S. Bae" <chang.seok.bae@...el.com>,
        Andy Lutomirski <luto@...nel.org>
Cc:     tdevries@...e.com, x86-ml <x86@...nel.org>,
        lkml <linux-kernel@...r.kernel.org>
Subject: gdbserver + fsgsbase kaputt

Hi,

so there's a breakage of a use case with gdbserver on fsgsbase machines,
see

https://sourceware.org/bugzilla/show_bug.cgi?id=26804

Tom has an even simpler reproducer:

$ cat test.c
int
main (void)
{
  return 0;
}
$ gcc test.c -m32
$ gdbserver localhost:12345 a.out
... other terminal ...
$ gdb -batch -q -ex "target remote localhost:12345" -ex continue
Program received signal SIGSEGV, Segmentation fault.
0xf7dd8bd2 in init_cacheinfo () at ../sysdeps/x86/cacheinfo.c:761

The correct output is, of course:

...
[Inferior 1 (process 1860) exited normally]

I tried to bisect this but it led me to:

  b745cfba44c1 ("x86/cpu: Enable FSGSBASE on 64bit by default and add a chicken bit")

which simply enables fsgsbase so I could've made a small mistake in the
bisection.

I say small because booting with "nofsgsbase" cures it so it must be
something fsgsbase + ptrace especially since the symptom is a corrupted
stack canary in %gs...

Thx.

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette