linux-kernel - Re: [REGRESSION] x86/cpu fsgsbase breaks TLS in 32 bit rr tracees on a 64 bit system

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Message-ID: <CAP045Aqhox6YSdk0v_YZWY=y7Ps4ZfH779MG-W4a=gc+cYEY+Q@mail.gmail.com>
Date:   Tue, 25 Aug 2020 09:32:08 -0700
From:   Kyle Huey <me@...ehuey.com>
To:     Andy Lutomirski <luto@...capital.net>
Cc:     Andy Lutomirski <luto@...nel.org>,
        "H. Peter Anvin" <hpa@...or.com>,
        "Robert O'Callahan" <robert@...llahan.org>,
        "Bae, Chang Seok" <chang.seok.bae@...el.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...nel.org>,
        Andi Kleen <ak@...ux.intel.com>,
        "Shankar, Ravi V" <ravi.v.shankar@...el.com>,
        LKML <linux-kernel@...r.kernel.org>,
        "Hansen, Dave" <dave.hansen@...el.com>
Subject: Re: [REGRESSION] x86/cpu fsgsbase breaks TLS in 32 bit rr tracees on
 a 64 bit system

On Tue, Aug 25, 2020 at 9:12 AM Andy Lutomirski <luto@...capital.net> wrote:
> I don’t like this at all. Your behavior really shouldn’t depend on
> whether the new instructions are available.  Also, some day I would
> like to change Linux to have the new behavior even if FSGSBASE
> instructions are not available, and this will break rr again.  (The
> current !FSGSBASE behavior is an ugly optimization of dubious value.
> I would not go so far as to describe it as correct.)

Ok.

> I would suggest you do one of the following things:
>
> 1. Use int $0x80 directly to load 32-bit regs into a child.  This
> might dramatically simplify your code and should just do the right
> thing.

I don't know what that means.

> 2. Something like your patch but make it unconditional.
>
> 3. Ask for, and receive, real kernel support for setting FS and GS in
> the way that 32-bit code expects.

I think the easiest way forward for us would be a PTRACE_GET/SETREGSET
like operation that operates on the regsets according to the
*tracee*'s bitness (rather than the tracer, as it works currently).
Does that sound workable?

- Kyle