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: <F1D9A90A-A2D5-47E4-8E33-410460FCD6CC@zytor.com>
Date:   Tue, 25 Aug 2020 08:13:51 -0700
From:   hpa@...or.com
To:     Andy Lutomirski <luto@...nel.org>
CC:     "Robert O'Callahan" <robert@...llahan.org>,
        "Bae, Chang Seok" <chang.seok.bae@...el.com>,
        Kyle Huey <me@...ehuey.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 August 24, 2020 5:30:56 PM PDT, Andy Lutomirski <luto@...nel.org> wrote:
>On Mon, Aug 24, 2020 at 4:52 PM H. Peter Anvin <hpa@...or.com> wrote:
>>
>> On 2020-08-24 14:10, Andy Lutomirski wrote:
>> >
>> > PTRACE_READ_SEGMENT_DESCRIPTOR to read a segment descriptor.
>> >
>> > PTRACE_SET_FS / PTRACE_SET_GS: Sets FS or GS and updates the base
>accordingly.
>> >
>> > PTRACE_READ_SEGMENT_BASE: pass in a segment selector, get a base
>out.
>> > You would use this to populate the base fields.
>> >
>> > or perhaps a ptrace SETREGS variant that tries to preserve the old
>> > base semantics and magically sets the bases to match the selectors
>if
>> > the selectors are nonzero.
>> >
>> > Do any of these choices sound preferable to any of you?
>> >
>>
>> My suggestion would be to export the GDT and LDT as a (readonly or
>mostly
>> readonly) regset(s) rather than adding entirely new operations. We
>could allow
>> the LDT and the per-thread GDT entries to be written, subject to the
>same
>> limitations as the corresponding system calls.
>>
>
>That seems useful, although we'd want to do some extensive
>sanitization of the GDT.  But maybe it's obnoxious to ask Kyle and
>Robert to parse the GDT, LDT, and selector just to emulate the
>demented pre-5.9 ptrace() behavior.
>
>--Andy

We only want to allow the same access that user space gets, that's exactly the sanitization we need.
-- 
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