linux-kernel - Re: general protection fault in perf_misc

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Message-ID: <20200921221336.GN5901@zn.tnic>
Date:   Tue, 22 Sep 2020 00:13:36 +0200
From:   Borislav Petkov <bp@...en8.de>
To:     Nick Desaulniers <ndesaulniers@...gle.com>
Cc:     Dmitry Vyukov <dvyukov@...gle.com>,
        syzbot <syzbot+ce179bc99e64377c24bc@...kaller.appspotmail.com>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        "H. Peter Anvin" <hpa@...or.com>, Jiri Olsa <jolsa@...hat.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Mark Rutland <mark.rutland@....com>,
        Ingo Molnar <mingo@...hat.com>,
        Namhyung Kim <namhyung@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        syzkaller-bugs <syzkaller-bugs@...glegroups.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        the arch/x86 maintainers <x86@...nel.org>,
        clang-built-linux <clang-built-linux@...glegroups.com>
Subject: Re: general protection fault in perf_misc_flags

On Mon, Sep 21, 2020 at 01:59:43PM -0700, Nick Desaulniers wrote:
> Right, the two sequences above look almost the same, except those 4
> bytes of zeros (the disassembler gets confused about the rest, but
> it's the same byte sequence otherwise).  Are the two disassemblies a
> comparison of the code at runtime vs. compile-time? 

Yes.

> If so, how did you disassemble the runtime code?

./scripts/decodecode < /tmp/splat

where /tmp/splat contains the line starting with "Code:". Make sure you
have only one "Code:"-line, otherwise you'll see the code of the *last*
Code: line only.

> If runtime and compile time differ, I suspect some kind of runtime
> patching.

If it is, it ain't patching at the right place. :)

But no, that function is pretty simple and looking at its asm, there's
no asm goto() or alternatives in there. But that .config might add them.
It adds a lot of calls to *ASAN helpers and whatnot.

> I wonder if we calculated the address of a static_key wrong
> (asm goto). What function am I looking at the disassembly of?
> perf_misc_flags() in arch/x86/events/core.c?

Yes.

> With this config?
> https://syzkaller.appspot.com/x/.config?x=cd992d74d6c7e62 (though I
> don't see _any_ asm goto in the IR for this file built with this
> config).

Right, there should be none.

> If this is deterministically reproducible, I suppose we
> could set a watchpoint on the address being overwritten?

Sounds like worth a try. I'll go sleep instead, tho. :)

Gnight and good luck.

-- 
Regards/Gruss,
    Boris.

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