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From:   Andy Lutomirski <luto@amacapital.net>
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Subject: Re: general protection fault in perf_misc_flags
Date:   Mon, 28 Sep 2020 14:19:57 -0700
Message-Id: <15CE600C-3006-4C4D-A65E-9B608D55DD35@amacapital.net>
References: <CAKwvOdmTm2rVdc2JTSVVadKP3DONRcPXSE-s3tFPqHgCSieH7Q@mail.gmail.com>
Cc:     Dmitry Vyukov <dvyukov@google.com>, Borislav Petkov <bp@alien8.de>,
        Alexander Potapenko <glider@google.com>,
        Marco Elver <elver@google.com>,
        syzbot <syzbot+ce179bc99e64377c24bc@syzkaller.appspotmail.com>,
        Arnaldo Carvalho de Melo <acme@kernel.org>,
        Alexander Shishkin <alexander.shishkin@linux.intel.com>,
        "H. Peter Anvin" <hpa@zytor.com>, Jiri Olsa <jolsa@redhat.com>,
        LKML <linux-kernel@vger.kernel.org>,
        Mark Rutland <mark.rutland@arm.com>,
        Ingo Molnar <mingo@redhat.com>,
        Namhyung Kim <namhyung@kernel.org>,
        Peter Zijlstra <peterz@infradead.org>,
        syzkaller-bugs <syzkaller-bugs@googlegroups.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        the arch/x86 maintainers <x86@kernel.org>,
        clang-built-linux <clang-built-linux@googlegroups.com>
In-Reply-To: <CAKwvOdmTm2rVdc2JTSVVadKP3DONRcPXSE-s3tFPqHgCSieH7Q@mail.gmail.com>
To:     Nick Desaulniers <ndesaulniers@google.com>
Precedence: bulk


> On Sep 28, 2020, at 1:52 PM, Nick Desaulniers <ndesaulniers@google.com> wr=
ote:
>=20
> =EF=BB=BFOn Mon, Sep 28, 2020 at 3:34 AM 'Dmitry Vyukov' via Clang Built L=
inux
> <clang-built-linux@googlegroups.com> wrote:
>>=20
>>> On Mon, Sep 28, 2020 at 10:54 AM Borislav Petkov <bp@alien8.de> wrote:
>>>=20
>>> On Mon, Sep 28, 2020 at 10:40:19AM +0200, Dmitry Vyukov wrote:
>>>> I meant the kernel self-corrupts itself, that just wasn't detected by
>>>> KASAN, page protections, etc.
>>>=20
>>> Well, Nick already asked this but we're marking all kernel text RO early=

>>> during boot. So it either is happening before that or something else
>>> altogether is going on.
>=20
>> On Sun, Sep 27, 2020 at 11:06 PM 'Dmitry Vyukov' via Clang Built Linux
>> <clang-built-linux@googlegroups.com> wrote:
>>=20
>> Interestingly there is a new crash, which looks similar:
>>=20
>> general protection fault in map_vdso
>> https://syzkaller.appspot.com/bug?extid=3Dc2ae01c2b1b385384a06
>>=20
>> The code is also with 4 0's:
>> Code: 00 00 00 48 b8 00 00 00 00 00 fc ff df 41 57 49 89 ff 41 56 41
>> 55 41 54 55 65 48 8b 2c 25 c0 fe 01 00 48 8d bd 28 04 00 00 53 <48> 00
>> 00 00 00 fa 48 83 ec 10 48 c1 ea 03 80 3c 02 00 0f 85 51 02
>>=20
>> But it happened with gcc.
>>=20
>> Also I found this older one:
>> general protection fault in map_vdso_randomized
>> https://syzkaller.appspot.com/bug?id=3D8366fd024559946137b9db23b26fd2235d=
43b383
>>=20
>> which also has code smashed and happened with gcc:
>> Code: 00 fc ff df 48 89 f9 48 c1 e9 03 80 3c 01 00 0f 85 eb 00 00 00
>> 65 48 8b 1c 25 c0 fe 01 00 48 8d bb 28 04 00 00 41 2b 54 24 20 <00> 00
>> 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
>=20
> If this is related to vdso's, they seem mapped as `r-xp` (not `w):
> $ sudo cat /proc/1/maps | grep vdso
> 7ffc667f5000-7ffc667f7000 r-xp 00000000 00:00 0                          [=
vdso]
>=20
> map_vdso() in arch/x86/entry/vdso/vma.c doesn't map the VMA as
> writable, but it uses VM_MAYWRITE with a comment about GDB setting
> breakpoints.
>=20
> So it sounds like the page protections on the vdso can be changed at
> runtime (via mprotect).  Maybe syzkaller is tickling that first?
>=20
> map_vdso_randomized() does call map_vdso().  Maybe if we mprotect the
> vdso to be writable, it may be easier to spot the write.
>=20
>=20

The kernel shouldn=E2=80=99t be executing the vDSO code.  Unless I=E2=80=99v=
e misread it, Te crash is that the map_vdso() text itself was corrupted.  Th=
is isn=E2=80=99t the same thing.

The VM_MAYWRITE means that a program may CoW the page and write to the copy,=
 which still won=E2=80=99t allow changing the vDSO text or executing it insi=
de the kernel.=