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From:   Andrey Konovalov <andreyknvl@gmail.com>
Date:   Tue, 6 Sep 2022 16:39:57 +0200
Message-ID: <CA+fCnZf-GOZpnBRLkRPrL7FUwYrAb-bu=PV0zMfbKOuNYrZ00A@mail.gmail.com>
Subject: Re: [syzbot] KASAN: invalid-access Read in copy_page
To:     Catalin Marinas <catalin.marinas@arm.com>
Cc:     Dmitry Vyukov <dvyukov@google.com>,
        Linux ARM <linux-arm-kernel@lists.infradead.org>,
        LKML <linux-kernel@vger.kernel.org>,
        syzkaller-bugs <syzkaller-bugs@googlegroups.com>,
        tongtiangen@huawei.com,
        Vincenzo Frascino <vincenzo.frascino@arm.com>,
        Kefeng Wang <wangkefeng.wang@huawei.com>,
        Will Deacon <will@kernel.org>,
        syzbot <syzbot+c2c79c6d6eddc5262b77@syzkaller.appspotmail.com>,
        Evgenii Stepanov <eugenis@google.com>,
        Peter Collingbourne <pcc@google.com>
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On Tue, Sep 6, 2022 at 4:29 PM Catalin Marinas <catalin.marinas@arm.com> wrote:
>
> > > Does it take long to reproduce this kasan warning?
> >
> > syzbot finds several such cases every day (200 crashes for the past 35 days):
> > https://syzkaller.appspot.com/bug?extid=c2c79c6d6eddc5262b77
> > So once it reaches the tested tree, we should have an answer within a day.

To be specific, this syzkaller instance fuzzes the mainline, so the
patch with the WARN_ON needs to end up there.

If this is unacceptable, perhaps, we could switch the MTE syzkaller
instance to the arm64 testing tree.

> That's good to know. BTW, does syzkaller write tags in mmap'ed pages or
> only issues random syscalls?

syzkaller doesn't write tags. Or, at least, shouldn't. Theoretically
it could come up with same way to generate instructions that write
tags, but this is unlikely.

> I'm trying to figure out whether tag 0xf2
> was written by the kernel without updating the corresponding
> page_kasan_tag() or it was syzkaller recolouring the page.

Just in case, I want to point out that the kasantag == 0xa from the
page flags matches the pointer tag 0xf5 in the report. The tag value
is stored bitwise-inverted in the page flags. Not that this matters in
this case though.