Messages in this thread | | | Date | Tue, 6 Sep 2022 17:23:39 +0100 | From | Catalin Marinas <> | Subject | Re: [syzbot] KASAN: invalid-access Read in copy_page |
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On Tue, Sep 06, 2022 at 04:39:57PM +0200, Andrey Konovalov wrote: > 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.
It needs some more digging first. My first guess was that a PROT_MTE page was mapped into the user address space and the task repainted it but I don't think that's the case.
> > 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.
Yeah. And colouring an entire page with the same tag is even less likely.
> > 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.
Yes, I'm aware of this. So copy_page() tries to read from page_address(src) with kasantag == 0xa (real tag 0xf5) while the in-memory tag is 0xf2. Since the user didn't repaint the page, I'm trying to figure out what set the tags to 0xf2 while leaving the page_kasan_tag() to 0xf5. Some of the page_kasan_tag_reset() calls in the past could have hidden a different issue.
Since I can't find the kernel boot log for these runs, is there any kind of swap enabled? I'm trying to narrow down where the problem may be.
-- Catalin
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