Re: [syzbot] [mm?] kernel BUG in sanity_check_pinned_pages

[David Hildenbrand]

On 23.06.25 11:53, Alexander Potapenko wrote:
> On Mon, Jun 23, 2025 at 11:29 AM 'David Hildenbrand' via
> syzkaller-bugs <syzkaller-bugs@xxxxxxxxxxxxxxxx> wrote:
> > On 21.06.25 23:52, syzbot wrote:
> > > syzbot has found a reproducer for the following issue on:
> > >
> > > HEAD commit:    9aa9b43d689e Merge branch 'for-next/core' into for-kernelci
> > > git tree:       git://git.kernel.org/pub/scm/linux/kernel/git/arm64/linux.git for-kernelci
> > > console output: https://syzkaller.appspot.com/x/log.txt?x=1525330c580000
> > > kernel config:  https://syzkaller.appspot.com/x/.config?x=27f179c74d5c35cd
> > > dashboard link: https://syzkaller.appspot.com/bug?extid=1d335893772467199ab6
> > > compiler:       Debian clang version 20.1.6 (++20250514063057+1e4d39e07757-1~exp1~20250514183223.118), Debian LLD 20.1.6
> > > userspace arch: arm64
> > > syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=16d73370580000
> > > C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=160ef30c580000
> >
> > There is not that much magic in there, I'm afraid.
> >
> > fork() is only used to spin up guests, but before the memory region of
> > interest is actually allocated, IIUC. No threading code that races.
> >
> > IIUC, it triggers fairly fast on aarch64. I've left it running for a
> > while on x86_64 without any luck.
> >
> > So maybe this is really some aarch64-special stuff (pointer tagging?).
> >
> > In particular, there is something very weird in the reproducer:
> >
> >     syscall(__NR_madvise, /*addr=*/0x20a93000ul, /*len=*/0x4000ul,
> >             /*advice=MADV_HUGEPAGE|0x800000000*/ 0x80000000eul);
> >
> > advise is supposed to be a 32bit int. What does the magical
> > "0x800000000" do?
>
> I am pretty sure this is a red herring.
> Syzkaller sometimes mutates integer flags, even if the result makes no
> sense - because sometimes it can trigger interesting bugs.
> This `advice` argument will be discarded by is_valid_madvise(),
> resulting in -EINVAL.

I thought the same, but likely the upper bits are discarded, and we end 
up with __NR_madvise succeeding.

The kernel config has

	CONFIG_TRANSPARENT_HUGEPAGE_MADVISE=y

So without MADV_HUGEPAGE, we wouldn't get a THP in the first place.

So likely this is really just like dropping the "0x800000000"

Anyhow, I managed to reproduce in the VM using the provided rootfs on 
aarch64. It triggers immediately, so no races involved.

Running the reproducer on a Fedora 42 debug-kernel in the hypervisor 
does not trigger.

--
Cheers,

David / dhildenb