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

[Alexander Potapenko <glider@google.com>]

On Mon, Jun 23, 2025 at 11:29 AM 'David Hildenbrand' via
syzkaller-bugs <syzkaller-bugs@googlegroups.com> 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.