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

[David Hildenbrand <david@redhat.com>]

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@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.

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