linux-kernel - Re: [syzbot] [mm?] WARNING in copy_huge

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Message-ID: <4f96130c-12b7-4afa-ada3-bec354576112@redhat.com>
Date: Tue, 24 Sep 2024 16:45:00 +0200
From: David Hildenbrand <david@...hat.com>
To: syzbot <syzbot+bf2c35fa302ebe3c7471@...kaller.appspotmail.com>,
 akpm@...ux-foundation.org, bp@...en8.de, dave.hansen@...ux.intel.com,
 hpa@...or.com, jgg@...pe.ca, leitao@...ian.org,
 linux-kernel@...r.kernel.org, linux-mm@...ck.org, mingo@...hat.com,
 rppt@...nel.org, syzkaller-bugs@...glegroups.com, tglx@...utronix.de,
 x86@...nel.org, peterx@...hat.com
Subject: Re: [syzbot] [mm?] WARNING in copy_huge_pmd

On 23.09.24 14:18, syzbot wrote:
> Hello,
> 
> syzbot found the following issue on:
> 
> HEAD commit:    88264981f208 Merge tag 'sched_ext-for-6.12' of git://git.k..
> git tree:       upstream
> console+strace: https://syzkaller.appspot.com/x/log.txt?x=16c36c27980000
> kernel config:  https://syzkaller.appspot.com/x/.config?x=e851828834875d6f
> dashboard link: https://syzkaller.appspot.com/bug?extid=bf2c35fa302ebe3c7471
> compiler:       Debian clang version 15.0.6, GNU ld (GNU Binutils for Debian) 2.40
> syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=12773080580000
> C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=16ed5e9f980000
> 
> Downloadable assets:
> disk image: https://storage.googleapis.com/syzbot-assets/0e011ac37c93/disk-88264981.raw.xz
> vmlinux: https://storage.googleapis.com/syzbot-assets/f5c65577e19e/vmlinux-88264981.xz
> kernel image: https://storage.googleapis.com/syzbot-assets/984d963c8ea1/bzImage-88264981.xz
> 
> The issue was bisected to:
> 
> commit 75182022a0439788415b2dd1db3086e07aa506f7
> Author: Peter Xu <peterx@...hat.com>
> Date:   Mon Aug 26 20:43:51 2024 +0000
> 
>      mm/x86: support large pfn mappings
> 
> bisection log:  https://syzkaller.appspot.com/x/bisect.txt?x=17df9c27980000
> final oops:     https://syzkaller.appspot.com/x/report.txt?x=143f9c27980000
> console output: https://syzkaller.appspot.com/x/log.txt?x=103f9c27980000
> 
> IMPORTANT: if you fix the issue, please add the following tag to the commit:
> Reported-by: syzbot+bf2c35fa302ebe3c7471@...kaller.appspotmail.com
> Fixes: 75182022a043 ("mm/x86: support large pfn mappings")
> 
> ------------[ cut here ]------------
> WARNING: CPU: 1 PID: 5508 at mm/huge_memory.c:1602 copy_huge_pmd+0x102c/0x1c60 mm/huge_memory.c:1602

This is the

VM_WARN_ON_ONCE(is_cow_mapping(src_vma->vm_flags) && pmd_write(pmd))

So we have a special-marked PMD in a COW mapping.

The reproducer seems to involve fuse, but not sure if that makes a 
difference here.

-- 
Cheers,

David / dhildenb