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From:   Linus Torvalds <torvalds@linux-foundation.org>
Date:   Wed, 23 Mar 2022 20:05:11 -0700
Message-ID: <CAHk-=wgW1hEUaQeX41+3w7AGsXkeE1XOXCMndXs3kFp-XjSVzQ@mail.gmail.com>
Subject: Re: [syzbot] general protection fault in list_lru_add
To:     Muchun Song <songmuchun@bytedance.com>
Cc:     syzbot <syzbot+f8c45ccc7d5d45fc5965@syzkaller.appspotmail.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Linux-MM <linux-mm@kvack.org>,
        syzkaller-bugs <syzkaller-bugs@googlegroups.com>
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On Wed, Mar 23, 2022 at 7:19 PM Muchun Song <songmuchun@bytedance.com> wrote:
>
> After this commit, the rules of dentry allocations changed.
> The dentry should be allocated by kmem_cache_alloc_lru()

Yeah, I looked at that, but I can't find any way there could be other
allocations - not only are there strict rules how to initialize
everything, but the dentries are free'd using

        kmem_cache_free(dentry_cache, dentry);

and as a result if they were allocated any other way I would expect
things would go south very quickly.

The only other thing I could come up with is some breakage in the
superblock lifetime so that &dentry->d_sb->s_dentry_lru would have
problems, but again, this is *such* core code and not some unusual
path, that I would be very very surprised if it wouldn't have
triggered other issues long long ago.

That's why I'd be more inclined to worry about the list_lru code being
somehow broken.

               Linus