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From:   Linus Torvalds <torvalds@linux-foundation.org>
Date:   Mon, 29 Nov 2021 13:53:01 -0800
Message-ID: <CAHk-=wjZ6zME2SzohM1P_-B0BNi2JJgvz22ypF-EuAQiVKipRg@mail.gmail.com>
Subject: Re: [PATCH 3/3] btrfs: Avoid live-lock in search_ioctl() on hardware
 with sub-page faults
To:     Catalin Marinas <catalin.marinas@arm.com>
Cc:     Andreas Gruenbacher <agruenba@redhat.com>,
        Matthew Wilcox <willy@infradead.org>,
        Josef Bacik <josef@toxicpanda.com>,
        David Sterba <dsterba@suse.com>,
        Al Viro <viro@zeniv.linux.org.uk>,
        Andrew Morton <akpm@linux-foundation.org>,
        Will Deacon <will@kernel.org>,
        linux-fsdevel <linux-fsdevel@vger.kernel.org>,
        LKML <linux-kernel@vger.kernel.org>,
        Linux ARM <linux-arm-kernel@lists.infradead.org>,
        linux-btrfs <linux-btrfs@vger.kernel.org>
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On Mon, Nov 29, 2021 at 12:56 PM Catalin Marinas
<catalin.marinas@arm.com> wrote:
>
> For arm64 at least __put_user() does the access_ok() check. I thought
> only unsafe_put_user() should skip the checks. If __put_user() can write
> arbitrary memory, we may have a bigger problem.

That's literally be the historical difference between __put_user() and
put_user() - the access check.

> I think that would be useful, though it doesn't solve the potential
> livelock with sub-page faults.

I was assuming we'd just do the sub-page faults.

In fact, I was assuming we'd basically just replace all the PAGE_ALIGN
and PAGE_SIZE with SUBPAGE_{ALIGN,SIZE}, together with something like

        if (size > PAGE_SIZE)
                size = PAGE_SIZE;

to limit that size thing (or possibly make that "min size" be a
parameter, so that people who have things like that "I need at least
this initial structure to be copied" issue can document their minimum
size needs).

            Linus