linux-kernel - Re: [PATCH] x86: add back the alignment of the destination to 8 bytes in copy_user

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <CAHk-=wg6aT1ytSaBouOSpu71ZFMY8ct1a6=HOQC0CZuHxQqK2w@mail.gmail.com>
Date: Mon, 17 Mar 2025 14:29:05 -0700
From: Linus Torvalds <torvalds@...ux-foundation.org>
To: David Laight <david.laight.linux@...il.com>
Cc: "Herton R. Krzesinski" <herton@...hat.com>, x86@...nel.org, tglx@...utronix.de, mingo@...hat.com, 
	bp@...en8.de, dave.hansen@...ux.intel.com, hpa@...or.com, 
	linux-kernel@...r.kernel.org, olichtne@...hat.com, atomasov@...hat.com, 
	aokuliar@...hat.com
Subject: Re: [PATCH] x86: add back the alignment of the destination to 8 bytes
 in copy_user_generic()

On Mon, 17 Mar 2025 at 06:16, David Laight <david.laight.linux@...il.com> wrote:
>
> You can also something similar for any trailing bytes.
> If you are feeling 'brave' copy the last 8 bytes first.

I think that would be a mistake.

Not only does ti cause bad patterns on page faults - we should recover
ok from it (the exception will go back and do the copy in the right
order one byte at a time in the "copy_user_tail" code) - but even in
the absence of page faults it quite possibly messes with CPU
prefetching and write buffer coalescing etc if you hop around like
that.

It *might* be worth trying doing last unaligned part the same way my
patch does the first one - by just doing a full-word write at the end,
offset backwards. That avoids the byte-at-a-time tail case.

I'm not convinced it's worth it, but if somebody spends the effort on
a patch and on benchmarking...

            Linus