linux-kernel - Re: [PATCH] rwonce: fix crash by removing READ

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: <20250326225444.GA1743548@ax162>
Date: Wed, 26 Mar 2025 15:54:44 -0700
From: Nathan Chancellor <nathan@...nel.org>
To: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: Arnd Bergmann <arnd@...db.de>, Jann Horn <jannh@...gle.com>,
	Marco Elver <elver@...gle.com>,
	Linux-Arch <linux-arch@...r.kernel.org>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] rwonce: fix crash by removing READ_ONCE() for unaligned
 read

On Wed, Mar 26, 2025 at 03:41:34PM -0700, Linus Torvalds wrote:
> That said, this whole thing worries me. The fact that the compiler
> magically makes READ_ONCE() require alignment that it normally doesn't
> require seems like a bug waiting to happen somewhere else.

For the record, I do not think it is the compiler doing this, it is the
arm64 code after commit e35123d83ee3 ("arm64: lto: Strengthen
READ_ONCE() to acquire when CONFIG_LTO=y") back in 5.11.

> Because I do think that we might want READ_ONCE() on unaligned data in
> general. Should said places generally use "get_unaligned()"? Sure. And
> are unaligned accesses potentially non-atomic anyway because of
> hardware? Also sure.
> 
> But one reason for READ_ONCE() isn't for some kind of hardware
> atomicity, it is to avoid any ToCToU issues due to compilers doing bad
> things.
> 
> And then this seems to be a serious issue with the whole "READ_ONCE()
> now requires alignment that it didn't require before".
> 
> Put another way: I wonder what other cases may lurk around this all...

That change has caused only one issue that I know of, which was fixed by
commit d3f450533bbc ("efi: tpm: Avoid READ_ONCE() for accessing the
event log"). I have not seen any since then until this point and I do
daily boots of -next with LTO enabled on both of my arm64 test machines.

Cheers,
Nathan