linux-kernel - Re: [PATCH] tools/nolibc: x86: Remove `r8`, `r9` and `r10` from the clobber list

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: <20211013142433.GB8557@1wt.eu>
Date:   Wed, 13 Oct 2021 16:24:33 +0200
From:   Willy Tarreau <w@....eu>
To:     Borislav Petkov <bp@...en8.de>
Cc:     Ammar Faizi <ammar.faizi@...dents.amikom.ac.id>,
        Paul Walmsley <paul.walmsley@...ive.com>,
        Palmer Dabbelt <palmer@...belt.com>,
        Albert Ou <aou@...s.berkeley.edu>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Andy Lutomirski <luto@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, x86@...nel.org,
        "H. Peter Anvin" <hpa@...or.com>, Michael Matz <matz@...e.de>
Subject: Re: [PATCH] tools/nolibc: x86: Remove `r8`, `r9` and `r10` from the
 clobber list

On Wed, Oct 13, 2021 at 04:20:55PM +0200, Borislav Petkov wrote:
> On Wed, Oct 13, 2021 at 04:07:23PM +0200, Willy Tarreau wrote:
> > Yes I agree with the "potentially" here. If it can potentially be (i.e.
> > the kernel is allowed by contract to later change the way it's currently
> > done) then we have to save them even if it means lower code efficiency.
> >
> > If, however, the kernel performs such savings on purpose because it is
> > willing to observe a stricter saving than the AMD64 ABI, we can follow
> > it but only once it's written down somewhere that it is by contract and
> > will not change.
> 
> Right, and Micha noted that such a change to the document can be done.

great.

> And we're basically doing that registers restoring anyway, in POP_REGS.

That's what I based my analysis on when I wanted to verify Ammar's
finding. I would tend to think that if we're burning cycles popping
plenty of registers it's probably for a reason, maybe at least a good
one, which is that it's the only way to make sure we're not leaking
internal kernel data! This is not a concern for kernel->kernel nor
user->user calls but for user->kernel calls it definitely is one, and
I don't think we could relax that series of pop without causing leaks
anyway.

Willy