From:   Thomas Gleixner <tglx@linutronix.de>
To:     "Maciej W. Rozycki" <macro@orcam.me.uk>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        "H. Peter Anvin" <hpa@zytor.com>
Cc:     "Jason A. Donenfeld" <Jason@zx2c4.com>,
        Kees Cook <keescook@chromium.org>, x86@kernel.org,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH v3] x86: Use `get_random_u8' for kernel stack offset
 randomization
In-Reply-To: <alpine.DEB.2.21.2301302011150.55843@angie.orcam.me.uk>
References: <alpine.DEB.2.21.2301302011150.55843@angie.orcam.me.uk>
Date:   Mon, 13 Feb 2023 20:03:34 +0100
Message-ID: <874jrptmk9.ffs@tglx>
MIME-Version: 1.0
Content-Type: text/plain
Precedence: bulk

On Mon, Jan 30 2023 at 21:30, Maciej W. Rozycki wrote:
>
> Therefore switch to our generic entropy source and use `get_random_u8' 
> instead, which according to Jason A. Donenfeld is supposed to be fast 
> enough:
>
> "Generally it's very very fast, as most cases wind up being only a
> memcpy -- in this case, a single byte copy. So by and large it should
> be suitable. It's fast enough now that most networking things are able
> to use it. And lots of other places where you'd want really high
> performance. So I'd expect it's okay to use here too. And if it is too
> slow, we should figure out how to make it faster. But I don't suspect
> it'll be too slow."

Please provide numbers on contemporary hardware.

Up to that point, it's easy enough to just disable that randomization on
32bit.

Thanks,

        tglx