linux-kernel - Re: [PATCH v2 3/3] treewide: use get_random_u32

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: <Y3WW2lOgoYLKQeve@zx2c4.com>
Date:   Thu, 17 Nov 2022 03:05:14 +0100
From:   "Jason A. Donenfeld" <Jason@...c4.com>
To:     Kees Cook <keescook@...omium.org>
Cc:     linux-kernel@...r.kernel.org, patches@...ts.linux.dev,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Jakub Kicinski <kuba@...nel.org>,
        Russell King <linux@...linux.org.uk>,
        Catalin Marinas <catalin.marinas@....com>,
        Thomas Bogendoerfer <tsbogend@...ha.franken.de>,
        Heiko Carstens <hca@...ux.ibm.com>,
        Herbert Xu <herbert@...dor.apana.org.au>,
        Christoph Böhmwalder 
        <christoph.boehmwalder@...bit.com>,
        Jani Nikula <jani.nikula@...ux.intel.com>,
        Jason Gunthorpe <jgg@...dia.com>,
        Sakari Ailus <sakari.ailus@...ux.intel.com>,
        "Martin K . Petersen" <martin.petersen@...cle.com>,
        Theodore Ts'o <tytso@....edu>,
        Andreas Dilger <adilger.kernel@...ger.ca>,
        Jaegeuk Kim <jaegeuk@...nel.org>,
        Richard Weinberger <richard@....at>,
        "Darrick J . Wong" <djwong@...nel.org>,
        SeongJae Park <sj@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        Helge Deller <deller@....de>, netdev@...r.kernel.org,
        linux-crypto@...r.kernel.org, linux-block@...r.kernel.org,
        linux-fsdevel@...r.kernel.org, linux-media@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, loongarch@...ts.linux.dev,
        linux-mips@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org,
        linux-mmc@...r.kernel.org, linux-parisc@...r.kernel.org,
        ydroneaud@...eya.com
Subject: Re: [PATCH v2 3/3] treewide: use get_random_u32_between() when
 possible

On Thu, Nov 17, 2022 at 12:55:47AM +0100, Jason A. Donenfeld wrote:
> 1) How/whether to make f(0, UR2_MAX) safe,
>    - without additional 64-bit arithmetic,
>    - minimizing the number of branches.
>    I have a few ideas I'll code golf for a bit.
> I think I can make progress with (1) alone by fiddling around with
> godbolt enough, like usual.

The code gen is definitely worse.

Original half-open interval:

    return floor + get_random_u32_below(ceil - floor);

Suggested fully closed interval:
	
    ceil = ceil - floor + 1;
    return likely(ceil) ? floor + get_random_u32_below(ceil) : get_random_u32();

Is the worse code gen actually worth it? Options:

 a) Decide worse codegen is worth it.
 b) Declare f(0, U32_MAX) undefined and just not handle it.
 c) Stick with original half-open interval that doesn't have this problem.

Jason