linux-kernel - Re: [PATCH v14 2/7] mm: add VM_DROPPABLE for designating always lazily freeable mappings

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Message-ID: <10302240-51ec-0854-2c86-16752d67a9be@opteya.com>
Date:   Thu, 5 Jan 2023 22:57:48 +0100
From:   Yann Droneaud <ydroneaud@...eya.com>
To:     "Jason A. Donenfeld" <Jason@...c4.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Andy Lutomirski <luto@...nel.org>, Ingo Molnar <mingo@...nel.org>,
        linux-kernel@...r.kernel.org, patches@...ts.linux.dev,
        tglx@...utronix.de, linux-crypto@...r.kernel.org,
        linux-api@...r.kernel.org, x86@...nel.org,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Adhemerval Zanella Netto <adhemerval.zanella@...aro.org>,
        Carlos O'Donell <carlos@...hat.com>,
        Florian Weimer <fweimer@...hat.com>,
        Arnd Bergmann <arnd@...db.de>, Jann Horn <jannh@...gle.com>,
        Christian Brauner <brauner@...nel.org>, linux-mm@...ck.org
Subject: Re: [PATCH v14 2/7] mm: add VM_DROPPABLE for designating always
 lazily freeable mappings

Hi,

Le 03/01/2023 à 21:44, Jason A. Donenfeld a écrit :
> On Tue, Jan 03, 2023 at 12:15:57PM -0800, Linus Torvalds wrote:
>> On Tue, Jan 3, 2023 at 12:03 PM Jason A. Donenfeld <Jason@...c4.com> wrote:
>>> That buffering cannot be done safely currently
>> .. again, this is "your semantics" (the (b) in my humbug list), not
>> necessarily reality for anybody else.
> Yea that's fair. Except, of course, I maintain that my semantics are
> important ones. :)


I concur.

To hold secret material, we need MADV_WIPEONFORK | MADV_DONTDUMP and the side effect of mlock() (pages' content never written to swap), inherited across fork().
And I want mlock() without paying the price.

Jason's proposed semantics, which I call MADV_WIPEONSWAP, provide a mean to hold /unlimited/ amount secrets in userspace memory (not limited by RLIMIT_MEMLOCK).
The only constraint for userspace is to handle the case pages are wiped, which is already the case of userspace arc4random()'s implementation.

Regards.

-- 
Yann Droneaud
OPTEYA