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Date:   Thu, 5 Jan 2023 22:57:48 +0100
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Subject: Re: [PATCH v14 2/7] mm: add VM_DROPPABLE for designating always
 lazily freeable mappings
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To:     "Jason A. Donenfeld" <Jason@zx2c4.com>,
        Linus Torvalds <torvalds@linux-foundation.org>
Cc:     Andy Lutomirski <luto@kernel.org>, Ingo Molnar <mingo@kernel.org>,
        linux-kernel@vger.kernel.org, patches@lists.linux.dev,
        tglx@linutronix.de, linux-crypto@vger.kernel.org,
        linux-api@vger.kernel.org, x86@kernel.org,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Adhemerval Zanella Netto <adhemerval.zanella@linaro.org>,
        Carlos O'Donell <carlos@redhat.com>,
        Florian Weimer <fweimer@redhat.com>,
        Arnd Bergmann <arnd@arndb.de>, Jann Horn <jannh@google.com>,
        Christian Brauner <brauner@kernel.org>, linux-mm@kvack.org
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From:   Yann Droneaud <ydroneaud@opteya.com>
Organization: OPTEYA
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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@zx2c4.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