linux-kernel - Re: [PATCH 1/2] mseal: Fix is_madv

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Message-ID: <CAKbZUD3kNTKfV5yMQW7YYbRx2XJiMNM4owGiYue1Y-A=yVbWog@mail.gmail.com>
Date: Wed, 7 Aug 2024 20:25:45 +0100
From: Pedro Falcato <pedro.falcato@...il.com>
To: Andrew Morton <akpm@...ux-foundation.org>
Cc: Shuah Khan <shuah@...nel.org>, "Liam R. Howlett" <Liam.Howlett@...cle.com>, 
	Jeff Xu <jeffxu@...omium.org>, Kees Cook <kees@...nel.org>, linux-mm@...ck.org, 
	linux-kernel@...r.kernel.org, linux-kselftest@...r.kernel.org, 
	stable@...r.kernel.org
Subject: Re: [PATCH 1/2] mseal: Fix is_madv_discard()

On Wed, Aug 7, 2024 at 7:58 PM Andrew Morton <akpm@...ux-foundation.org> wrote:
>
> On Wed,  7 Aug 2024 18:33:35 +0100 Pedro Falcato <pedro.falcato@...il.com> wrote:
>
> > is_madv_discard did its check wrong. MADV_ flags are not bitwise,
> > they're normal sequential numbers. So, for instance:
> >       behavior & (/* ... */ | MADV_REMOVE)
> >
> > tagged both MADV_REMOVE and MADV_RANDOM (bit 0 set) as
> > discard operations. This is obviously incorrect, so use
> > a switch statement instead.
>
> Please describe the userspace-visible runtime effects of this bug?

The kernel could erroneously block certain madvises (e.g MADV_RANDOM
or MADV_HUGEPAGE) on sealed VMAs due to them sharing bits with blocked
MADV operations (e.g REMOVE or WIPEONFORK).

Thanks,
Pedro