Re: [PATCH 1/2] mseal: Fix is_madv_discard()

[Pedro Falcato]

On Wed, Aug 7, 2024 at 7:58 PM Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> wrote:
>
> On Wed,  7 Aug 2024 18:33:35 +0100 Pedro Falcato <pedro.falcato@xxxxxxxxx> 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