linux-hardening - Re: [PATCH v3 2/5] mm/mseal: update madvise() logic

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Message-ID: <ab45d2f2-a34e-4ec7-8a49-03d166e6215a@redhat.com>
Date: Thu, 24 Jul 2025 20:56:19 +0200
From: David Hildenbrand <david@...hat.com>
To: Jeff Xu <jeffxu@...omium.org>,
 Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>,
 "Liam R . Howlett" <Liam.Howlett@...cle.com>,
 Vlastimil Babka <vbabka@...e.cz>, Jann Horn <jannh@...gle.com>,
 Pedro Falcato <pfalcato@...e.de>, linux-mm@...ck.org,
 linux-kernel@...r.kernel.org, Kees Cook <kees@...nel.org>,
 linux-hardening@...r.kernel.org
Subject: Re: [PATCH v3 2/5] mm/mseal: update madvise() logic

> 
> To summarize all the discussion points so far:
> 1. It's questionable behavior for madvise to allow destructive
> behavior for read-only anonymous mappings, regardless of mseal state.
 > 2. We could potentially fix point 1 within madvise itself, without> 
involving mseal, as Linus desires.

IIUC: disallow madvise(MADV_DONTNEED) without PROT_WRITE.

I am 99.99999% sure that that would break user case, unfortunately.

> 3. Android userspace uses destructive madvise to free up RAM, but I
> need to take a closer look at the patterns and usage to understand why
> they do that.

I am shocked that you question why they would use MADV_DONTNEED instead 
of ...

 > 4. We could ask applications to switch to non-destructive madvise,> 
like MADV_COLD or MADV_PAGEOUT. Or, another option is that we could
> switch the kernel to use non-destructive madvise implicitly for
> destructive madvise in suitable situations.

... MADV_COLD / MADV_PAGEOUT.

I am also shocked that you think asking apps to switch would not make us 
break user space.

> 5. We could investigate more based on vma->anon_vma

Or we do what sealing is supposed to do.

With the hope that this sealing fix here would not break user space.

-- 
Cheers,

David / dhildenb