linux-kernel - Re: [PATCH v2 2/5] mm/mseal: update madvise() logic

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Message-ID: <4e098a16-1213-44d1-9585-88f9041fff57@redhat.com>
Date: Wed, 16 Jul 2025 15:37:19 +0200
From: David Hildenbrand <david@...hat.com>
To: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>,
 Andrew Morton <akpm@...ux-foundation.org>
Cc: "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, Jeff Xu <jeffxu@...omium.org>
Subject: Re: [PATCH v2 2/5] mm/mseal: update madvise() logic

On 15.07.25 15:37, Lorenzo Stoakes wrote:
> The madvise() logic is inexplicably performed in mm/mseal.c - this ought to
> be located in mm/madvise.c.
> 
> Additionally can_modify_vma_madv() is inconsistently named and, in
> combination with is_ro_anon(), is very confusing logic.
> 
> Put a static function in mm/madvise.c instead - can_madvise_modify() - that
> spells out exactly what's happening. Also explicitly check for an anon VMA.
> 
> Also add commentary to explain what's going on.
> 
> Essentially - we disallow discarding of data in mseal()'d mappings in
> instances where the user couldn't otherwise write to that data.
> 
> Shared mappings are always backed, so no discard will actually truly
> discard the data. Read-only anonymous and MAP_PRIVATE file-backed mappings
> are the ones we are interested in.
> 
> We make a change to the logic here to correct a mistake - we must disallow
> discard of read-only MAP_PRIVATE file-backed mappings, which previously we
> were not.
> 
> The justification for this change is to account for the case where:
> 
> 1. A MAP_PRIVATE R/W file-backed mapping is established.
> 2. The mapping is written to, which backs it with anonymous memory.
> 3. The mapping is mprotect()'d read-only.
> 4. The mapping is mseal()'d.
> 
> If we were to now allow discard of this data, it would mean mseal() would
> not prevent the unrecoverable discarding of data and it was thus violate
> the semantics of sealed VMAs.
> 
> Finally, update the mseal tests, which were asserting previously that a
> read-only MAP_PRIVATE file-backed mapping could be discarded.
> 
> Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
> Reviewed-by: Liam R. Howlett <Liam.Howlett@...cle.com>
> Reviewed-by: Pedro Falcato <pfalcato@...e.de>
> ---

Acked-by: David Hildenbrand <david@...hat.com>

-- 
Cheers,

David / dhildenb