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

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Message-ID: <d242b6c4-8be4-4294-a399-ead2339f08b1@redhat.com>
Date: Fri, 25 Jul 2025 09:41:42 +0200
From: David Hildenbrand <david@...hat.com>
To: Kees Cook <kees@...nel.org>
Cc: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>,
 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, Jeff Xu <jeffxu@...omium.org>
Subject: Re: [PATCH v3 2/5] mm/mseal: update madvise() logic

On 25.07.25 00:47, David Hildenbrand wrote:
> On 25.07.25 00:29, Kees Cook wrote:
>> On Thu, Jul 24, 2025 at 11:41:04PM +0200, David Hildenbrand wrote:
>>> On 24.07.25 23:32, David Hildenbrand wrote:
>>>>> As an aside, why should discard work in this case even without step 4?
>>>>> Wouldn't setting "read-only" imply you don't want the memory to change
>>>>> out from under you? I guess I'm not clear on the semantics: how do memory
>>>>> protection bits map to madvise actions like this?
>>>>
>>>> They generally don't affect MADV_DONTNEED behavior. The only documented
>>>> (man page) reason for EPERM in the man page is related to MADV_HWPOISON.
>>>>
>>>
>>> (Exception: MADV_POPULATE_READ/MADV_POPULATE_WRITE requires corresponding
>>> permissions)
>>
>> Shouldn't an MADV action that changes memory contents require the W bit
>> though?
> 

Pondering about this some more, at least MADV_DONTNEED is mostly a 
cheaper way of doing mmap(MAP_FIXED): in other word, zap everything but 
leave the original mapping unchanged.

So if you allow for mmap(MAP_FIXED) -- ignore any permissions bits, of 
course -- nothing really wrong about allowing MADV_DONTNEED.

With mseal(), it got all weird I am afraid, because we have this 
exception list, and apparently, it has holes. :(

-- 
Cheers,

David / dhildenb