linux-kernel - Re: [PATCH 1/2] arm64/pageattr: Propagate return value from __change_memory

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Message-ID: <01c00163-ec81-41ba-ba4d-c3425e2109f7@arm.com>
Date: Thu, 20 Nov 2025 09:40:38 +0530
From: Dev Jain <dev.jain@....com>
To: Catalin Marinas <catalin.marinas@....com>
Cc: will@...nel.org, ryan.roberts@....com, rppt@...nel.org,
 shijie@...amperecomputing.com, yang@...amperecomputing.com,
 linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] arm64/pageattr: Propagate return value from
 __change_memory_common

On 19/11/25 11:33 pm, Catalin Marinas wrote:
> On Wed, Nov 12, 2025 at 11:57:15AM +0530, Dev Jain wrote:
>> The rodata=on security measure requires that any code path which does
>> vmalloc -> set_memory_ro/set_memory_rox must protect the linear map alias
>> too. Therefore, if such a call fails, we must abort set_memory_* and caller
>> must take appropriate action; currently we are suppressing the error, and
>> there is a real chance of such an error arising post commit a166563e7ec3
>> ("arm64: mm: support large block mapping when rodata=full"). Therefore,
>> propagate any error to the caller.
>>
>> Fixes: a166563e7ec3 ("arm64: mm: support large block mapping when rodata=full")
>> Signed-off-by: Dev Jain <dev.jain@....com>
>> ---
>> v1 of this patch: https://lore.kernel.org/all/20251103061306.82034-1-dev.jain@arm.com/
>> I have dropped stable since no real chance of failure was there.
> I couldn't figure out from the comments on v1 whether Will's concern was
> addressed:
>
> https://lore.kernel.org/all/aQn4EwKar66UZ7rz@willie-the-truck/
>
> IOW, is the patch necessary? What are the failure scenarios and what

The patch is necessary because rodata=on requires us to proceed with
set_memory_* *only* if the linear map alias permission change was successful.
If we still go ahead with the set_memory_*, you now have memory in
RO state, whereas the linear map alias is in RW state, defeating the purpose
of rodata = on.

> does the caller do? It's good to propagate an error to the caller but
> patch also changes the current behaviour by bailing out earlier.

The caller will get to know that set_memory_* failed. Since we may have
bailed out partially in between the linear map alias permission change,
we must also ensure that we change that memory back to RW - vm_reset_perms()
will ensure that, and that change back to RW is *guaranteed* to succeed
in the calls to set_area_direct_map() in vm_reset_perms(): as explained
in the next patch, since that memory is non-RW, it is guaranteed to be
pte-mapped.

Now, it is the responsibility of the caller to pass VM_FLUSH_RESET_PERMS
*before* it does set_memory_ro/set_memory_rox. If it doesn't, then the
code is broken as is (Ryan noted earlier how this is a fragility in the
vmalloc API and we should do something about it).

>