Message-ID: <4ad29a03-2166-4963-ba4f-6b0350c255c5@arm.com>
Date: Wed, 12 Nov 2025 09:20:41 +0530
From: Dev Jain <dev.jain@....com>
To: Ryan Roberts <ryan.roberts@....com>,
 Yang Shi <yang@...amperecomputing.com>, Will Deacon <will@...nel.org>
Cc: catalin.marinas@....com, rppt@...nel.org, shijie@...amperecomputing.com,
 linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
 stable@...r.kernel.org
Subject: Re: [PATCH] arm64/pageattr: Propagate return value from
 __change_memory_common


On 11/11/25 11:22 pm, Ryan Roberts wrote:
> On 11/11/2025 05:12, Dev Jain wrote:
>> On 11/11/25 10:38 am, Yang Shi wrote:
>>>
>>> On 11/10/25 8:55 PM, Dev Jain wrote:
>>>> On 11/11/25 10:14 am, Yang Shi wrote:
>>>>>
>>>>> On 11/10/25 8:37 PM, Dev Jain wrote:
>>>>>> On 11/11/25 9:47 am, Yang Shi wrote:
>>>>>>>
>>>>>>> On 11/10/25 7:39 PM, Dev Jain wrote:
>>>>>>>> On 05/11/25 9:27 am, Dev Jain wrote:
>>>>>>>>> On 04/11/25 6:26 pm, Will Deacon wrote:
>>>>>>>>>> On Tue, Nov 04, 2025 at 09:06:12AM +0530, Dev Jain wrote:
>>>>>>>>>>> On 04/11/25 12:15 am, Yang Shi wrote:
>>>>>>>>>>>> On 11/3/25 7:16 AM, Will Deacon wrote:
>>>>>>>>>>>>> On Mon, Nov 03, 2025 at 11:43:06AM +0530, Dev Jain wrote:
>>>>>>>>>>>>>> Post a166563e7ec3 ("arm64: mm: support large block mapping when
>>>>>>>>>>>>>> rodata=full"),
>>>>>>>>>>>>>> __change_memory_common has a real chance of failing due to split
>>>>>>>>>>>>>> failure.
>>>>>>>>>>>>>> Before that commit, this line was introduced in c55191e96caa,
>>>>>>>>>>>>>> still having
>>>>>>>>>>>>>> a chance of failing if it needs to allocate pagetable memory in
>>>>>>>>>>>>>> apply_to_page_range, although that has never been observed to be true.
>>>>>>>>>>>>>> In general, we should always propagate the return value to the caller.
>>>>>>>>>>>>>>
>>>>>>>>>>>>>> Cc: stable@...r.kernel.org
>>>>>>>>>>>>>> Fixes: c55191e96caa ("arm64: mm: apply r/o permissions of VM
>>>>>>>>>>>>>> areas to its linear alias as well")
>>>>>>>>>>>>>> Signed-off-by: Dev Jain <dev.jain@....com>
>>>>>>>>>>>>>> ---
>>>>>>>>>>>>>> Based on Linux 6.18-rc4.
>>>>>>>>>>>>>>
>>>>>>>>>>>>>>     arch/arm64/mm/pageattr.c | 5 ++++-
>>>>>>>>>>>>>>     1 file changed, 4 insertions(+), 1 deletion(-)
>>>>>>>>>>>>>>
>>>>>>>>>>>>>> diff --git a/arch/arm64/mm/pageattr.c b/arch/arm64/mm/pageattr.c
>>>>>>>>>>>>>> index 5135f2d66958..b4ea86cd3a71 100644
>>>>>>>>>>>>>> --- a/arch/arm64/mm/pageattr.c
>>>>>>>>>>>>>> +++ b/arch/arm64/mm/pageattr.c
>>>>>>>>>>>>>> @@ -148,6 +148,7 @@ static int change_memory_common(unsigned
>>>>>>>>>>>>>> long addr, int numpages,
>>>>>>>>>>>>>>         unsigned long size = PAGE_SIZE * numpages;
>>>>>>>>>>>>>>         unsigned long end = start + size;
>>>>>>>>>>>>>>         struct vm_struct *area;
>>>>>>>>>>>>>> +    int ret;
>>>>>>>>>>>>>>         int i;
>>>>>>>>>>>>>>           if (!PAGE_ALIGNED(addr)) {
>>>>>>>>>>>>>> @@ -185,8 +186,10 @@ static int change_memory_common(unsigned
>>>>>>>>>>>>>> long addr, int numpages,
>>>>>>>>>>>>>>         if (rodata_full && (pgprot_val(set_mask) == PTE_RDONLY ||
>>>>>>>>>>>>>>                     pgprot_val(clear_mask) == PTE_RDONLY)) {
>>>>>>>>>>>>>>             for (i = 0; i < area->nr_pages; i++) {
>>>>>>>>>>>>>> - __change_memory_common((u64)page_address(area->pages[i]),
>>>>>>>>>>>>>> +            ret =
>>>>>>>>>>>>>> __change_memory_common((u64)page_address(area->pages[i]),
>>>>>>>>>>>>>>                                PAGE_SIZE, set_mask, clear_mask);
>>>>>>>>>>>>>> +            if (ret)
>>>>>>>>>>>>>> +                return ret;
>>>>>>>>>>>>> Hmm, this means we can return failure half-way through the
>>>>>>>>>>>>> operation. Is
>>>>>>>>>>>>> that something callers are expecting to handle? If so, how can they
>>>>>>>>>>>>> tell
>>>>>>>>>>>>> how far we got?
>>>>>>>>>>>> IIUC the callers don't have to know whether it is half-way or not
>>>>>>>>>>>> because the callers will change the permission back (e.g. to RW) for the
>>>>>>>>>>>> whole range when freeing memory.
>>>>>>>>>>> Yes, it is the caller's responsibility to set VM_FLUSH_RESET_PERMS flag.
>>>>>>>>>>> Upon vfree(), it will change the direct map permissions back to RW.
>>>>>>>>>> Ok, but vfree() ends up using update_range_prot() to do that and if we
>>>>>>>>>> need to worry about that failing (as per your commit message), then
>>>>>>>>>> we're in trouble because the calls to set_area_direct_map() are unchecked.
>>>>>>>>>>
>>>>>>>>>> In other words, this patch is either not necessary or it is incomplete.
>>>>>>>>> Here is the relevant email, in the discussion between Ryan and Yang:
>>>>>>>>>
>>>>>>>>> https://lore.kernel.org/all/fe52a1d8-5211-4962-afc8-
>>>>>>>>> c3f9caf64119@...amperecomputing.com/
>>>>>>>>>
>>>>>>>>> We had concluded that all callers of set_memory_ro() or set_memory_rox()
>>>>>>>>> (which require the
>>>>>>>>> linear map perm change back to default, upon vfree() ) will call it for
>>>>>>>>> the entire region (vm_struct).
>>>>>>>>> So, when we do the set_direct_map_invalid_noflush, it is guaranteed that
>>>>>>>>> the region has already
>>>>>>>>> been split. So this call cannot fail.
>>>>>>>>>
>>>>>>>>> https://lore.kernel.org/all/f8898c87-8f49-4ef2-86ae-
>>>>>>>>> b60bcf67658c@...amperecomputing.com/
>>>>>>>>>
>>>>>>>>> This email notes that there is some code doing set_memory_rw() and
>>>>>>>>> unnecessarily setting the VM_FLUSH_RESET_PERMS
>>>>>>>>> flag, but in that case we don't care about the
>>>>>>>>> set_direct_map_invalid_noflush call failing because the protections
>>>>>>>>> are already RW.
>>>>>>>>>
>>>>>>>>> Although we had also observed that all of this is fragile and depends on
>>>>>>>>> the caller doing the
>>>>>>>>> correct thing. The real solution should be somehow getting rid of the
>>>>>>>>> BBM style invalidation.
>>>>>>>>> Ryan had proposed some methods in that email thread.
>>>>>>>>>
>>>>>>>>> One solution which I had thought of, is that, observe that we are doing
>>>>>>>>> an overkill by
>>>>>>>>> setting the linear map to invalid and then default, for the *entire*
>>>>>>>>> region. What we
>>>>>>>>> can do is iterate over the linear map alias of the vm_struct *area and
>>>>>>>>> only change permission
>>>>>>>>> back to RW for the pages which are *not* RW. And, those relevant
>>>>>>>>> mappings are guaranteed to
>>>>>>>>> be split because they were changed from RW to not RW.
>>>>>>>> @Yang and Ryan,
>>>>>>>>
>>>>>>>> I saw Yang's patch here:
>>>>>>>> https://lore.kernel.org/all/20251023204428.477531-1-
>>>>>>>> yang@...amperecomputing.com/
>>>>>>>> and realized that currently we are splitting away the linear map alias of
>>>>>>>> the *entire* region.
>>>>>>>>
>>>>>>>> Shouldn't this then imply that set_direct_map_invalid_noflush will never
>>>>>>>> fail, since even
>>>>>>>>
>>>>>>>> a set_memory_rox() call on a single page will split the linear map for
>>>>>>>> the entire region,
>>>>>>>>
>>>>>>>> and thus there is no fragility here which we were discussing about? I may
>>>>>>>> be forgetting
>>>>>>>>
>>>>>>>> something, this linear map stuff is confusing enough already.
>>>>>>> It still may fail due to page table allocation failure when doing split.
>>>>>>> But it is still fine. We may run into 3 cases:
>>>>>>>
>>>>>>> 1. set_memory_rox succeed to split the whole range, then
>>>>>>> set_direct_map_invalid_noflush() will succeed too
>>>>>>> 2. set_memory_rox fails to split, for example, just change partial range
>>>>>>> permission due to page table allocation failure, then
>>>>>>> set_direct_map_invalid_noflush() may
>>>>>>>     a. successfully change the permission back to default till where
>>>>>>> set_memory_rox fails at since that range has been successfully split. It
>>>>>>> is ok since the remaining range is actually not changed to ro by
>>>>>>> set_memory_rox at all
>>>>>>>     b. successfully change the permission back to default for the whole
>>>>>>> range (for example, memory pressure is mitigated when
>>>>>>> set_direct_map_invalid_noflush() is called). It is definitely fine as well
>>>>>> Correct, what I mean to imply here is that, your patch will break this? If
>>>>>> set_memory_* is applied on x till y, your patch changes the linear map alias
>>>>>>
>>>>>> only from x till y - set_direct_map_invalid_noflush instead operates on 0
>>>>>> till size - 1, where 0 <=x <=y <= size - 1. So, it may encounter a -ENOMEM
>>>>>>
>>>>>> on [0, x) range while invalidating, and that is *not* okay because we must
>>>>>> reset back [0, x) to default?
>>>>> I see your point now. But I think the callers need to guarantee they call
>>>>> set_memory_rox and set_direct_map_invalid_noflush on the same range, right?
>>>>> Currently kernel just calls them on the whole area.
>>>> Nope. The fact that the kernel changes protections, and undoes the changed
>>>> protections, on the *entire* alias of the vm_struct region, protects us from
>>>> the fragility we were talking about earlier.
>>> This is what I meant "kernel just calls them on the whole area".
>>>
>>>> Suppose you have a range from 0 till size - 1, and you call set_memory_* on a
>>>> random point (page) p. The argument we discussed above is independent of p,
>>>> which lets us drop our
>>>>
>>>> previous erroneous conclusion that all of this works because no caller does a
>>>> partial set_memory_*.
>>> Sorry I don't follow you. What "erroneous conclusion" do you mean? You can
>>> call set_memory_* on a random point, but set_direct_map_invalid_noflush()
>>> should be called on the random point too. The current code of
>>> set_area_direct_map() doesn't consider this case because there is no such
>>> call. Is this what you meant?
>>
>> I was referring to the discussion in the linear map work - I think we had
>> concluded that we don't need to worry about the BBM style invalidation failing,
>> *because* no one does a partial set_memory_*.
>>
>> What I am saying - we don't care whether caller does a partial or a full
>> set_memory_*, we are still safe, because the linear map alias change on both
>> sides (set_memory_* -> __change_memory_common, and vm_reset_perms ->
>> set_area_direct_map() )
>>
>> operate on the entire region.
> I'm thoughoughly confused again. I thought we had concluded this was all safe
> when discussing in the context of the "block mapping the linear map" series. But
> now I'm a bit unclear on whether we have a bug. I think I'm hearing that we
> don't need this patch and Dev will submit an alternative which just adds some
> comments to explain why this is safe?

We need this patch because currently we will suppress a linear map alias change -ENOMEM,
and the caller will see that set_memory_* succeeded. So basically now we have done
set_memory_* without having the corresponding security measure succeed.

The comment refers to the linear map series - we should document why we don't care about
-ENOMEM failure of set_direct_map_invalid_noflush().

>
> Thanks,
> Ryan
>
>
>>
>>>>
>>>> I would like to send a patch clearly documenting this behaviour, assuming no
>>>> one else finds a hole in this reasoning.
>>> Proper comment to explain the subtle behavior is definitely welcome.
>>>
>>> Thanks,
>>> Yang
>>>
>>>>
>>>>> Thanks,
>>>>> Yang
>>>>>
>>>>>>
>>>>>>> Hopefully I don't miss anything.
>>>>>>>
>>>>>>> Thanks,
>>>>>>> Yang
>>>>>>>
>>>>>>>
>>>>>>>>
>>>>>>>>>> Will

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