[PATCH] arm64/pageattr: Propagate return value from __change_memory_common
Yang Shi
yang at os.amperecomputing.com
Mon Nov 10 21:08:38 PST 2025
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 at vger.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 at arm.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@os.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@os.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@os.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 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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