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Internal X-MS-Exchange-CrossTenant-OriginalArrivalTime: 11 Nov 2025 23:59:37.7681 (UTC) X-MS-Exchange-CrossTenant-FromEntityHeader: Hosted X-MS-Exchange-CrossTenant-Id: 3bc2b170-fd94-476d-b0ce-4229bdc904a7 X-MS-Exchange-CrossTenant-MailboxType: HOSTED X-MS-Exchange-CrossTenant-UserPrincipalName: SAb7WzKNdDtla/vZFQcboL5YoRR9kHi5V4PR+STlGgOhw7N/BN+OtJTelj3Svdsa8DysABOKFvVwMltwxZadffm/Z9fJX4e7Yz6sbgwnr6M= X-MS-Exchange-Transport-CrossTenantHeadersStamped: MWHPR01MB8697 On 11/11/25 9:52 AM, 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@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 >>>>>>>>>>>>>> --- >>>>>>>>>>>>>> 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 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? IIUC, it is still all safe. I think Dev's patch is right. We should check the return value of __change_memory_common() because page table split may fail. I had the return value check in my old patches which called page table split function outside __change_memory_common(). W/o this patch, the callers of set_memory_*() may continue to run with some memory in wrong permissions, for example, RW memory but they are supposed to be RO, instead of jumping to error handling path. Thanks, Yang > > 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