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Received: from localhost ([::1] helo=bombadil.infradead.org) by bombadil.infradead.org with esmtp (Exim 4.98.2 #2 (Red Hat Linux)) id 1vIg8H-00000006VsU-1mmi; Tue, 11 Nov 2025 04:37:29 +0000 Received: from foss.arm.com ([217.140.110.172]) by bombadil.infradead.org with esmtp (Exim 4.98.2 #2 (Red Hat Linux)) id 1vIg8E-00000006Vrc-1Wwr for linux-arm-kernel@lists.infradead.org; Tue, 11 Nov 2025 04:37:27 +0000 Received: from usa-sjc-imap-foss1.foss.arm.com (unknown [10.121.207.14]) by usa-sjc-mx-foss1.foss.arm.com (Postfix) with ESMTP id C5E122F; Mon, 10 Nov 2025 20:37:16 -0800 (PST) Received: from [10.164.136.36] (unknown [10.164.136.36]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 8BEF33F66E; Mon, 10 Nov 2025 20:37:21 -0800 (PST) Message-ID: <938fc839-b27a-484f-a49c-6dc05b3e9983@arm.com> Date: Tue, 11 Nov 2025 10:07:18 +0530 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH] arm64/pageattr: Propagate return value from __change_memory_common To: Yang Shi , Will Deacon Cc: catalin.marinas@arm.com, ryan.roberts@arm.com, rppt@kernel.org, shijie@os.amperecomputing.com, linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org, stable@vger.kernel.org References: <20251103061306.82034-1-dev.jain@arm.com> <586b8d19-a5d2-4248-869b-98f39b792acb@arm.com> <17eed751-e1c5-4ea5-af1d-e96da16d5e26@arm.com> Content-Language: en-US From: Dev Jain In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20251110_203726_485592_EAC12850 X-CRM114-Status: GOOD ( 34.85 ) X-BeenThere: linux-arm-kernel@lists.infradead.org X-Mailman-Version: 2.1.34 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+linux-arm-kernel=archiver.kernel.org@lists.infradead.org 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? > > Hopefully I don't miss anything. > > Thanks, > Yang > > >> >> >>> >>>> >>>> Will >>> >