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[2003:d8:2f03:5b00:f549:a879:b2d3:73ee]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-450d7f8f194sm13143635e9.4.2025.05.30.02.27.42 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Fri, 30 May 2025 02:27:43 -0700 (PDT) Message-ID: <9c32dbf6-527f-465b-9010-f5b22cbac075@redhat.com> Date: Fri, 30 May 2025 11:27:42 +0200 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [RFC PATCH] mm/mmap: Fix uprobe anon page be overwritten when expanding vma during mremap To: Lorenzo Stoakes Cc: Pu Lehui , Oleg Nesterov , mhiramat@kernel.org, peterz@infradead.org, Liam.Howlett@oracle.com, akpm@linux-foundation.org, vbabka@suse.cz, jannh@google.com, pfalcato@suse.de, linux-mm@kvack.org, linux-kernel@vger.kernel.org, pulehui@huawei.com References: <13c5fe73-9e11-4465-b401-fc96a22dc5d1@redhat.com> <4cbc1e43-ea46-44de-9e2b-1c62dcd2b6d5@huaweicloud.com> <20250526154850.GA4156@redhat.com> <06bd94c0-fefe-4bdc-8483-2d9b6703c3d6@redhat.com> <57533126-eb30-4b56-bc4d-2f27514ae5ad@huaweicloud.com> <956124be-c73c-4023-9edd-25372f3f865a@huaweicloud.com> <172df994-7f24-4fbb-876c-4fab22937177@redhat.com> <205f8165-449c-441f-8ee9-58f69d23dbeb@lucifer.local> From: David Hildenbrand Content-Language: en-US Autocrypt: addr=david@redhat.com; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit On 30.05.25 11:03, Lorenzo Stoakes wrote: > On Fri, May 30, 2025 at 10:50:25AM +0200, David Hildenbrand wrote: >> On 30.05.25 10:41, Lorenzo Stoakes wrote: >>> On Fri, May 30, 2025 at 10:33:16AM +0200, David Hildenbrand wrote: >>>> On 29.05.25 18:07, Pu Lehui wrote: >>>>> >>>>> On 2025/5/28 17:03, David Hildenbrand wrote: >>>>>> On 27.05.25 15:38, Pu Lehui wrote: >>>>>>> Hi David, >>>>>>> >>>>>>> On 2025/5/27 2:46, David Hildenbrand wrote: >>>>>>>> On 26.05.25 17:48, Oleg Nesterov wrote: >>>>>>>>> Hi Lehui, >>>>>>>>> >>>>>>>>> As I said, I don't understand mm/, so can't comment, but... >>>>>>>>> >>>>>>>>> On 05/26, Pu Lehui wrote: >>>>>>>>>> >>>>>>>>>> To make things simpler, perhaps we could try post-processing, that is: >>>>>>>>>> >>>>>>>>>> diff --git a/mm/mremap.c b/mm/mremap.c >>>>>>>>>> index 83e359754961..46a757fd26dc 100644 >>>>>>>>>> --- a/mm/mremap.c >>>>>>>>>> +++ b/mm/mremap.c >>>>>>>>>> @@ -240,6 +240,11 @@ static int move_ptes(struct >>>>>>>>>> pagetable_move_control >>>>>>>>>> *pmc, >>>>>>>>>>                   if (pte_none(ptep_get(old_pte))) >>>>>>>>>>                           continue; >>>>>>>>>> >>>>>>>>>> +               /* skip move pte when expanded range has uprobe */ >>>>>>>>>> +               if (unlikely(pte_present(*new_pte) && >>>>>>>>>> +                            vma_has_uprobes(pmc->new, new_addr, >>>>>>>>>> new_addr + >>>>>>>>>> PAGE_SIZE))) >>>>>>>>>> +                       continue; >>>>>>>>>> + >>>>>>>>> >>>>>>>>> I was thinking about >>>>>>>>> >>>>>>>>>      WARN_ON(!pte_none(*new_pte)) >>>>>>>>> >>>>>>>>> at the start of the main loop. >>>>>>>>> >>>>>>>>> Obviously not to fix the problem, but rather to make it more explicit. >>>>>>>> >>>>>>>> Yeah, WARN_ON_ONCE(). >>>>>>>> >>>>>>>> We really should fix the code to not install uprobes into the area we >>>>>>>> are moving. >>>>>>> Alright, so let's try this direction. >>>>>>> >>>>>>>> >>>>>>>> Likely, the correct fix will be to pass the range as well to >>>>>>>> uprobe_mmap(), and passing that range to build_probe_list(). >>>>>>> >>>>>>> It will be great. But IIUC, the range we expand to is already included >>>>>>> when entering uprobe_mmap and also build_probe_list. >>>>>> >>>>>> Right, you'd have to communicate that information through all layers >>>>>> (expanded range). >>>>>> >>>>>> As an alternative, maybe we can really call handle_vma_uprobe() after >>>>>> moving the pages. >>>>> >>>>> Hi David, >>>>> >>>>> Not sure if this is possible, but I think it would be appropriate to not >>>>> handle this uprobe_mmap at the source, and maybe we should make it clear >>>>> that new_pte must be NULL when move_ptes, otherwise it should be an >>>>> exception? >>>> >>>> Yeah, we should ay least document that if we find any non-none pte in the >>>> range we are moving to, we have a big problem. > > By the way I agree with this. > >>>> >>>> I think the main issue is that vma_complete() calls uprobe_mmap() before >>>> moving the page tables over. >>> >>> Well vma_complete() is not _normally_ invoked before moving page tables, >>> it's mremap that's making things strange :) >>> >>> That's why I think my suggested approach of specifically indicating that we >>> want different behaviour for mremap is a reasonable one here, as it special >>> cases things for this case. >>> >>> However... >>> >>>> >>>> If we could defer the uprobe_mmap() call, we might be good. >>>> >>>> The entry point is copy_vma_and_data(), where we call copy_vma() before >>>> move_page_tables(). >>>> >>>> copy_vma() should trigger the uprobe_mmap() through vma_merge_new_range(). >>>> >>>> I wonder if there might be a clean way to move the uprobe_mmap() out of >>>> vma_complete(). (or at least specify to skip it because it will be done >>>> manually). >>> >>> ...I would also love to see some means of not having to invoke >>> uprobe_mmap() in the VMA code, but I mean _at all_. >>> >>> But that leads into my desire to not do: >>> >>> if (blah blah) >>> some_specific_hardcoded_case(); >>> >>> I wish we had a better means of hooking stuff like this. >>> >>> However I don't think currently we can reasonably do so, as in all other >>> merge cases we _do_ want to invoke it. >> >> "all other" -- not so sure. >> >> Why would we invoke uprobe when merging VMAs after mprotect, mremap, >> madvise, ordinary mremap where we are not mapping anything new but just ... >> merging VMAs? >> >> Really, we need to invoke uprobe only when adding new VMAs or extending >> existing VMAs -- mapping new file ranges some way. >> >> Or am I missing something important? > > Well, this is where my limited knowledge of uprobe comes in. Let me try to summarize it: Essentially, what it does is go over the VMA to find where to install breakpoints. A breakpoint is essentially faulting in the file page, to then trigger a COW fault to get an anonymous page instead that we can modify to ... install the breakpoint. So wherever we have a breakpoint, we want to have an anonymous page mapped later. That is only required when we map a new file range. When ordinarily merging/splitting/moving, we already called uprobe before and installed the breakpoints. Calling uprobe_mmap() when we already installed breakpoints is not really problematic, only suboptimal. Calling uprobe_mmap() before moving page tables is bad. -- Cheers, David / dhildenb