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([2a09:80c0:192:0:5dac:bf3d:c41:c3e7]) by smtp.gmail.com with ESMTPSA id a640c23a62f3a-ab7e1ce7ab7sm106831966b.171.2025.02.11.04.09.37 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Tue, 11 Feb 2025 04:09:39 -0800 (PST) Message-ID: Date: Tue, 11 Feb 2025 13:09:37 +0100 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: [REGRESSION] NULL pointer dereference on ARM (AT91SAM9G25) during compaction To: Qi Zheng Cc: "Russell King (Oracle)" , Ezra Buehler , linux-mm@kvack.org, Andrew Morton , "Mike Rapoport (Microsoft)" , Muchun Song , Vlastimil Babka , Ryan Roberts , "Vishal Moola (Oracle)" , Hugh Dickins , Matthew Wilcox , Peter Xu , Nicolas Ferre , Alexandre Belloni , Claudiu Beznea , open list , linux-arm-kernel@lists.infradead.org References: <5d50d714-197f-44c0-94e0-ff70ee51e866@bytedance.com> <34bcf011-b4ac-479c-92ce-852623e73039@redhat.com> <3f7babee-b232-4e6b-a896-947150dcd1ef@bytedance.com> <2b0bb476-5bd6-489a-9b9e-7aa20964abfa@redhat.com> <4e298f68-36ff-496a-81d2-7124f792180d@bytedance.com> 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 11.02.25 10:43, Qi Zheng wrote: > > > On 2025/2/11 17:37, David Hildenbrand wrote: >> On 11.02.25 10:29, Qi Zheng wrote: >>> >>> >>> On 2025/2/11 17:14, David Hildenbrand wrote: >>>> On 11.02.25 04:45, Qi Zheng wrote: >>>>> Hi Russell, >>>>> >>>>> On 2025/2/11 01:03, Russell King (Oracle) wrote: >>>>>> On Mon, Feb 10, 2025 at 05:49:38PM +0100, Ezra Buehler wrote: >>>>>>> When running vanilla Linux 6.13 or newer (6.14-rc2) on the >>>>>>> AT91SAM9G25-based GARDENA smart Gateway, we are seeing a NULL pointer >>>>>>> dereference resulting in a kernel panic. The culprit seems to be >>>>>>> commit >>>>>>> fc9c45b71f43 ("arm: adjust_pte() usepte_offset_map_rw_nolock()"). >>>>>>> Reverting the commit apparently fixes the issue. >>>>>> >>>>>> The blamed commit is buggy: >>>>>> >>>>>> arch/arm/include/asm/tlbflush.h: >>>>>> #define update_mmu_cache(vma, addr, ptep) \ >>>>>>            update_mmu_cache_range(NULL, vma, addr, ptep, 1) >>>>>> >>>>>> So vmf can be NULL. This didn't used to matter before this commit, >>>>>> because vmf was not used by ARM's update_mmu_cache_range(). However, >>>>>> the commit introduced a dereference of it, which now causes a NULL >>>>>> point dereference. >>>>>> >>>>>> Not sure what the correct solution is, but at a guess, both: >>>>>> >>>>>>      if (ptl != vmf->ptl) >>>>>> >>>>>> need to become: >>>>>> >>>>>>      if (!vmf || ptl != vmf->ptl) >>>>> >>>>> No, we can't do that, because without using split PTE locks, we would >>>>> use shared mm->page_table_lock, which would create a deadlock. >>>> >>>> Maybe we can simply special-case on CONFIG_SPLIT_PTE_PTLOCKS ? >>>> >>>> if (IS_ENABLED(CONFIG_SPLIT_PTE_PTLOCKS)) { >>> >>> In this case, if two vmas map the same PTE page, then the same PTE lock >>> will be held repeatedly. Right? >> >> Hmm, the comment says: >> >>         /* >>          * This is called while another page table is mapped, so we >>          * must use the nested version.  This also means we need to >>          * open-code the spin-locking. >>          */ >> >> "another page table" implies that it cannot be the same. But maybe that >> comment was also wrong? > > I don't see make_coherent() ensuring this when traversing vma. Right, we could just have the same file range mapped MAP_SHARED into the same page table using two VMAs ... I suspect writing a reproducer for the deadlock should be easy. I therefore propose the following changes: > > diff --git a/arch/arm/mm/fault-armv.c b/arch/arm/mm/fault-armv.c > index 2bec87c3327d2..dddbca9a2597e 100644 > --- a/arch/arm/mm/fault-armv.c > +++ b/arch/arm/mm/fault-armv.c > @@ -61,8 +61,41 @@ static int do_adjust_pte(struct vm_area_struct *vma, > unsigned long address, > return ret; > } > > +#if defined(CONFIG_SPLIT_PTE_PTLOCKS) > +/* > + * If we are using split PTE locks, then we need to take the pte > + * lock here. Otherwise we are using shared mm->page_table_lock > + * which is already locked, thus cannot take it. > + */ > +static inline bool do_pte_lock(spinlock_t *ptl, pmd_t pmdval, pmd_t *pmd) > +{ > + /* > + * Use nested version here to indicate that we are already > + * holding one similar spinlock. > + */ > + spin_lock_nested(ptl, SINGLE_DEPTH_NESTING); > + if (unlikely(!pmd_same(pmdval, pmdp_get_lockless(pmd)))) { > + spin_unlock(ptl); > + return false; > + } > + > + return true; > +} > + > +static inline void do_pte_unlock(spinlock_t *ptl) > +{ > + spin_unlock(ptl); > +} > +#else /* !defined(CONFIG_SPLIT_PTE_PTLOCKS) */ > +static inline bool do_pte_lock(spinlock_t *ptl) > +{ > + return true; > +} > +static inline void do_pte_unlock(spinlock_t *ptl) {} > +#endif /* defined(CONFIG_SPLIT_PTE_PTLOCKS) */ > + > static int adjust_pte(struct vm_area_struct *vma, unsigned long address, > - unsigned long pfn, struct vm_fault *vmf) > + unsigned long pfn) > { > spinlock_t *ptl; > pgd_t *pgd; > @@ -99,23 +132,14 @@ static int adjust_pte(struct vm_area_struct *vma, > unsigned long address, > if (!pte) > return 0; > > - /* > - * If we are using split PTE locks, then we need to take the page > - * lock here. Otherwise we are using shared mm->page_table_lock > - * which is already locked, thus cannot take it. > - */ > - if (ptl != vmf->ptl) { > - spin_lock_nested(ptl, SINGLE_DEPTH_NESTING); > - if (unlikely(!pmd_same(pmdval, pmdp_get_lockless(pmd)))) { > - pte_unmap_unlock(pte, ptl); > - goto again; > - } > + if (!do_pte_lock(ptl, pmdval, pmd)) { > + pte_unmap(pte); > + goto again; > } > > ret = do_adjust_pte(vma, address, pfn, pte); > > - if (ptl != vmf->ptl) > - spin_unlock(ptl); > + do_pte_unlock(ptl); > pte_unmap(pte); > > return ret; > @@ -123,16 +147,17 @@ static int adjust_pte(struct vm_area_struct *vma, > unsigned long address, > > static void > make_coherent(struct address_space *mapping, struct vm_area_struct *vma, > - unsigned long addr, pte_t *ptep, unsigned long pfn, > - struct vm_fault *vmf) > + unsigned long addr, pte_t *ptep, unsigned long pfn) > { > struct mm_struct *mm = vma->vm_mm; > struct vm_area_struct *mpnt; > unsigned long offset; > + unsigned long start; > pgoff_t pgoff; > int aliases = 0; > > pgoff = vma->vm_pgoff + ((addr - vma->vm_start) >> PAGE_SHIFT); > + start = ALIGN_DOWN(addr, PMD_SIZE); > > /* > * If we have any shared mappings that are in the same mm > @@ -141,6 +166,8 @@ make_coherent(struct address_space *mapping, struct > vm_area_struct *vma, > */ > flush_dcache_mmap_lock(mapping); > vma_interval_tree_foreach(mpnt, &mapping->i_mmap, pgoff, pgoff) { > + unsigned long mpnt_addr; > + > /* > * If this VMA is not in our MM, we can ignore it. > * Note that we intentionally mask out the VMA > @@ -151,7 +178,14 @@ make_coherent(struct address_space *mapping, struct > vm_area_struct *vma, > if (!(mpnt->vm_flags & VM_MAYSHARE)) > continue; > offset = (pgoff - mpnt->vm_pgoff) << PAGE_SHIFT; > - aliases += adjust_pte(mpnt, mpnt->vm_start + offset, > pfn, vmf); > + mpnt_addr = mpnt->vm_start + offset; > + /* > + * If mpnt_addr and addr are mapped to the same PTE page, > + * also skip this vma. > + */ > + if (mpnt_addr >= start && mpnt_addr - start < PMD_SIZE) > + continue; Hmm, but is skipping the right thing to do? Maybe you would want to indicate to adjust_pte() whether it must take the PTL or not. I did not study that code in detail, though ... -- Cheers, David / dhildenb