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b=Or+Lqa0GPXuwhvc+Harb5abpmSPoTuUf2HI0IyojIXiPH0jLGYTjqp8a8DHkhelsu 8biX6EZ8sp5UVaio8Sh2Sb+uvejPipX0B1Gy2MjaTB354kSWur8rWWXMasrT0j+NDW KshrSVaBse17Aj1tg24RVAjkzQBBdp2Jw2mSLz0hhFHwh50wBIHb90Qc1SBlIhr1tI ilusTfU/q2Je9wQFaLubIR6ARnBym+9ASXkQYxu8LcDeK/fN7VRaDyfwMERbnBYyj3 dXz4AIEROKIH1e6oo35hXvAJanCX0GG5Y2TVSus53vwgcmL53qgKdX1lzTYQxSQSeT mpjLdKzJKLL0A== Message-ID: <32fe40e2-854a-47ec-9d95-93e23f9a7af4@kernel.org> Date: Thu, 1 Oct 2026 22:22:29 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH] mm: don't ioremap COWed anon pages in generic_access_phys() To: Nguyen Ngoc Thang , akpm@linux-foundation.org Cc: ljs@kernel.org, liam@infradead.org, vbabka@kernel.org, rppt@kernel.org, surenb@google.com, mhocko@suse.com, peterx@redhat.com, dave.hansen@linux.intel.com, linux-mm@kvack.org, linux-kernel@vger.kernel.org, syzbot+49b1021becba70c1f3f6@syzkaller.appspotmail.com References: <699ae98c.050a0220.340abe.0d31.GAE@google.com> <20261001152524.171115-1-ngocthang2710.1999@gmail.com> From: "David Hildenbrand (Arm)" Content-Language: en-US Autocrypt: addr=david@kernel.org; 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charset=UTF-8 Content-Transfer-Encoding: 7bit X-Stat-Signature: qpgn6naao9d1dfmyqyj9to9pfsxx4bd6 X-Rspam-User: X-Rspamd-Server: rspam09 X-Rspamd-Queue-Id: D11294000A X-HE-Tag: 1790886158-415153 X-HE-Meta: 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 PIGJftW5 m48onw0rAaRNzWN0h1JVTVPmHsUfm6ATx28zXLWykuy9TD4lOOBI3OAHZWPTaLGdTxeTQ5rzHUJEBbjTwR4zUJAOKSlvX/jbo/QpGcnOao51ftdGUJ4emhJ026HwlXzDXeTonRxtJ1CGEL88p6jKKH7lltlF6a7c8LkvDMfRloUBMpJwqxSK0NvMw3l00B9nLrf+Pm1esBFiHgLkVbpPnEYrkl/OAU00WX99eERKMnTXpdnsqppdwizLWgTl3Myxyk4avPEU3UWlTYFPl6A1OVcrqX09Kh5IcckjbsuPmZaOq5WMV26x42drEqdyRMTjPxLgvLvLoE5OPuRKdKmeScaPJP3SUTF+C4Bctt2l64gNFgdvzpUWQlpamsDjF3z9W5L08kzw32hCDUBa7l1rHf434HKS5MzKcGXRS3JaV3wFL2LYv+6UsA0q+7OW4T5s7xEV73qxRtvhnLySxFPUBNWla2w== Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 10/1/26 17:25, Nguyen Ngoc Thang wrote: > A MAP_PRIVATE mapping of iomem (e.g. a PCI sysfs resourceN file) is a > COW pfnmap: a write fault replaces the pfn with an anonymous page, which > remap_pfn_range() allows by keeping vm_pgoff equal to the base pfn. > > generic_access_phys() does not tell those COWed pages apart from the > original pfns and ioremaps whatever the PTE points to. Reading such an > address via /proc/pid/mem or ptrace then ioremaps RAM: > > ioremap on RAM at 0x0000000045623000 - 0x0000000045623fff > WARNING: arch/x86/mm/ioremap.c:216 at __ioremap_caller.isra.0+0x4c2/0x5f0 > Call Trace: > generic_access_phys+0x130/0x4d0 mm/memory.c:7178 > kernfs_vma_access+0x1ce/0x280 fs/kernfs/file.c:437 > __access_remote_vm+0x58f/0x890 mm/memory.c:7256 > mem_rw+0x2a1/0x670 fs/proc/base.c:912 > Ack, apart from that, nothing bad should happen. > Reject COWed pages using the same linearity rule vm_normal_page() uses. Hm I don't enjoy that. What about letting follow_pfnmap_start() just perform a vm_normal_page() etc and indicate that information to the caller in the follow_pfnmap_args() ? We kind-of have that information in the form of follow_pfnmap_args.special ... but it's not available on all architectures. And it shouldn't exist. I was thinking a while ago about disallowing follow_pfnmap_start() entirely on anon folios, but the VM_IO thingy made me assume that there are some odd users (kvm/vfio) that actually need CoWed folios. Anyhow, this is what I think we should do for now: >From 0b75f30b137612f18cdee28c8c0dfa96870868c2 Mon Sep 17 00:00:00 2001 From: "David Hildenbrand (Arm)" Date: Thu, 1 Oct 2026 22:19:57 +0200 Subject: [PATCH] tmp Signed-off-by: David Hildenbrand (Arm) --- include/linux/mm.h | 4 +++- mm/memory.c | 48 ++++++++++++++++++++++++++++++++++++++++------ 2 files changed, 45 insertions(+), 7 deletions(-) diff --git a/include/linux/mm.h b/include/linux/mm.h index c49ef99b4413..b90e547797a9 100644 --- a/include/linux/mm.h +++ b/include/linux/mm.h @@ -3193,6 +3193,8 @@ struct folio *vm_normal_folio_pmd(struct vm_area_struct *vma, unsigned long addr, pmd_t pmd); struct page *vm_normal_page_pmd(struct vm_area_struct *vma, unsigned long addr, pmd_t pmd); +struct folio *vm_normal_folio_pud(struct vm_area_struct *vma, + unsigned long addr, pud_t pud); struct page *vm_normal_page_pud(struct vm_area_struct *vma, unsigned long addr, pud_t pud); @@ -3245,7 +3247,7 @@ struct follow_pfnmap_args { unsigned long addr_mask; pgprot_t pgprot; bool writable; - bool special; + bool cow; }; int follow_pfnmap_start(struct follow_pfnmap_args *args); void follow_pfnmap_end(struct follow_pfnmap_args *args); diff --git a/mm/memory.c b/mm/memory.c index 926276d41920..764b79204ab7 100644 --- a/mm/memory.c +++ b/mm/memory.c @@ -843,7 +843,7 @@ struct page *vm_normal_page_pmd(struct vm_area_struct *vma, unsigned long addr, * @addr: The address where the @pmd is mapped. * @pmd: The PMD. * - * Get the "struct folio" associated with a PTE. See __vm_normal_page() + * Get the "struct folio" associated with a PMD. See __vm_normal_page() * for details on "normal" and "special" mappings. * * Return: Returns the "struct folio" if this is a "normal" mapping. Returns @@ -879,6 +879,28 @@ struct page *vm_normal_page_pud(struct vm_area_struct *vma, return __vm_normal_page(vma, addr, pud_pfn(pud), pud_special(pud), &entry, sizeof(entry), PGTABLE_LEVEL_PUD); } + +/** + * vm_normal_folio_pud() - Get the "struct folio" associated with a PUD + * @vma: The VMA mapping the @pud. + * @addr: The address where the @pud is mapped. + * @pud: The PUD. + * + * Get the "struct folio" associated with a PUD. See __vm_normal_page() + * for details on "normal" and "special" mappings. + * + * Return: Returns the "struct folio" if this is a "normal" mapping. Returns + * NULL if this is a "special" mapping. + */ +struct folio *vm_normal_folio_pud(struct vm_area_struct *vma, + unsigned long addr, pud_t pud) +{ + struct page *page = vm_normal_page_pud(vma, addr, pud); + + if (page) + return page_folio(page); + return NULL; +} #endif /** @@ -6947,7 +6969,7 @@ static inline void pfnmap_args_setup(struct follow_pfnmap_args *args, spinlock_t *lock, pte_t *ptep, pgprot_t pgprot, unsigned long pfn_base, unsigned long addr_mask, bool writable, - bool special) + struct folio *folio) { args->lock = lock; args->ptep = ptep; @@ -6955,7 +6977,7 @@ static inline void pfnmap_args_setup(struct follow_pfnmap_args *args, args->addr_mask = addr_mask; args->pgprot = pgprot; args->writable = writable; - args->special = special; + args->cow = folio && folio_test_anon(folio); } static inline void pfnmap_lockdep_assert(struct vm_area_struct *vma) @@ -7008,6 +7030,7 @@ int follow_pfnmap_start(struct follow_pfnmap_args *args) struct vm_area_struct *vma = args->vma; unsigned long address = args->address; struct mm_struct *mm = vma->vm_mm; + struct folio *folio = NULL; spinlock_t *lock; pgd_t *pgdp; p4d_t *p4dp, p4d; @@ -7047,9 +7070,12 @@ int follow_pfnmap_start(struct follow_pfnmap_args *args) spin_unlock(lock); goto retry; } + + if (vma_is_cow_mapping(vma)) + folio = vm_normal_folio_pud(vma, address, pud); pfnmap_args_setup(args, lock, NULL, pud_pgprot(pud), pud_pfn(pud), PUD_MASK, pud_write(pud), - pud_special(pud)); + folio); return 0; } @@ -7068,9 +7094,12 @@ int follow_pfnmap_start(struct follow_pfnmap_args *args) spin_unlock(lock); goto retry; } + + if (vma_is_cow_mapping(vma)) + folio = vm_normal_folio_pmd(vma, address, pmd); pfnmap_args_setup(args, lock, NULL, pmd_pgprot(pmd), pmd_pfn(pmd), PMD_MASK, pmd_write(pmd), - pmd_special(pmd)); + folio); return 0; } @@ -7080,9 +7109,12 @@ int follow_pfnmap_start(struct follow_pfnmap_args *args) pte = ptep_get(ptep); if (!pte_present(pte)) goto unlock; + + if (vma_is_cow_mapping(vma)) + folio = vm_normal_folio(vma, address, pte); pfnmap_args_setup(args, lock, ptep, pte_pgprot(pte), pte_pfn(pte), PAGE_MASK, pte_write(pte), - pte_special(pte)); + folio); return 0; unlock: pte_unmap_unlock(ptep, lock); @@ -7145,6 +7177,10 @@ int generic_access_phys(struct vm_area_struct *vma, unsigned long addr, writable = args.writable; follow_pfnmap_end(&args); + /* Reject CoW'ed anonymous folios. */ + if (args.cow) + return -EINVAL; + if ((write & FOLL_WRITE) && !writable) return -EINVAL; -- 2.43.0 -- Cheers, David