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Tue, 04 Aug 2026 05:05:37 -0700 (PDT) X-Received: by 2002:a05:600c:6298:b0:499:484a:7644 with SMTP id 5b1f17b1804b1-499484a7693mr122465915e9.9.1785845136990; Tue, 04 Aug 2026 05:05:36 -0700 (PDT) Received: from [192.168.10.48] ([151.95.34.92]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-49807b8d04fsm453070475e9.3.2026.08.04.05.05.35 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 04 Aug 2026 05:05:36 -0700 (PDT) From: Paolo Bonzini To: linux-kernel@vger.kernel.org, kvm@vger.kernel.org Cc: Alex Williamson , bcm-kernel-feedback-list@broadcom.com, Boris Brezillon , Christian Koenig , David Hildenbrand , dri-devel@lists.freedesktop.org, Fei Li , Huang Rui , linux-mm@kvack.org, linux-s390@vger.kernel.org, Michal Hocko , Peter Xu , Sergio Lopez , Sean Christopherson , Thomas Zimmermann , stable@vger.kernel.org Subject: [PATCH v2 2/6] drm/shmem_helper: use vmf_insert_pfn_mkwrite() Date: Tue, 4 Aug 2026 14:05:24 +0200 Message-ID: <20260804120529.1730187-3-pbonzini@redhat.com> X-Mailer: git-send-email 2.55.0 In-Reply-To: <20260804120529.1730187-1-pbonzini@redhat.com> References: <20260804120529.1730187-1-pbonzini@redhat.com> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit This ensures that KVM or VFIO correctly see a writable PTE when they request one. Otherwise, a guest write to an unpopulated PTE from a mapping backed by a DRM GEM BO triggers a VM exit with EFAULT. The code actually is simpler, because the same logic already applied to the hugepage mapping case using vmf_insert_pfn_pmd(). Reported-by: Sergio Lopez Link: https://lore.kernel.org/kvm/20260729072044.25796-1-slp@redhat.com/ Tested-by: Sergio Lopez Reviewed-by: Boris Brezillon Fixes: 28e3918179aa ("drm/gem-shmem: Track folio accessed/dirty status in mmap") Cc: stable@vger.kernel.org Signed-off-by: Paolo Bonzini --- drivers/gpu/drm/drm_gem_shmem_helper.c | 38 ++++++++++++++------------ 1 file changed, 20 insertions(+), 18 deletions(-) diff --git a/drivers/gpu/drm/drm_gem_shmem_helper.c b/drivers/gpu/drm/drm_gem_shmem_helper.c index c989459eb215..c81be3e97317 100644 --- a/drivers/gpu/drm/drm_gem_shmem_helper.c +++ b/drivers/gpu/drm/drm_gem_shmem_helper.c @@ -589,11 +589,25 @@ static void drm_gem_shmem_record_mkwrite(struct vm_fault *vmf) folio_mark_dirty(page_folio(shmem->pages[page_offset])); } +/* + * Because the vm_ops have a .pfn_mkwrite() callback, vma_set_page_prot() + * has cleared the write bit from vma->vm_page_prot. vmf_insert_pfn() + * would install a read-only entry even for a write fault, relying on a + * second fault to reach .pfn_mkwrite() and upgrade it, but that second + * fault never happens for fixup_user_fault() callers that directly + * walk the page tables with follow_pfnmap_start(). To ensure that + * they don't see the read-only entry, pass FAULT_FLAG_WRITE info down + * to install a writable entry right away. Because .pfn_mkwrite() is + * not invoked, record the write afterwards. + */ static vm_fault_t try_insert_pfn(struct vm_fault *vmf, unsigned int order, unsigned long pfn) { + bool write = vmf->flags & FAULT_FLAG_WRITE; + vm_fault_t ret = VM_FAULT_FALLBACK; + if (!order) { - return vmf_insert_pfn(vmf->vma, vmf->address, pfn); + ret = vmf_insert_pfn_mkwrite(vmf->vma, vmf->address, pfn, write); #ifdef CONFIG_ARCH_SUPPORTS_PMD_PFNMAP } else if (order == PMD_ORDER) { unsigned long paddr = pfn << PAGE_SHIFT; @@ -601,27 +615,15 @@ static vm_fault_t try_insert_pfn(struct vm_fault *vmf, unsigned int order, if (aligned && folio_test_pmd_mappable(page_folio(pfn_to_page(pfn)))) { - vm_fault_t ret; - pfn &= PMD_MASK >> PAGE_SHIFT; - - /* Unlike PTEs which are automatically upgraded to - * writeable entries, the PMD upgrades go through - * .huge_fault(). Make sure we pass the "write" info - * along in that case. - * This also means we have to record the write fault - * here, instead of in .pfn_mkwrite(). - */ - ret = vmf_insert_pfn_pmd(vmf, pfn, - vmf->flags & FAULT_FLAG_WRITE); - if (ret == VM_FAULT_NOPAGE && (vmf->flags & FAULT_FLAG_WRITE)) - drm_gem_shmem_record_mkwrite(vmf); - - return ret; + ret = vmf_insert_pfn_pmd(vmf, pfn, write); } #endif } - return VM_FAULT_FALLBACK; + + if (ret == VM_FAULT_NOPAGE && write) + drm_gem_shmem_record_mkwrite(vmf); + return ret; } static vm_fault_t drm_gem_shmem_any_fault(struct vm_fault *vmf, unsigned int order) -- 2.55.0