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SUUrMFhczjE1jGUM0taixf0lIk5dRYyLHslUlpTDFYHSjDih3IvEqdKAmUd1S75hS5/mel5bsieg0y2B8pa8FbGFWorO9jBNwfIw6cyyWtSJt7hKrMcE+UOVJxEyVVhSIfnaDvPILHgWhyJSG7vtZCcnNKVCnhDtZijc3oxAQcjeony5QPI61TWkda/GZChECSnjahxpz33siSV952VBThfxt+30dT6y01dRrQ0tRGP6gfOHF8H14Y7JUfkKrNS70H3M/yLsNc9Y//gIgEx2qS+bE1P8rIUadQTklU5XgIRkxl3W2Jx871ir0mi+TdhsqhDI3bEySY0ixNEEOmx5AG5WKc8rdPDm/LMBABdD3pMYLuVp8PVjcnbk90gK7B9IZk+p727+vXFfsU8HrDag6W8sk866aX09dxQ2mi9EdBCAst5PtIlEBTqyN9nGbTOAGvgbbEJ4CnVTFoog= Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 7/24/26 3:29 PM, Rik van Riel wrote: ... > +struct page *get_user_page_vma(struct vm_area_struct *vma, unsigned long addr, ... > + ret = check_vma_flags(vma, gup_flags, VM_IO | VM_PFNMAP); > + if (ret) > + goto fail; > + pfnmap = vma->vm_flags & (VM_IO | VM_PFNMAP); This treats VM_IO and VM_PFNMAP as the same exception, but they have different meanings. VM_IO tells MM callers not to inspect the mapped pages because accesses can have side effects. It does not imply that the VMA contains raw PFNs! For example, sg_mmap() creates a VM_IO-only VMA whose fault handler installs ordinary pages from the SCSI reserve buffer. If such a page is already present, follow_page_mask() returns it before the pfnmap test below is reached. Patch 6 then lets /proc/PID/mem and ptrace copy that page directly, even though sg provides no ->access callback and the operation fails today. Writes can also modify the buffer when the mapping is writable. Existing ptrace authorization still applies, so this is not a privilege bypass, but it removes the VM_IO exclusion for a driver that did not opt in through ->access. An untouched page in the same VMA still fails because pfnmap is also true for VM_IO and this function refuses to fault it. The result therefore depends on whether the target previously touched the page. The required exception is narrower. A remap_pfn_range() VMA has both VM_IO and VM_PFNMAP, but VM_PFNMAP is what identifies the mapping whose COWed page should be returned. So if you use the FOLL_PFNMAP_COW flag suggested on patch 4, and test only VM_PFNMAP here: ret = check_vma_flags(vma, gup_flags); if (ret) goto fail; pfnmap = vma->vm_flags & VM_PFNMAP; Patch 6 can set FOLL_PFNMAP_COW for __access_remote_vm(). check_vma_flags() then permits VM_IO | VM_PFNMAP for that caller while continuing to reject VM_IO-only mappings. vm_normal_page() supplies the remaining distinction: it returns the COWed normal page and rejects the raw PFN. Something like that. thanks, -- John Hubbard