Messages in this thread Patch in this message | | | Date | Wed, 13 Dec 2017 13:22:11 +0100 | From | Peter Zijlstra <> | Subject | Re: [patch 05/16] mm: Allow special mappings with user access cleared |
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On Tue, Dec 12, 2017 at 10:00:08AM -0800, Andy Lutomirski wrote: > On Tue, Dec 12, 2017 at 9:32 AM, Thomas Gleixner <tglx@linutronix.de> wrote: > > From: Peter Zijstra <peterz@infradead.org> > > > > In order to create VMAs that are not accessible to userspace create a new > > VM_NOUSER flag. This can be used in conjunction with > > install_special_mapping() to inject 'kernel' data into the userspace map. > > > > Similar to how arch_vm_get_page_prot() allows adding _PAGE_flags to > > pgprot_t, introduce arch_vm_get_page_prot_excl() which masks > > _PAGE_flags from pgprot_t and use this to implement VM_NOUSER for x86. > > How does this interact with get_user_pages(), etc?
So I went through that code and I think I found a bug related to this.
get_user_pages_fast() will ultimately end up doing pte_access_permitted() before getting the page, follow_page OTOH does not do this, which makes for a curious difference between the two.
So I'm thinking we want the below irrespective of the VM_NOUSER patch, but with VM_NOUSER it would mean write(2) will no longer be able to access the page.
diff --git a/mm/gup.c b/mm/gup.c index dfcde13f289a..b852f37a2b0c 100644 --- a/mm/gup.c +++ b/mm/gup.c @@ -153,6 +153,11 @@ static struct page *follow_page_pte(struct vm_area_struct *vma, } if (flags & FOLL_GET) { + if (!pte_access_permitted(pte, !!(flags & FOLL_WRITE))) { + page = ERR_PTR(-EFAULT); + goto out; + } + get_page(page); /* drop the pgmap reference now that we hold the page */
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