On Mon, Jan 24, 2022, Paolo Bonzini wrote:
On 1/24/22 18:26, Tadeusz Struk wrote:
Syzbot reported an use-after-free bug in update_accessed_dirty_bits(). Fix this by checking if the memremap'ed pointer is still valid.
access_ok only checks that the pointer is in the userspace range. Is this correct? And if so, what are the exact circumstances in which access_ok returns a non-NULL but also non-userspace address?
I "objected" to this patch in its initial posting[*]. AFAICT adding access_ok() is just masking a more egregious bug where interpretation of vm_pgoff as a PFN base is flat out wrong except for select backing stores that use VM_PFNMAP. In other words, the vm_pgoff hack works for the /dev/mem use case, but it is wrong in general.
[*] https://lore.kernel.org/all/Ybp0naX%2FZTG9FNEa@google.com
Thanks,
Paolo
Cc: Paolo Bonzini pbonzini@redhat.com Cc: Sean Christopherson seanjc@google.com Cc: Vitaly Kuznetsov vkuznets@redhat.com Cc: Wanpeng Li wanpengli@tencent.com Cc: Jim Mattson jmattson@google.com Cc: Joerg Roedel joro@8bytes.org Cc: Thomas Gleixner tglx@linutronix.de Cc: Ingo Molnar mingo@redhat.com Cc: Borislav Petkov bp@alien8.de Cc: Dave Hansen dave.hansen@linux.intel.com Cc: x86@kernel.org Cc: "H. Peter Anvin" hpa@zytor.com Cc: kvm@vger.kernel.org Cc: stable@vger.kernel.org Fixes: bd53cb35a3e9 ("X86/KVM: Handle PFNs outside of kernel reach when touching GPTEs") Link: https://syzkaller.appspot.com/bug?id=6cb6102a0a7b0c52060753dd62d070a1d1e7134... Reported-by: syzbot+6cde2282daa792c49ab8@syzkaller.appspotmail.com Signed-off-by: Tadeusz Struk tadeusz.struk@linaro.org
arch/x86/kvm/mmu/paging_tmpl.h | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-)
diff --git a/arch/x86/kvm/mmu/paging_tmpl.h b/arch/x86/kvm/mmu/paging_tmpl.h index 5b5bdac97c7b..d25b72d7b1b1 100644 --- a/arch/x86/kvm/mmu/paging_tmpl.h +++ b/arch/x86/kvm/mmu/paging_tmpl.h @@ -174,7 +174,7 @@ static int FNAME(cmpxchg_gpte)(struct kvm_vcpu *vcpu, struct kvm_mmu *mmu, pfn = ((vaddr - vma->vm_start) >> PAGE_SHIFT) + vma->vm_pgoff; paddr = pfn << PAGE_SHIFT; table = memremap(paddr, PAGE_SIZE, MEMREMAP_WB);
if (!table) {
if (!table || !access_ok(table, PAGE_SIZE)) { mmap_read_unlock(current->mm); return -EFAULT;