On Sat, May 08, 2021 at 09:46:41AM -0700, Linus Torvalds wrote:
I think follow_pfn() is ok for the actual "this is not a 'struct page' backed area", and disabling that case is wrong even going forward.
Every place we've audited using follow_pfn() has been shown to have some use-after-free bugs like Daniel describes, and a failure to check permissions bug too.
All the other follow_pfn() users were moved to follow_pte() to fix the permissions check and this shifts the use-after-free bug away from being inside an MM API and into the caller mis-using the API by, say, extracting and using the PFN outside the pte lock.
eg look at how VFIO wrongly uses follow_pte():
static int follow_fault_pfn() ret = follow_pte(vma->vm_mm, vaddr, &ptep, &ptl); *pfn = pte_pfn(*ptep); pte_unmap_unlock(ptep, ptl);
// no protection that pte_pfn() is still valid! use_pfn(*pfn)
v4l is the only user that still has the missing permissions check security bug too - so there is no outcome that should keep follow_pfn() in the tree.
At worst v4l should change to follow_pte() and use it wrongly like VFIO. At best we should delete all the v4l stuff.
Daniel I suppose we missed this relation to follow_pte(), so I agree that keeping a unsafe_follow_pfn() around is not good.
Regards, Jason