Re: [PATCH RESEND] KVM: x86/mmu: fix UAF in paging_update_accessed_dirty_bits

[Sean Christopherson <seanjc@google.com>]

On Mon, Jan 24, 2022, Tadeusz Struk wrote:
> On 1/24/22 10:29, Sean Christopherson wrote:
> > 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.
> > 
> 
> The issue here is not related to /dev/mem, but binder allocated memory, which is
> yet another special mapping use case. In this case the condition
> 
> if (!vma || !(vma->vm_flags & VM_PFNMAP))
> 
> doesn't cover this special mappings. Adding the access_ok() was my something
> that fixed the use-after-free issue for me, and since I didn't have anything
> better I thought I will send an RFC to start some discussion.
> After some more debugging I came up with the bellow.
> Will that be more acceptable?

I'm pretty sure anything that keeps the vm_pgoff "logic" is a band-aid.  But I'm 99%
sure we can simply do cmpxchg directly on the user address, we just need to get
support for that, which has happily been posted[*].  I'll give that a shot tomorrow,
I want to convert similar code in the emulator, it'd be very nice to purge all of
this crud.

[*] https://lore.kernel.org/all/20220120160822.852009966@infradead.org