On Thu, 2019-10-10 at 01:08 +0200, speck for Paolo Bonzini wrote:
> On 10/10/19 00:42, speck for Ben Hutchings wrote:
> > I've started trying to backport the NX patches to stable branches, and
> > I think I can take them back as far as 4.9.
> > 
> > However, kvm_mmu_zap_all() is a relatively new addition and looks hard
> > to backport.  I intend to make the nx_huge_pages parameter read-only
> > (0444 permissions) and delete this "if (new_val != old_val)" block. 
> > Does that seem reasonable?
> 
> Just replace it with kvm_mmu_invalidate_zap_all_pages (which will be in 
> v6, because we have since brought it back from the dead).
> 
> You also have to backport commit 833b45de69a6 ("kvm: x86, powerpc: do 
> not allow clearing largepages debugfs entry", 2019-09-30) and make the 
> new statistic 0444 as well.  This is the only other change in v6 for now.
> 
> This is the list of prerequisites I had prepared, it should go back to 4.3 or so:

Wow, thanks for this.  I still don't think it's worthwile to backport
this mitigation to stable branches older than 4.9 - they never got KVM
mitigations for L1TF so they're already unsuitable for hosting
untrusted guests.  After excluding commits older than 4.9, the list is
then:

[...]
> e08d26f0712532c79b5ba6200862eaf2036f8df6 kvm: x86: simplify ept_misconfig
> 9b8ebbdb74b5ad76b9dfd8b101af17839174b126 kvm: x86: extend usage of RET_MMIO_PF_* constants
> 42522d08cdba6d8be4247e4f0770f39f4708b71f KVM: MMU: drop vcpu param in gpte_access
> 0d9ce162cf46c99628cc5da9510b959c7976735b kvm: Convert kvm_lock to a mutex
[...]
> 43fdcda96e2550c6d1c46fb8a78801aa2f7276ed kvm: mmu: Do not release the page inside mmu_set_spte() 
> 9de2b2120668d2874570b14105e49235097b70c2 KVM: x86: make FNAME(fetch) and __direct_map more similar

Commit hash should be 3fcf2d1bdeb6a513523cb2c77012a6b047aa859c?

> d679b32611c0102ce33b9e1a4e4b94854ed1812a KVM: x86: remove now unneeded hugepage gfn adjustment 
> 1e823556fd3af3635e174f570d0b85b4e72b2b1c KVM: x86: change kvm_mmu_page_get_gfn BUG_ON to WARN_ON

Commit hash should be e9f2a760b158551bfbef6db31d2cae45ab8072e5?

> bf9af89c4146978000eba9b0a1eb43540d893223 KVM: x86: add tracepoints around __direct_map and FNAME(fetch)

Commit hash should be 335e192a3fa415e1202c8b9ecdaaecd643f823cc?

> 833b45de69a6016c4b0cebe6765d526a31a81580 kvm: x86, powerpc: do not allow clearing largepages debugfs entry
> 
> None of the backports should be particularly tricky.

In my previous attempt I found it helpful to pick these two that you
didn't mention:

3ff519f29d98 KVM: x86: adjust kvm_mmu_page member to save 8 bytes
00ae831dfe44 x86/cpu: Add Atom Tremont (Jacobsville)

Ben.

-- 
Ben Hutchings
The obvious mathematical breakthrough [to break modern encryption]
would be development of an easy way to factor large prime numbers.
                                                           - Bill Gates