3.16.84-rc1 review patch. If anyone has any objections, please let me know.
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From: Sean Christopherson sean.j.christopherson@intel.com
commit fcfbc617547fc6d9552cb6c1c563b6a90ee98085 upstream.
When reading/writing using the guest/host cache, check for a bad hva before checking for a NULL memslot, which triggers the slow path for handing cross-page accesses. Because the memslot is nullified on error by __kvm_gfn_to_hva_cache_init(), if the bad hva is encountered after crossing into a new page, then the kvm_{read,write}_guest() slow path could potentially write/access the first chunk prior to detecting the bad hva.
Arguably, performing a partial access is semantically correct from an architectural perspective, but that behavior is certainly not intended. In the original implementation, memslot was not explicitly nullified and therefore the partial access behavior varied based on whether the memslot itself was null, or if the hva was simply bad. The current behavior was introduced as a seemingly unintentional side effect in commit f1b9dd5eb86c ("kvm: Disallow wraparound in kvm_gfn_to_hva_cache_init"), which justified the change with "since some callers don't check the return code from this function, it sit seems prudent to clear ghc->memslot in the event of an error".
Regardless of intent, the partial access is dependent on _not_ checking the result of the cache initialization, which is arguably a bug in its own right, at best simply weird.
Fixes: 8f964525a121 ("KVM: Allow cross page reads and writes from cached translations.") Cc: Jim Mattson jmattson@google.com Cc: Andrew Honig ahonig@google.com Signed-off-by: Sean Christopherson sean.j.christopherson@intel.com Signed-off-by: Paolo Bonzini pbonzini@redhat.com [bwh: Backported to 3.16: adjust context] Signed-off-by: Ben Hutchings ben@decadent.org.uk --- virt/kvm/kvm_main.c | 12 ++++++------ 1 file changed, 6 insertions(+), 6 deletions(-)
--- a/virt/kvm/kvm_main.c +++ b/virt/kvm/kvm_main.c @@ -1596,12 +1596,12 @@ int kvm_write_guest_cached(struct kvm *k if (slots->generation != ghc->generation) kvm_gfn_to_hva_cache_init(kvm, ghc, ghc->gpa, ghc->len);
- if (unlikely(!ghc->memslot)) - return kvm_write_guest(kvm, ghc->gpa, data, len); - if (kvm_is_error_hva(ghc->hva)) return -EFAULT;
+ if (unlikely(!ghc->memslot)) + return kvm_write_guest(kvm, ghc->gpa, data, len); + r = __copy_to_user((void __user *)ghc->hva, data, len); if (r) return -EFAULT; @@ -1622,12 +1622,12 @@ int kvm_read_guest_cached(struct kvm *kv if (slots->generation != ghc->generation) kvm_gfn_to_hva_cache_init(kvm, ghc, ghc->gpa, ghc->len);
- if (unlikely(!ghc->memslot)) - return kvm_read_guest(kvm, ghc->gpa, data, len); - if (kvm_is_error_hva(ghc->hva)) return -EFAULT;
+ if (unlikely(!ghc->memslot)) + return kvm_read_guest(kvm, ghc->gpa, data, len); + r = __copy_from_user(data, (void __user *)ghc->hva, len); if (r) return -EFAULT;