On 09/25/2018 07:48 AM, Sean Christopherson wrote:
> One defense against L1TF in KVM is to always set the upper five bits
> of the *legal* physical address in the SPTEs for non-present and
> reserved SPTEs, e.g. MMIO SPTEs. In the MMIO case, the GFN of the
> MMIO SPTE may overlap with the upper five bits that are being usurped
> to defend against L1TF. To preserve the GFN, the bits of the GFN that
> overlap with the repurposed bits are shifted left into the reserved
> bits, i.e. the GFN in the SPTE will be split into high and low parts.
> When retrieving the GFN from the MMIO SPTE, e.g. to check for an MMIO
> access, get_mmio_spte_gfn() unshifts the affected bits and restores
> the original GFN for comparison. Unfortunately, get_mmio_spte_gfn()
> neglects to mask off the reserved bits in the SPTE that were used to
> store the upper chunk of the GFN. As a result, KVM fails to detect
> MMIO accesses whose GPA overlaps the repurprosed bits, which in turn
> causes guest panics and hangs.
>
> Fix the bug by generating a mask that covers the lower chunk of the
> GFN, i.e. the bits that aren't shifted by the L1TF mitigation. The
> alternative approach would be to explicitly zero the five reserved
> bits that are used to store the upper chunk of the GFN, but that
> requires additional run-time computation and makes an already-ugly
> bit of code even more inscrutable.
Thanks for debugging and fixing this.
>
> Reported-by: Sakari Ailus <sakari.ailus@xxxxxx>
> Fixes: d9b47449c1a1 ("kvm: x86: Set highest physical address bits in non-present/reserved SPTEs")
> Cc: Junaid Shahid <junaids@xxxxxxxxxx>
> Cc: Jim Mattson <jmattson@xxxxxxxxxx>
> Cc: stable@xxxxxxxxxxxxxxx
> Signed-off-by: Sean Christopherson <sean.j.christopherson@xxxxxxxxx>
> ---
> arch/x86/kvm/mmu.c | 23 +++++++++++++++++++----
> 1 file changed, 19 insertions(+), 4 deletions(-)
>
> diff --git a/arch/x86/kvm/mmu.c b/arch/x86/kvm/mmu.c
> index a282321329b5..be5cb753fb42 100644
> --- a/arch/x86/kvm/mmu.c
> +++ b/arch/x86/kvm/mmu.c
> @@ -249,6 +249,17 @@ static u64 __read_mostly shadow_nonpresent_or_rsvd_mask;
> */
> static const u64 shadow_nonpresent_or_rsvd_mask_len = 5;
>
> +/*
> + * In some cases, we need to preserve the GFN of a non-present or reserved
> + * SPTE when we usurp the upper five bits of the physical address space to
> + * defend against L1TF, e.g. for MMIO SPTEs. To preserve the GFN, we'll
> + * shift bits of the GFN that overlap with shadow_nonpresent_or_rsvd_mask
> + * left into the reserved bits, i.e. the GFN in the SPTE will be split into
> + * high and low parts. This mask covers the lower bits of the GFN.
> + */
> +static u64 __read_mostly shadow_nonpresent_or_rsvd_lower_gpa_mask;
> +
> +
> static void mmu_spte_set(u64 *sptep, u64 spte);
> static union kvm_mmu_page_role
> kvm_mmu_calc_root_page_role(struct kvm_vcpu *vcpu);
> @@ -357,9 +368,7 @@ static bool is_mmio_spte(u64 spte)
>
> static gfn_t get_mmio_spte_gfn(u64 spte)
> {
> - u64 mask = generation_mmio_spte_mask(MMIO_GEN_MASK) | shadow_mmio_mask |
> - shadow_nonpresent_or_rsvd_mask;
> - u64 gpa = spte & ~mask;
> + u64 gpa = spte & shadow_nonpresent_or_rsvd_lower_gpa_mask;
>
> gpa |= (spte >> shadow_nonpresent_or_rsvd_mask_len)
> & shadow_nonpresent_or_rsvd_mask;
> @@ -423,6 +432,8 @@ EXPORT_SYMBOL_GPL(kvm_mmu_set_mask_ptes);
>
> static void kvm_mmu_reset_all_pte_masks(void)
> {
> + u8 low_gpa_bits;
> +
> shadow_user_mask = 0;
> shadow_accessed_mask = 0;
> shadow_dirty_mask = 0;
> @@ -437,12 +448,16 @@ static void kvm_mmu_reset_all_pte_masks(void)
> * appropriate mask to guard against L1TF attacks. Otherwise, it is
> * assumed that the CPU is not vulnerable to L1TF.
> */
> + low_gpa_bits = boot_cpu_data.x86_phys_bits;
> if (boot_cpu_data.x86_phys_bits <
> - 52 - shadow_nonpresent_or_rsvd_mask_len)
> + 52 - shadow_nonpresent_or_rsvd_mask_len) {
> shadow_nonpresent_or_rsvd_mask =
> rsvd_bits(boot_cpu_data.x86_phys_bits -
> shadow_nonpresent_or_rsvd_mask_len,
> boot_cpu_data.x86_phys_bits - 1);
> + low_gpa_bits -= shadow_nonpresent_or_rsvd_mask_len;
> + }
> + shadow_nonpresent_or_rsvd_lower_gpa_mask = (1ULL << low_gpa_bits) - 1;
I think that it might be slightly better to do something like:
+ shadow_nonpresent_or_rsvd_lower_gpa_mask = rsvd_bits(PAGE_SHIFT, low_gpa_bits - 1);
Of course, it doesn't matter for get_mmio_spte_gfn() because that already shifts by PAGE_SHIFT, but could matter if this were to get used somewhere else.
> }
>
> static int is_cpuid_PSE36(void)
>
Reviewed-by: Junaid Shahid <junaids@xxxxxxxxxx>
