Re: [PATCH 22/24] kvm: x86/mmu: Flush TLBs after zap in TDP MMU PF handler

Sean Christopherson
2021-01-21T01:00:52Z

On Tue, Jan 12, 2021, Ben Gardon wrote:
> When the TDP MMU is allowed to handle page faults in parallel there is
> the possiblity of a race where an SPTE is cleared and then imediately
> replaced with a present SPTE pointing to a different PFN, before the
> TLBs can be flushed. This race would violate architectural specs. Ensure
> that the TLBs are flushed properly before other threads are allowed to
> install any present value for the SPTE.
>
> Reviewed-by: Peter Feiner <pfeiner@google.com>
>
> Signed-off-by: Ben Gardon <bgardon@google.com>
> ---
> arch/x86/kvm/mmu/spte.h | 16 +++++++++-
> arch/x86/kvm/mmu/tdp_mmu.c | 62 ++++++++++++++++++++++++++++++++------
> 2 files changed, 68 insertions(+), 10 deletions(-)
>
> diff --git a/arch/x86/kvm/mmu/spte.h b/arch/x86/kvm/mmu/spte.h
> index 2b3a30bd38b0..ecd9bfbccef4 100644
> --- a/arch/x86/kvm/mmu/spte.h
> +++ b/arch/x86/kvm/mmu/spte.h
> @@ -130,6 +130,20 @@ extern u64 __read_mostly shadow_nonpresent_or_rsvd_mask;
> PT64_EPT_EXECUTABLE_MASK)
> #define SHADOW_ACC_TRACK_SAVED_BITS_SHIFT PT64_SECOND_AVAIL_BITS_SHIFT
>
> +/*
> + * If a thread running without exclusive control of the MMU lock must perform a
> + * multi-part operation on an SPTE, it can set the SPTE to FROZEN_SPTE as a
> + * non-present intermediate value. This will guarantee that other threads will
> + * not modify the spte.
> + *
> + * This constant works because it is considered non-present on both AMD and
> + * Intel CPUs and does not create a L1TF vulnerability because the pfn section
> + * is zeroed out.
> + *
> + * Only used by the TDP MMU.
> + */
> +#define FROZEN_SPTE (1ull << 59)

I dislike FROZEN, for similar reasons that I disliked "disconnected". The SPTE
isn't frozen in the sense that it's temporarily immutable, rather it's been
removed but hasn't been flushed and so can't yet be reused. Given that
FROZEN_SPTEs are treated as not-preset SPTEs, there's zero chance that this can
be extended in the future to be a generic temporarily freeze mechanism.

Mabye REMOVED_SPTE to match earlier feedback?

> +
> /*
> * 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
> @@ -187,7 +201,7 @@ static inline bool is_access_track_spte(u64 spte)
>
> static inline int is_shadow_present_pte(u64 pte)

Waaaay off topic, I'm going to send a patch to have this, and any other pte
helpers that return an int, return a bool. While futzing around with ideas I
managed to turn this into a nop by doing

return pte & SPTE_PRESENT;

which is guaranteed to be 0 if SPTE_PRESENT is a bit > 31. I'm sure others will
point out that I'm a heathen for not doing !!(pte & SPTE_PRESENT), but still...

> {
> - return (pte != 0) && !is_mmio_spte(pte);
> + return (pte != 0) && !is_mmio_spte(pte) && (pte != FROZEN_SPTE);

For all other checks, I'd strongly prefer to add a helper, e.g. is_removed_spte()
or whatever. That way changing the implementation won't be as painful, and we
can add assertions and whatnot if we break things. Especially since FROZEN_SPTE
is a single bit, which makes it look like a flag even though it's used as a full
64-bit constant.

For this, I worry that is_shadow_present_pte() is getting bloated. It's also a
bit unfortunate that it's bloated for the old MMU, without any benefit. That
being said, most that bloat is from the existing MMIO checks. Looking
elsewhere, TDX's SEPT also has a similar concept that may or may not need to
hook is_shadow_present_pte().

Rather than bundle MMIO SPTEs into the access-tracking flags and have a bunch of
special cases for not-present SPTEs, what if we add an explicit flag to mark
SPTEs as present (or not-present)? Defining SPTE_PRESENT instead of
SPTE_NOT_PRESENT might require a few more changes, but it would be the most
optimal for is_shadow_present_pte().

I'm thinking something like this (completely untested):

diff --git a/arch/x86/kvm/mmu/spte.c b/arch/x86/kvm/mmu/spte.c
index c51ad544f25b..86f6c84569c4 100644
--- a/arch/x86/kvm/mmu/spte.c
+++ b/arch/x86/kvm/mmu/spte.c
@@ -38,7 +38,7 @@ static u64 generation_mmio_spte_mask(u64 gen)
u64 mask;

WARN_ON(gen & ~MMIO_SPTE_GEN_MASK);
- BUILD_BUG_ON((MMIO_SPTE_GEN_HIGH_MASK | MMIO_SPTE_GEN_LOW_MASK) & SPTE_SPECIAL_MASK);
+ BUILD_BUG_ON((MMIO_SPTE_GEN_HIGH_MASK | MMIO_SPTE_GEN_LOW_MASK) & SPTE_MMIO);

mask = (gen << MMIO_SPTE_GEN_LOW_SHIFT) & MMIO_SPTE_GEN_LOW_MASK;
mask |= (gen << MMIO_SPTE_GEN_HIGH_SHIFT) & MMIO_SPTE_GEN_HIGH_MASK;
@@ -86,7 +86,7 @@ int make_spte(struct kvm_vcpu *vcpu, unsigned int pte_access, int level,
bool can_unsync, bool host_writable, bool ad_disabled,
u64 *new_spte)
{
- u64 spte = 0;
+ u64 spte = SPTE_PRESENT;
int ret = 0;

if (ad_disabled)
@@ -247,7 +247,7 @@ void kvm_mmu_set_mmio_spte_mask(u64 mmio_value, u64 access_mask)
BUG_ON((u64)(unsigned)access_mask != access_mask);
WARN_ON(mmio_value & (shadow_nonpresent_or_rsvd_mask << SHADOW_NONPRESENT_OR_RSVD_MASK_LEN));
WARN_ON(mmio_value & shadow_nonpresent_or_rsvd_lower_gfn_mask);
- shadow_mmio_value = mmio_value | SPTE_MMIO_MASK;
+ shadow_mmio_value = mmio_value | SPTE_MMIO;
shadow_mmio_access_mask = access_mask;
}
EXPORT_SYMBOL_GPL(kvm_mmu_set_mmio_spte_mask);
diff --git a/arch/x86/kvm/mmu/spte.h b/arch/x86/kvm/mmu/spte.h
index ecd9bfbccef4..465e43d34034 100644
--- a/arch/x86/kvm/mmu/spte.h
+++ b/arch/x86/kvm/mmu/spte.h
@@ -5,18 +5,15 @@

#include "mmu_internal.h"

+/* Software available bits for present SPTEs. */
#define PT_FIRST_AVAIL_BITS_SHIFT 10
#define PT64_SECOND_AVAIL_BITS_SHIFT 54

-/*
- * The mask used to denote special SPTEs, which can be either MMIO SPTEs or
- * Access Tracking SPTEs.
- */
+/* The mask used to denote Access Tracking SPTEs. Note, val=3 is available. */
#define SPTE_SPECIAL_MASK (3ULL << 52)
#define SPTE_AD_ENABLED_MASK (0ULL << 52)
#define SPTE_AD_DISABLED_MASK (1ULL << 52)
#define SPTE_AD_WRPROT_ONLY_MASK (2ULL << 52)
-#define SPTE_MMIO_MASK (3ULL << 52)

#ifdef CONFIG_DYNAMIC_PHYSICAL_MASK
#define PT64_BASE_ADDR_MASK (physical_mask & ~(u64)(PAGE_SIZE-1))
@@ -55,12 +52,16 @@
#define SPTE_HOST_WRITEABLE (1ULL << PT_FIRST_AVAIL_BITS_SHIFT)
#define SPTE_MMU_WRITEABLE (1ULL << (PT_FIRST_AVAIL_BITS_SHIFT + 1))

+#define SPTE_REMOVED BIT_ULL(60)
+#define SPTE_MMIO BIT_ULL(61)
+#define SPTE_PRESENT BIT_ULL(62)
+
/*
* Due to limited space in PTEs, the MMIO generation is a 18 bit subset of
* the memslots generation and is derived as follows:
*
* Bits 0-8 of the MMIO generation are propagated to spte bits 3-11
- * Bits 9-17 of the MMIO generation are propagated to spte bits 54-62
+ * Bits 9-17 of the MMIO generation are propagated to spte bits 52-60
*
* The KVM_MEMSLOT_GEN_UPDATE_IN_PROGRESS flag is intentionally not included in
* the MMIO generation number, as doing so would require stealing a bit from
@@ -73,8 +74,8 @@
#define MMIO_SPTE_GEN_LOW_START 3
#define MMIO_SPTE_GEN_LOW_END 11

-#define MMIO_SPTE_GEN_HIGH_START PT64_SECOND_AVAIL_BITS_SHIFT
-#define MMIO_SPTE_GEN_HIGH_END 62
+#define MMIO_SPTE_GEN_HIGH_START 52
+#define MMIO_SPTE_GEN_HIGH_END 60

#define MMIO_SPTE_GEN_LOW_MASK GENMASK_ULL(MMIO_SPTE_GEN_LOW_END, \
MMIO_SPTE_GEN_LOW_START)
@@ -162,7 +163,7 @@ extern u8 __read_mostly shadow_phys_bits;

static inline bool is_mmio_spte(u64 spte)
{
- return (spte & SPTE_SPECIAL_MASK) == SPTE_MMIO_MASK;
+ return spte & SPTE_MMIO;
}

static inline bool sp_ad_disabled(struct kvm_mmu_page *sp)
@@ -199,9 +200,9 @@ static inline bool is_access_track_spte(u64 spte)
return !spte_ad_enabled(spte) && (spte & shadow_acc_track_mask) == 0;
}

-static inline int is_shadow_present_pte(u64 pte)
+static inline bool is_shadow_present_pte(u64 pte)
{
- return (pte != 0) && !is_mmio_spte(pte) && (pte != FROZEN_SPTE);
+ return pte & SPTE_PRESENT;
}

static inline int is_large_pte(u64 pte)

> }
>
> static inline int is_large_pte(u64 pte)
> diff --git a/arch/x86/kvm/mmu/tdp_mmu.c b/arch/x86/kvm/mmu/tdp_mmu.c
> index 7b12a87a4124..5c9d053000ad 100644
> --- a/arch/x86/kvm/mmu/tdp_mmu.c
> +++ b/arch/x86/kvm/mmu/tdp_mmu.c
> @@ -429,15 +429,19 @@ static void __handle_changed_spte(struct kvm *kvm, int as_id, gfn_t gfn,
> */
> if (!was_present && !is_present) {
> /*
> - * If this change does not involve a MMIO SPTE, it is
> - * unexpected. Log the change, though it should not impact the
> - * guest since both the former and current SPTEs are nonpresent.
> + * If this change does not involve a MMIO SPTE or FROZEN_SPTE,

For comments and error message, I think we should avoid using the exact constant
name, and instead call them "removed SPTE", similar to MMIO SPTE. That will
help reduce thrash and/or stale comments if the name changes.

> + * it is unexpected. Log the change, though it should not
> + * impact the guest since both the former and current SPTEs
> + * are nonpresent.
> */
> - if (WARN_ON(!is_mmio_spte(old_spte) && !is_mmio_spte(new_spte)))
> + if (WARN_ON(!is_mmio_spte(old_spte) &&
> + !is_mmio_spte(new_spte) &&
> + new_spte != FROZEN_SPTE))
> pr_err("Unexpected SPTE change! Nonpresent SPTEs\n"
> "should not be replaced with another,\n"
> "different nonpresent SPTE, unless one or both\n"
> - "are MMIO SPTEs.\n"
> + "are MMIO SPTEs, or the new SPTE is\n"
> + "FROZEN_SPTE.\n"
> "as_id: %d gfn: %llx old_spte: %llx new_spte: %llx level: %d",
> as_id, gfn, old_spte, new_spte, level);
> return;