[PATCH v4 0/3] KVM: arm64: Fixes for parallel faults series

[PATCH v4 0/3] KVM: arm64: Fixes for parallel faults series

[Oliver Upton]

Small set of fixes for the parallel faults series. Most importantly,
stop taking the RCU read lock for walking hyp stage-1. For the sake of
consistency, take a pointer to kvm_pgtable_walker in
kvm_dereference_pteref() as well.

Tested on an Ampere Altra system with kvm-arm.mode={nvhe,protected} and
lockdep. Applies on top of the parallel faults series picked up last
week.

v3: https://lore.kernel.org/kvmarm/20221116165655.2649475-1-oliver.upton@linux.dev/

v3 -> v4:
 - Return an error instead of WARN() in hyp for shared walks (Will)

Oliver Upton (3):
  KVM: arm64: Take a pointer to walker data in kvm_dereference_pteref()
  KVM: arm64: Don't acquire RCU read lock for exclusive table walks
  KVM: arm64: Reject shared table walks in the hyp code

 arch/arm64/include/asm/kvm_pgtable.h | 159 +++++++++++++++------------
 arch/arm64/kvm/hyp/pgtable.c         |  13 ++-
 2 files changed, 96 insertions(+), 76 deletions(-)

-- 
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[PATCH v4 1/3] KVM: arm64: Take a pointer to walker data in kvm_dereference_pteref()

[Oliver Upton]

Rather than passing through the state of the KVM_PGTABLE_WALK_SHARED
flag, just take a pointer to the whole walker structure instead. Move
around struct kvm_pgtable and the RCU indirection such that the
associated ifdeffery remains in one place while ensuring the walker +
flags definitions precede their use.

No functional change intended.

Signed-off-by: Oliver Upton <oliver.upton@linux.dev>
---
 arch/arm64/include/asm/kvm_pgtable.h | 144 ++++++++++++++-------------
 arch/arm64/kvm/hyp/pgtable.c         |   6 +-
 2 files changed, 76 insertions(+), 74 deletions(-)

diff --git a/arch/arm64/include/asm/kvm_pgtable.h b/arch/arm64/include/asm/kvm_pgtable.h
index a874ce0ce7b5..f23af693e3c5 100644
--- a/arch/arm64/include/asm/kvm_pgtable.h
+++ b/arch/arm64/include/asm/kvm_pgtable.h
@@ -37,54 +37,6 @@ static inline u64 kvm_get_parange(u64 mmfr0)
 
 typedef u64 kvm_pte_t;
 
-/*
- * RCU cannot be used in a non-kernel context such as the hyp. As such, page
- * table walkers used in hyp do not call into RCU and instead use other
- * synchronization mechanisms (such as a spinlock).
- */
-#if defined(__KVM_NVHE_HYPERVISOR__) || defined(__KVM_VHE_HYPERVISOR__)
-
-typedef kvm_pte_t *kvm_pteref_t;
-
-static inline kvm_pte_t *kvm_dereference_pteref(kvm_pteref_t pteref, bool shared)
-{
-	return pteref;
-}
-
-static inline void kvm_pgtable_walk_begin(void) {}
-static inline void kvm_pgtable_walk_end(void) {}
-
-static inline bool kvm_pgtable_walk_lock_held(void)
-{
-	return true;
-}
-
-#else
-
-typedef kvm_pte_t __rcu *kvm_pteref_t;
-
-static inline kvm_pte_t *kvm_dereference_pteref(kvm_pteref_t pteref, bool shared)
-{
-	return rcu_dereference_check(pteref, !shared);
-}
-
-static inline void kvm_pgtable_walk_begin(void)
-{
-	rcu_read_lock();
-}
-
-static inline void kvm_pgtable_walk_end(void)
-{
-	rcu_read_unlock();
-}
-
-static inline bool kvm_pgtable_walk_lock_held(void)
-{
-	return rcu_read_lock_held();
-}
-
-#endif
-
 #define KVM_PTE_VALID			BIT(0)
 
 #define KVM_PTE_ADDR_MASK		GENMASK(47, PAGE_SHIFT)
@@ -212,29 +164,6 @@ enum kvm_pgtable_prot {
 typedef bool (*kvm_pgtable_force_pte_cb_t)(u64 addr, u64 end,
 					   enum kvm_pgtable_prot prot);
 
-/**
- * struct kvm_pgtable - KVM page-table.
- * @ia_bits:		Maximum input address size, in bits.
- * @start_level:	Level at which the page-table walk starts.
- * @pgd:		Pointer to the first top-level entry of the page-table.
- * @mm_ops:		Memory management callbacks.
- * @mmu:		Stage-2 KVM MMU struct. Unused for stage-1 page-tables.
- * @flags:		Stage-2 page-table flags.
- * @force_pte_cb:	Function that returns true if page level mappings must
- *			be used instead of block mappings.
- */
-struct kvm_pgtable {
-	u32					ia_bits;
-	u32					start_level;
-	kvm_pteref_t				pgd;
-	struct kvm_pgtable_mm_ops		*mm_ops;
-
-	/* Stage-2 only */
-	struct kvm_s2_mmu			*mmu;
-	enum kvm_pgtable_stage2_flags		flags;
-	kvm_pgtable_force_pte_cb_t		force_pte_cb;
-};
-
 /**
  * enum kvm_pgtable_walk_flags - Flags to control a depth-first page-table walk.
  * @KVM_PGTABLE_WALK_LEAF:		Visit leaf entries, including invalid
@@ -285,6 +214,79 @@ struct kvm_pgtable_walker {
 	const enum kvm_pgtable_walk_flags	flags;
 };
 
+/*
+ * RCU cannot be used in a non-kernel context such as the hyp. As such, page
+ * table walkers used in hyp do not call into RCU and instead use other
+ * synchronization mechanisms (such as a spinlock).
+ */
+#if defined(__KVM_NVHE_HYPERVISOR__) || defined(__KVM_VHE_HYPERVISOR__)
+
+typedef kvm_pte_t *kvm_pteref_t;
+
+static inline kvm_pte_t *kvm_dereference_pteref(struct kvm_pgtable_walker *walker,
+						kvm_pteref_t pteref)
+{
+	return pteref;
+}
+
+static inline void kvm_pgtable_walk_begin(void) {}
+static inline void kvm_pgtable_walk_end(void) {}
+
+static inline bool kvm_pgtable_walk_lock_held(void)
+{
+	return true;
+}
+
+#else
+
+typedef kvm_pte_t __rcu *kvm_pteref_t;
+
+static inline kvm_pte_t *kvm_dereference_pteref(struct kvm_pgtable_walker *walker,
+						kvm_pteref_t pteref)
+{
+	return rcu_dereference_check(pteref, !(walker->flags & KVM_PGTABLE_WALK_SHARED));
+}
+
+static inline void kvm_pgtable_walk_begin(void)
+{
+	rcu_read_lock();
+}
+
+static inline void kvm_pgtable_walk_end(void)
+{
+	rcu_read_unlock();
+}
+
+static inline bool kvm_pgtable_walk_lock_held(void)
+{
+	return rcu_read_lock_held();
+}
+
+#endif
+
+/**
+ * struct kvm_pgtable - KVM page-table.
+ * @ia_bits:		Maximum input address size, in bits.
+ * @start_level:	Level at which the page-table walk starts.
+ * @pgd:		Pointer to the first top-level entry of the page-table.
+ * @mm_ops:		Memory management callbacks.
+ * @mmu:		Stage-2 KVM MMU struct. Unused for stage-1 page-tables.
+ * @flags:		Stage-2 page-table flags.
+ * @force_pte_cb:	Function that returns true if page level mappings must
+ *			be used instead of block mappings.
+ */
+struct kvm_pgtable {
+	u32					ia_bits;
+	u32					start_level;
+	kvm_pteref_t				pgd;
+	struct kvm_pgtable_mm_ops		*mm_ops;
+
+	/* Stage-2 only */
+	struct kvm_s2_mmu			*mmu;
+	enum kvm_pgtable_stage2_flags		flags;
+	kvm_pgtable_force_pte_cb_t		force_pte_cb;
+};
+
 /**
  * kvm_pgtable_hyp_init() - Initialise a hypervisor stage-1 page-table.
  * @pgt:	Uninitialised page-table structure to initialise.
diff --git a/arch/arm64/kvm/hyp/pgtable.c b/arch/arm64/kvm/hyp/pgtable.c
index 5bca9610d040..b5b91a882836 100644
--- a/arch/arm64/kvm/hyp/pgtable.c
+++ b/arch/arm64/kvm/hyp/pgtable.c
@@ -188,7 +188,7 @@ static inline int __kvm_pgtable_visit(struct kvm_pgtable_walk_data *data,
 				      kvm_pteref_t pteref, u32 level)
 {
 	enum kvm_pgtable_walk_flags flags = data->walker->flags;
-	kvm_pte_t *ptep = kvm_dereference_pteref(pteref, flags & KVM_PGTABLE_WALK_SHARED);
+	kvm_pte_t *ptep = kvm_dereference_pteref(data->walker, pteref);
 	struct kvm_pgtable_visit_ctx ctx = {
 		.ptep	= ptep,
 		.old	= READ_ONCE(*ptep),
@@ -558,7 +558,7 @@ void kvm_pgtable_hyp_destroy(struct kvm_pgtable *pgt)
 	};
 
 	WARN_ON(kvm_pgtable_walk(pgt, 0, BIT(pgt->ia_bits), &walker));
-	pgt->mm_ops->put_page(kvm_dereference_pteref(pgt->pgd, false));
+	pgt->mm_ops->put_page(kvm_dereference_pteref(&walker, pgt->pgd));
 	pgt->pgd = NULL;
 }
 
@@ -1241,7 +1241,7 @@ void kvm_pgtable_stage2_destroy(struct kvm_pgtable *pgt)
 
 	WARN_ON(kvm_pgtable_walk(pgt, 0, BIT(pgt->ia_bits), &walker));
 	pgd_sz = kvm_pgd_pages(pgt->ia_bits, pgt->start_level) * PAGE_SIZE;
-	pgt->mm_ops->free_pages_exact(kvm_dereference_pteref(pgt->pgd, false), pgd_sz);
+	pgt->mm_ops->free_pages_exact(kvm_dereference_pteref(&walker, pgt->pgd), pgd_sz);
 	pgt->pgd = NULL;
 }
 
-- 
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[PATCH v4 2/3] KVM: arm64: Don't acquire RCU read lock for exclusive table walks

[Oliver Upton]

Marek reported a BUG resulting from the recent parallel faults changes,
as the hyp stage-1 map walker attempted to allocate table memory while
holding the RCU read lock:

  BUG: sleeping function called from invalid context at
  include/linux/sched/mm.h:274
  in_atomic(): 0, irqs_disabled(): 0, non_block: 0, pid: 1, name: swapper/0
  preempt_count: 0, expected: 0
  RCU nest depth: 1, expected: 0
  2 locks held by swapper/0/1:
    #0: ffff80000a8a44d0 (kvm_hyp_pgd_mutex){+.+.}-{3:3}, at:
  __create_hyp_mappings+0x80/0xc4
    #1: ffff80000a927720 (rcu_read_lock){....}-{1:2}, at:
  kvm_pgtable_walk+0x0/0x1f4
  CPU: 2 PID: 1 Comm: swapper/0 Not tainted 6.1.0-rc3+ #5918
  Hardware name: Raspberry Pi 3 Model B (DT)
  Call trace:
    dump_backtrace.part.0+0xe4/0xf0
    show_stack+0x18/0x40
    dump_stack_lvl+0x8c/0xb8
    dump_stack+0x18/0x34
    __might_resched+0x178/0x220
    __might_sleep+0x48/0xa0
    prepare_alloc_pages+0x178/0x1a0
    __alloc_pages+0x9c/0x109c
    alloc_page_interleave+0x1c/0xc4
    alloc_pages+0xec/0x160
    get_zeroed_page+0x1c/0x44
    kvm_hyp_zalloc_page+0x14/0x20
    hyp_map_walker+0xd4/0x134
    kvm_pgtable_visitor_cb.isra.0+0x38/0x5c
    __kvm_pgtable_walk+0x1a4/0x220
    kvm_pgtable_walk+0x104/0x1f4
    kvm_pgtable_hyp_map+0x80/0xc4
    __create_hyp_mappings+0x9c/0xc4
    kvm_mmu_init+0x144/0x1cc
    kvm_arch_init+0xe4/0xef4
    kvm_init+0x3c/0x3d0
    arm_init+0x20/0x30
    do_one_initcall+0x74/0x400
    kernel_init_freeable+0x2e0/0x350
    kernel_init+0x24/0x130
    ret_from_fork+0x10/0x20

Since the hyp stage-1 table walkers are serialized by kvm_hyp_pgd_mutex,
RCU protection really doesn't add anything. Don't acquire the RCU read
lock for an exclusive walk.

Reported-by: Marek Szyprowski <m.szyprowski@samsung.com>
Signed-off-by: Oliver Upton <oliver.upton@linux.dev>
---
 arch/arm64/include/asm/kvm_pgtable.h | 14 ++++++++------
 arch/arm64/kvm/hyp/pgtable.c         |  4 ++--
 2 files changed, 10 insertions(+), 8 deletions(-)

diff --git a/arch/arm64/include/asm/kvm_pgtable.h b/arch/arm64/include/asm/kvm_pgtable.h
index f23af693e3c5..4b6b52ebc11c 100644
--- a/arch/arm64/include/asm/kvm_pgtable.h
+++ b/arch/arm64/include/asm/kvm_pgtable.h
@@ -229,8 +229,8 @@ static inline kvm_pte_t *kvm_dereference_pteref(struct kvm_pgtable_walker *walke
 	return pteref;
 }
 
-static inline void kvm_pgtable_walk_begin(void) {}
-static inline void kvm_pgtable_walk_end(void) {}
+static inline void kvm_pgtable_walk_begin(struct kvm_pgtable_walker *walker) {}
+static inline void kvm_pgtable_walk_end(struct kvm_pgtable_walker *walker) {}
 
 static inline bool kvm_pgtable_walk_lock_held(void)
 {
@@ -247,14 +247,16 @@ static inline kvm_pte_t *kvm_dereference_pteref(struct kvm_pgtable_walker *walke
 	return rcu_dereference_check(pteref, !(walker->flags & KVM_PGTABLE_WALK_SHARED));
 }
 
-static inline void kvm_pgtable_walk_begin(void)
+static inline void kvm_pgtable_walk_begin(struct kvm_pgtable_walker *walker)
 {
-	rcu_read_lock();
+	if (walker->flags & KVM_PGTABLE_WALK_SHARED)
+		rcu_read_lock();
 }
 
-static inline void kvm_pgtable_walk_end(void)
+static inline void kvm_pgtable_walk_end(struct kvm_pgtable_walker *walker)
 {
-	rcu_read_unlock();
+	if (walker->flags & KVM_PGTABLE_WALK_SHARED)
+		rcu_read_unlock();
 }
 
 static inline bool kvm_pgtable_walk_lock_held(void)
diff --git a/arch/arm64/kvm/hyp/pgtable.c b/arch/arm64/kvm/hyp/pgtable.c
index b5b91a882836..d6f3753cb87e 100644
--- a/arch/arm64/kvm/hyp/pgtable.c
+++ b/arch/arm64/kvm/hyp/pgtable.c
@@ -289,9 +289,9 @@ int kvm_pgtable_walk(struct kvm_pgtable *pgt, u64 addr, u64 size,
 	};
 	int r;
 
-	kvm_pgtable_walk_begin();
+	kvm_pgtable_walk_begin(walker);
 	r = _kvm_pgtable_walk(pgt, &walk_data);
-	kvm_pgtable_walk_end();
+	kvm_pgtable_walk_end(walker);
 
 	return r;
 }
-- 
2.38.1.584.g0f3c55d4c2-goog


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[PATCH v4 3/3] KVM: arm64: Reject shared table walks in the hyp code

[Oliver Upton]

Exclusive table walks are the only supported table walk in the hyp, as
there is no construct like RCU available in the hypervisor code. Reject
any attempt to do a shared table walk by returning an error and allowing
the caller to clean up the mess.

Suggested-by: Will Deacon <will@kernel.org>
Signed-off-by: Oliver Upton <oliver.upton@linux.dev>
---
 arch/arm64/include/asm/kvm_pgtable.h | 17 +++++++++++++++--
 arch/arm64/kvm/hyp/pgtable.c         |  5 ++++-
 2 files changed, 19 insertions(+), 3 deletions(-)

diff --git a/arch/arm64/include/asm/kvm_pgtable.h b/arch/arm64/include/asm/kvm_pgtable.h
index 4b6b52ebc11c..d5cb01f8dc06 100644
--- a/arch/arm64/include/asm/kvm_pgtable.h
+++ b/arch/arm64/include/asm/kvm_pgtable.h
@@ -229,7 +229,18 @@ static inline kvm_pte_t *kvm_dereference_pteref(struct kvm_pgtable_walker *walke
 	return pteref;
 }
 
-static inline void kvm_pgtable_walk_begin(struct kvm_pgtable_walker *walker) {}
+static inline int kvm_pgtable_walk_begin(struct kvm_pgtable_walker *walker)
+{
+	/*
+	 * Due to the lack of RCU (or a similar protection scheme), only
+	 * non-shared table walkers are allowed in the hypervisor.
+	 */
+	if (walker->flags & KVM_PGTABLE_WALK_SHARED)
+		return -EPERM;
+
+	return 0;
+}
+
 static inline void kvm_pgtable_walk_end(struct kvm_pgtable_walker *walker) {}
 
 static inline bool kvm_pgtable_walk_lock_held(void)
@@ -247,10 +258,12 @@ static inline kvm_pte_t *kvm_dereference_pteref(struct kvm_pgtable_walker *walke
 	return rcu_dereference_check(pteref, !(walker->flags & KVM_PGTABLE_WALK_SHARED));
 }
 
-static inline void kvm_pgtable_walk_begin(struct kvm_pgtable_walker *walker)
+static inline int kvm_pgtable_walk_begin(struct kvm_pgtable_walker *walker)
 {
 	if (walker->flags & KVM_PGTABLE_WALK_SHARED)
 		rcu_read_lock();
+
+	return 0;
 }
 
 static inline void kvm_pgtable_walk_end(struct kvm_pgtable_walker *walker)
diff --git a/arch/arm64/kvm/hyp/pgtable.c b/arch/arm64/kvm/hyp/pgtable.c
index d6f3753cb87e..58dbe0ab567f 100644
--- a/arch/arm64/kvm/hyp/pgtable.c
+++ b/arch/arm64/kvm/hyp/pgtable.c
@@ -289,7 +289,10 @@ int kvm_pgtable_walk(struct kvm_pgtable *pgt, u64 addr, u64 size,
 	};
 	int r;
 
-	kvm_pgtable_walk_begin(walker);
+	r = kvm_pgtable_walk_begin(walker);
+	if (r)
+		return r;
+
 	r = _kvm_pgtable_walk(pgt, &walk_data);
 	kvm_pgtable_walk_end(walker);
 
-- 
2.38.1.584.g0f3c55d4c2-goog


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Re: [PATCH v4 0/3] KVM: arm64: Fixes for parallel faults series

[Will Deacon]

On Fri, Nov 18, 2022 at 06:22:19PM +0000, Oliver Upton wrote:
> Small set of fixes for the parallel faults series. Most importantly,
> stop taking the RCU read lock for walking hyp stage-1. For the sake of
> consistency, take a pointer to kvm_pgtable_walker in
> kvm_dereference_pteref() as well.
> 
> Tested on an Ampere Altra system with kvm-arm.mode={nvhe,protected} and
> lockdep. Applies on top of the parallel faults series picked up last
> week.
> 
> v3: https://lore.kernel.org/kvmarm/20221116165655.2649475-1-oliver.upton@linux.dev/
> 
> v3 -> v4:
>  - Return an error instead of WARN() in hyp for shared walks (Will)

For the series:

Acked-by: Will Deacon <will@kernel.org>

Thanks!

Will

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Re: [PATCH v4 0/3] KVM: arm64: Fixes for parallel faults series

[Marc Zyngier]

On Fri, 18 Nov 2022 18:22:19 +0000, Oliver Upton wrote:
> Small set of fixes for the parallel faults series. Most importantly,
> stop taking the RCU read lock for walking hyp stage-1. For the sake of
> consistency, take a pointer to kvm_pgtable_walker in
> kvm_dereference_pteref() as well.
> 
> Tested on an Ampere Altra system with kvm-arm.mode={nvhe,protected} and
> lockdep. Applies on top of the parallel faults series picked up last
> week.
> 
> [...]

Applied to next, thanks!

[1/3] KVM: arm64: Take a pointer to walker data in kvm_dereference_pteref()
      commit: 3a5154c723ba5ceb9ce374a7307e03263c03fd29
[2/3] KVM: arm64: Don't acquire RCU read lock for exclusive table walks
      commit: b7833bf202e3068abb77c642a0843f696e9c8d38
[3/3] KVM: arm64: Reject shared table walks in the hyp code
      commit: 5e806c5812e8012a83496cf96bdba266b3aec428

Cheers,

	M.
-- 
Without deviation from the norm, progress is not possible.



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