[PATCH 0/2] A couple of EL2 refcounts fixes

[PATCH 0/2] A couple of EL2 refcounts fixes

[Quentin Perret]

Hi all,

This addresses a couple of issues Will has found with the refcounting of
page-tables at EL2. Patch 01 fixes a nasty bug, and probably wants to go
in -stable. Patch 02 fixes a small inconsistency which made it harder to
find refcount-related bugs at EL2.

Feedback welcome !

Thanks,
Quentin

Quentin Perret (2):
  KVM: arm64: Fix host stage-2 PGD refcount
  KVM: arm64: Report corrupted refcount at EL2

 arch/arm64/kvm/hyp/include/nvhe/gfp.h |  1 +
 arch/arm64/kvm/hyp/nvhe/mem_protect.c |  6 +++++-
 arch/arm64/kvm/hyp/nvhe/page_alloc.c  | 15 +++++++++++++++
 3 files changed, 21 insertions(+), 1 deletion(-)

-- 
2.33.0.800.g4c38ced690-goog

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[PATCH 1/2] KVM: arm64: Fix host stage-2 PGD refcount

[Quentin Perret]

The KVM page-table library refcounts the pages of concatenated stage-2
PGDs individually. However, the host's stage-2 PGD is currently managed
by EL2 as a single high-order compound page, which can cause the
refcount of the tail pages to reach 0 when they really shouldn't, hence
corrupting the page-table.

Fix this by introducing a new hyp_split_page() helper in the EL2 page
allocator (matching EL1's split_page() function), and make use of it
from host_s2_zalloc_page().

Fixes: 1025c8c0c6ac ("KVM: arm64: Wrap the host with a stage 2")
Suggested-by: Will Deacon <will@kernel.org>
Signed-off-by: Quentin Perret <qperret@google.com>
---
 arch/arm64/kvm/hyp/include/nvhe/gfp.h |  1 +
 arch/arm64/kvm/hyp/nvhe/mem_protect.c |  6 +++++-
 arch/arm64/kvm/hyp/nvhe/page_alloc.c  | 14 ++++++++++++++
 3 files changed, 20 insertions(+), 1 deletion(-)

diff --git a/arch/arm64/kvm/hyp/include/nvhe/gfp.h b/arch/arm64/kvm/hyp/include/nvhe/gfp.h
index fb0f523d1492..0a048dc06a7d 100644
--- a/arch/arm64/kvm/hyp/include/nvhe/gfp.h
+++ b/arch/arm64/kvm/hyp/include/nvhe/gfp.h
@@ -24,6 +24,7 @@ struct hyp_pool {
 
 /* Allocation */
 void *hyp_alloc_pages(struct hyp_pool *pool, unsigned short order);
+void hyp_split_page(struct hyp_page *page);
 void hyp_get_page(struct hyp_pool *pool, void *addr);
 void hyp_put_page(struct hyp_pool *pool, void *addr);
 
diff --git a/arch/arm64/kvm/hyp/nvhe/mem_protect.c b/arch/arm64/kvm/hyp/nvhe/mem_protect.c
index bacd493a4eac..93a79736c283 100644
--- a/arch/arm64/kvm/hyp/nvhe/mem_protect.c
+++ b/arch/arm64/kvm/hyp/nvhe/mem_protect.c
@@ -35,7 +35,11 @@ const u8 pkvm_hyp_id = 1;
 
 static void *host_s2_zalloc_pages_exact(size_t size)
 {
-	return hyp_alloc_pages(&host_s2_pool, get_order(size));
+	void *addr = hyp_alloc_pages(&host_s2_pool, get_order(size));
+
+	hyp_split_page(hyp_virt_to_page(addr));
+
+	return addr;
 }
 
 static void *host_s2_zalloc_page(void *pool)
diff --git a/arch/arm64/kvm/hyp/nvhe/page_alloc.c b/arch/arm64/kvm/hyp/nvhe/page_alloc.c
index 41fc25bdfb34..a6e874e61a40 100644
--- a/arch/arm64/kvm/hyp/nvhe/page_alloc.c
+++ b/arch/arm64/kvm/hyp/nvhe/page_alloc.c
@@ -193,6 +193,20 @@ void hyp_get_page(struct hyp_pool *pool, void *addr)
 	hyp_spin_unlock(&pool->lock);
 }
 
+void hyp_split_page(struct hyp_page *p)
+{
+	unsigned short order = p->order;
+	unsigned int i;
+
+	p->order = 0;
+	for (i = 1; i < (1 << order); i++) {
+		struct hyp_page *tail = p + i;
+
+		tail->order = 0;
+		hyp_set_page_refcounted(tail);
+	}
+}
+
 void *hyp_alloc_pages(struct hyp_pool *pool, unsigned short order)
 {
 	unsigned short i = order;
-- 
2.33.0.800.g4c38ced690-goog

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[PATCH 2/2] KVM: arm64: Report corrupted refcount at EL2

[Quentin Perret]

Some of the refcount manipulation helpers used at EL2 are instrumented
to catch a corrupt state, but not all of them are treated equally. Let's
make things more consistent by instrumenting hyp_page_ref_dec_and_test()
as well.

Suggested-by: Will Deacon <will@kernel.org>
Signed-off-by: Quentin Perret <qperret@google.com>
---
 arch/arm64/kvm/hyp/nvhe/page_alloc.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/arch/arm64/kvm/hyp/nvhe/page_alloc.c b/arch/arm64/kvm/hyp/nvhe/page_alloc.c
index a6e874e61a40..0bd7701ad1df 100644
--- a/arch/arm64/kvm/hyp/nvhe/page_alloc.c
+++ b/arch/arm64/kvm/hyp/nvhe/page_alloc.c
@@ -152,6 +152,7 @@ static inline void hyp_page_ref_inc(struct hyp_page *p)
 
 static inline int hyp_page_ref_dec_and_test(struct hyp_page *p)
 {
+	BUG_ON(!p->refcount);
 	p->refcount--;
 	return (p->refcount == 0);
 }
-- 
2.33.0.800.g4c38ced690-goog

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[PATCH v2 0/2] A couple of EL2 refcounts fixes

[Quentin Perret]

Hi all,

This is v2 of the series previously posted here:

https://lore.kernel.org/kvmarm/20211004090328.540941-1-qperret@google.com/

This addresses a couple of issues Will has found with the refcounting of
page-tables at EL2. Patch 01 fixes a nasty bug, and probably wants to go
in -stable. Patch 02 fixes a small inconsistency which made it harder to
find refcount-related bugs at EL2.

Changes since v1:
 - applied Acked-by from Will;
 - better comment + commit message for patch 01 (Marc).

Thanks!
Quentin

Quentin Perret (2):
  KVM: arm64: Fix host stage-2 PGD refcount
  KVM: arm64: Report corrupted refcount at EL2

 arch/arm64/kvm/hyp/include/nvhe/gfp.h |  1 +
 arch/arm64/kvm/hyp/nvhe/mem_protect.c | 13 ++++++++++++-
 arch/arm64/kvm/hyp/nvhe/page_alloc.c  | 15 +++++++++++++++
 3 files changed, 28 insertions(+), 1 deletion(-)

-- 
2.33.0.800.g4c38ced690-goog

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Re: [PATCH v2 0/2] A couple of EL2 refcounts fixes

[Marc Zyngier]

On Tue, 5 Oct 2021 10:01:40 +0100, Quentin Perret wrote:
> This is v2 of the series previously posted here:
> 
> https://lore.kernel.org/kvmarm/20211004090328.540941-1-qperret@google.com/
> 
> This addresses a couple of issues Will has found with the refcounting of
> page-tables at EL2. Patch 01 fixes a nasty bug, and probably wants to go
> in -stable. Patch 02 fixes a small inconsistency which made it harder to
> find refcount-related bugs at EL2.
> 
> [...]

Applied to fixes, thanks!

[1/2] KVM: arm64: Fix host stage-2 PGD refcount
      commit: 1d58a17ef54599506d44c45ac95be27273a4d2b1
[2/2] KVM: arm64: Report corrupted refcount at EL2
      commit: 7615c2a514788559c6684234b8fc27f3a843c2c6

Cheers,

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


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[PATCH v2 1/2] KVM: arm64: Fix host stage-2 PGD refcount

[Quentin Perret]

The KVM page-table library refcounts the pages of concatenated stage-2
PGDs individually. However, when running KVM in protected mode, the
host's stage-2 PGD is currently managed by EL2 as a single high-order
compound page, which can cause the refcount of the tail pages to reach 0
when they shouldn't, hence corrupting the page-table.

Fix this by introducing a new hyp_split_page() helper in the EL2 page
allocator (matching the kernel's split_page() function), and make use of
it from host_s2_zalloc_pages_exact().

Fixes: 1025c8c0c6ac ("KVM: arm64: Wrap the host with a stage 2")
Acked-by: Will Deacon <will@kernel.org>
Suggested-by: Will Deacon <will@kernel.org>
Signed-off-by: Quentin Perret <qperret@google.com>
---
 arch/arm64/kvm/hyp/include/nvhe/gfp.h |  1 +
 arch/arm64/kvm/hyp/nvhe/mem_protect.c | 13 ++++++++++++-
 arch/arm64/kvm/hyp/nvhe/page_alloc.c  | 14 ++++++++++++++
 3 files changed, 27 insertions(+), 1 deletion(-)

diff --git a/arch/arm64/kvm/hyp/include/nvhe/gfp.h b/arch/arm64/kvm/hyp/include/nvhe/gfp.h
index fb0f523d1492..0a048dc06a7d 100644
--- a/arch/arm64/kvm/hyp/include/nvhe/gfp.h
+++ b/arch/arm64/kvm/hyp/include/nvhe/gfp.h
@@ -24,6 +24,7 @@ struct hyp_pool {
 
 /* Allocation */
 void *hyp_alloc_pages(struct hyp_pool *pool, unsigned short order);
+void hyp_split_page(struct hyp_page *page);
 void hyp_get_page(struct hyp_pool *pool, void *addr);
 void hyp_put_page(struct hyp_pool *pool, void *addr);
 
diff --git a/arch/arm64/kvm/hyp/nvhe/mem_protect.c b/arch/arm64/kvm/hyp/nvhe/mem_protect.c
index bacd493a4eac..34eeb524b686 100644
--- a/arch/arm64/kvm/hyp/nvhe/mem_protect.c
+++ b/arch/arm64/kvm/hyp/nvhe/mem_protect.c
@@ -35,7 +35,18 @@ const u8 pkvm_hyp_id = 1;
 
 static void *host_s2_zalloc_pages_exact(size_t size)
 {
-	return hyp_alloc_pages(&host_s2_pool, get_order(size));
+	void *addr = hyp_alloc_pages(&host_s2_pool, get_order(size));
+
+	hyp_split_page(hyp_virt_to_page(addr));
+
+	/*
+	 * The size of concatenated PGDs is always a power of two of PAGE_SIZE,
+	 * so there should be no need to free any of the tail pages to make the
+	 * allocation exact.
+	 */
+	WARN_ON(size != (PAGE_SIZE << get_order(size)));
+
+	return addr;
 }
 
 static void *host_s2_zalloc_page(void *pool)
diff --git a/arch/arm64/kvm/hyp/nvhe/page_alloc.c b/arch/arm64/kvm/hyp/nvhe/page_alloc.c
index 41fc25bdfb34..a6e874e61a40 100644
--- a/arch/arm64/kvm/hyp/nvhe/page_alloc.c
+++ b/arch/arm64/kvm/hyp/nvhe/page_alloc.c
@@ -193,6 +193,20 @@ void hyp_get_page(struct hyp_pool *pool, void *addr)
 	hyp_spin_unlock(&pool->lock);
 }
 
+void hyp_split_page(struct hyp_page *p)
+{
+	unsigned short order = p->order;
+	unsigned int i;
+
+	p->order = 0;
+	for (i = 1; i < (1 << order); i++) {
+		struct hyp_page *tail = p + i;
+
+		tail->order = 0;
+		hyp_set_page_refcounted(tail);
+	}
+}
+
 void *hyp_alloc_pages(struct hyp_pool *pool, unsigned short order)
 {
 	unsigned short i = order;
-- 
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[PATCH v2 2/2] KVM: arm64: Report corrupted refcount at EL2

[Quentin Perret]

Some of the refcount manipulation helpers used at EL2 are instrumented
to catch a corrupted state, but not all of them are treated equally. Let's
make things more consistent by instrumenting hyp_page_ref_dec_and_test()
as well.

Acked-by: Will Deacon <will@kernel.org>
Suggested-by: Will Deacon <will@kernel.org>
Signed-off-by: Quentin Perret <qperret@google.com>
---
 arch/arm64/kvm/hyp/nvhe/page_alloc.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/arch/arm64/kvm/hyp/nvhe/page_alloc.c b/arch/arm64/kvm/hyp/nvhe/page_alloc.c
index a6e874e61a40..0bd7701ad1df 100644
--- a/arch/arm64/kvm/hyp/nvhe/page_alloc.c
+++ b/arch/arm64/kvm/hyp/nvhe/page_alloc.c
@@ -152,6 +152,7 @@ static inline void hyp_page_ref_inc(struct hyp_page *p)
 
 static inline int hyp_page_ref_dec_and_test(struct hyp_page *p)
 {
+	BUG_ON(!p->refcount);
 	p->refcount--;
 	return (p->refcount == 0);
 }
-- 
2.33.0.800.g4c38ced690-goog

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[PATCH 0/2] A couple of EL2 refcounts fixes

[Quentin Perret]

Hi all,

This addresses a couple of issues Will has found with the refcounting of
page-tables at EL2. Patch 01 fixes a nasty bug, and probably wants to go
in -stable. Patch 02 fixes a small inconsistency which made it harder to
find refcount-related bugs at EL2.

Feedback welcome !

Thanks,
Quentin

Quentin Perret (2):
  KVM: arm64: Fix host stage-2 PGD refcount
  KVM: arm64: Report corrupted refcount at EL2

 arch/arm64/kvm/hyp/include/nvhe/gfp.h |  1 +
 arch/arm64/kvm/hyp/nvhe/mem_protect.c |  6 +++++-
 arch/arm64/kvm/hyp/nvhe/page_alloc.c  | 15 +++++++++++++++
 3 files changed, 21 insertions(+), 1 deletion(-)

-- 
2.33.0.800.g4c38ced690-goog

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[PATCH 1/2] KVM: arm64: Fix host stage-2 PGD refcount

[Quentin Perret]

The KVM page-table library refcounts the pages of concatenated stage-2
PGDs individually. However, the host's stage-2 PGD is currently managed
by EL2 as a single high-order compound page, which can cause the
refcount of the tail pages to reach 0 when they really shouldn't, hence
corrupting the page-table.

Fix this by introducing a new hyp_split_page() helper in the EL2 page
allocator (matching EL1's split_page() function), and make use of it
from host_s2_zalloc_page().

Fixes: 1025c8c0c6ac ("KVM: arm64: Wrap the host with a stage 2")
Suggested-by: Will Deacon <will@kernel.org>
Signed-off-by: Quentin Perret <qperret@google.com>
---
 arch/arm64/kvm/hyp/include/nvhe/gfp.h |  1 +
 arch/arm64/kvm/hyp/nvhe/mem_protect.c |  6 +++++-
 arch/arm64/kvm/hyp/nvhe/page_alloc.c  | 14 ++++++++++++++
 3 files changed, 20 insertions(+), 1 deletion(-)

diff --git a/arch/arm64/kvm/hyp/include/nvhe/gfp.h b/arch/arm64/kvm/hyp/include/nvhe/gfp.h
index fb0f523d1492..0a048dc06a7d 100644
--- a/arch/arm64/kvm/hyp/include/nvhe/gfp.h
+++ b/arch/arm64/kvm/hyp/include/nvhe/gfp.h
@@ -24,6 +24,7 @@ struct hyp_pool {
 
 /* Allocation */
 void *hyp_alloc_pages(struct hyp_pool *pool, unsigned short order);
+void hyp_split_page(struct hyp_page *page);
 void hyp_get_page(struct hyp_pool *pool, void *addr);
 void hyp_put_page(struct hyp_pool *pool, void *addr);
 
diff --git a/arch/arm64/kvm/hyp/nvhe/mem_protect.c b/arch/arm64/kvm/hyp/nvhe/mem_protect.c
index bacd493a4eac..93a79736c283 100644
--- a/arch/arm64/kvm/hyp/nvhe/mem_protect.c
+++ b/arch/arm64/kvm/hyp/nvhe/mem_protect.c
@@ -35,7 +35,11 @@ const u8 pkvm_hyp_id = 1;
 
 static void *host_s2_zalloc_pages_exact(size_t size)
 {
-	return hyp_alloc_pages(&host_s2_pool, get_order(size));
+	void *addr = hyp_alloc_pages(&host_s2_pool, get_order(size));
+
+	hyp_split_page(hyp_virt_to_page(addr));
+
+	return addr;
 }
 
 static void *host_s2_zalloc_page(void *pool)
diff --git a/arch/arm64/kvm/hyp/nvhe/page_alloc.c b/arch/arm64/kvm/hyp/nvhe/page_alloc.c
index 41fc25bdfb34..a6e874e61a40 100644
--- a/arch/arm64/kvm/hyp/nvhe/page_alloc.c
+++ b/arch/arm64/kvm/hyp/nvhe/page_alloc.c
@@ -193,6 +193,20 @@ void hyp_get_page(struct hyp_pool *pool, void *addr)
 	hyp_spin_unlock(&pool->lock);
 }
 
+void hyp_split_page(struct hyp_page *p)
+{
+	unsigned short order = p->order;
+	unsigned int i;
+
+	p->order = 0;
+	for (i = 1; i < (1 << order); i++) {
+		struct hyp_page *tail = p + i;
+
+		tail->order = 0;
+		hyp_set_page_refcounted(tail);
+	}
+}
+
 void *hyp_alloc_pages(struct hyp_pool *pool, unsigned short order)
 {
 	unsigned short i = order;
-- 
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Re: [PATCH 1/2] KVM: arm64: Fix host stage-2 PGD refcount

[Marc Zyngier]

Hi Quentin,

On Mon, 04 Oct 2021 10:03:13 +0100,
Quentin Perret <qperret@google.com> wrote:
> 
> The KVM page-table library refcounts the pages of concatenated stage-2
> PGDs individually. However, the host's stage-2 PGD is currently managed
> by EL2 as a single high-order compound page, which can cause the
> refcount of the tail pages to reach 0 when they really shouldn't, hence
> corrupting the page-table.

nit: this comment only applies to the protected mode, right? As far as
I can tell, 'classic' KVM is just fine.

> 
> Fix this by introducing a new hyp_split_page() helper in the EL2 page
> allocator (matching EL1's split_page() function), and make use of it

uber nit: split_page() is not an EL1 function. more of a standard
kernel function.

> from host_s2_zalloc_page().
> 
> Fixes: 1025c8c0c6ac ("KVM: arm64: Wrap the host with a stage 2")
> Suggested-by: Will Deacon <will@kernel.org>
> Signed-off-by: Quentin Perret <qperret@google.com>
> ---
>  arch/arm64/kvm/hyp/include/nvhe/gfp.h |  1 +
>  arch/arm64/kvm/hyp/nvhe/mem_protect.c |  6 +++++-
>  arch/arm64/kvm/hyp/nvhe/page_alloc.c  | 14 ++++++++++++++
>  3 files changed, 20 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/arm64/kvm/hyp/include/nvhe/gfp.h b/arch/arm64/kvm/hyp/include/nvhe/gfp.h
> index fb0f523d1492..0a048dc06a7d 100644
> --- a/arch/arm64/kvm/hyp/include/nvhe/gfp.h
> +++ b/arch/arm64/kvm/hyp/include/nvhe/gfp.h
> @@ -24,6 +24,7 @@ struct hyp_pool {
>  
>  /* Allocation */
>  void *hyp_alloc_pages(struct hyp_pool *pool, unsigned short order);
> +void hyp_split_page(struct hyp_page *page);
>  void hyp_get_page(struct hyp_pool *pool, void *addr);
>  void hyp_put_page(struct hyp_pool *pool, void *addr);
>  
> diff --git a/arch/arm64/kvm/hyp/nvhe/mem_protect.c b/arch/arm64/kvm/hyp/nvhe/mem_protect.c
> index bacd493a4eac..93a79736c283 100644
> --- a/arch/arm64/kvm/hyp/nvhe/mem_protect.c
> +++ b/arch/arm64/kvm/hyp/nvhe/mem_protect.c
> @@ -35,7 +35,11 @@ const u8 pkvm_hyp_id = 1;
>  
>  static void *host_s2_zalloc_pages_exact(size_t size)
>  {
> -	return hyp_alloc_pages(&host_s2_pool, get_order(size));
> +	void *addr = hyp_alloc_pages(&host_s2_pool, get_order(size));
> +
> +	hyp_split_page(hyp_virt_to_page(addr));

The only reason this doesn't lead to a subsequent memory leak is that
concatenated page tables are always a power of two, right?

If so, that deserves a comment, because I don't think this works in
the general case unless you actively free the pages that are between
size and (1 << order).

> +
> +	return addr;
>  }
>  
>  static void *host_s2_zalloc_page(void *pool)
> diff --git a/arch/arm64/kvm/hyp/nvhe/page_alloc.c b/arch/arm64/kvm/hyp/nvhe/page_alloc.c
> index 41fc25bdfb34..a6e874e61a40 100644
> --- a/arch/arm64/kvm/hyp/nvhe/page_alloc.c
> +++ b/arch/arm64/kvm/hyp/nvhe/page_alloc.c
> @@ -193,6 +193,20 @@ void hyp_get_page(struct hyp_pool *pool, void *addr)
>  	hyp_spin_unlock(&pool->lock);
>  }
>  
> +void hyp_split_page(struct hyp_page *p)
> +{
> +	unsigned short order = p->order;
> +	unsigned int i;
> +
> +	p->order = 0;
> +	for (i = 1; i < (1 << order); i++) {
> +		struct hyp_page *tail = p + i;
> +
> +		tail->order = 0;
> +		hyp_set_page_refcounted(tail);
> +	}
> +}
> +
>  void *hyp_alloc_pages(struct hyp_pool *pool, unsigned short order)
>  {
>  	unsigned short i = order;

Thanks,

	M.

-- 
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Re: [PATCH 1/2] KVM: arm64: Fix host stage-2 PGD refcount

[Quentin Perret]

Hey Marc,

On Monday 04 Oct 2021 at 10:55:13 (+0100), Marc Zyngier wrote:
> Hi Quentin,
> 
> On Mon, 04 Oct 2021 10:03:13 +0100,
> Quentin Perret <qperret@google.com> wrote:
> > 
> > The KVM page-table library refcounts the pages of concatenated stage-2
> > PGDs individually. However, the host's stage-2 PGD is currently managed
> > by EL2 as a single high-order compound page, which can cause the
> > refcount of the tail pages to reach 0 when they really shouldn't, hence
> > corrupting the page-table.
> 
> nit: this comment only applies to the protected mode, right? As far as
> I can tell, 'classic' KVM is just fine.

Correct, this really only applies to the host stage-2, which implies
we're in protected mode. I'll make that a bit more explicit.

> > Fix this by introducing a new hyp_split_page() helper in the EL2 page
> > allocator (matching EL1's split_page() function), and make use of it
> 
> uber nit: split_page() is not an EL1 function. more of a standard
> kernel function.

Fair enough :)

> > from host_s2_zalloc_page().
> > 
> > Fixes: 1025c8c0c6ac ("KVM: arm64: Wrap the host with a stage 2")
> > Suggested-by: Will Deacon <will@kernel.org>
> > Signed-off-by: Quentin Perret <qperret@google.com>
> > ---
> >  arch/arm64/kvm/hyp/include/nvhe/gfp.h |  1 +
> >  arch/arm64/kvm/hyp/nvhe/mem_protect.c |  6 +++++-
> >  arch/arm64/kvm/hyp/nvhe/page_alloc.c  | 14 ++++++++++++++
> >  3 files changed, 20 insertions(+), 1 deletion(-)
> > 
> > diff --git a/arch/arm64/kvm/hyp/include/nvhe/gfp.h b/arch/arm64/kvm/hyp/include/nvhe/gfp.h
> > index fb0f523d1492..0a048dc06a7d 100644
> > --- a/arch/arm64/kvm/hyp/include/nvhe/gfp.h
> > +++ b/arch/arm64/kvm/hyp/include/nvhe/gfp.h
> > @@ -24,6 +24,7 @@ struct hyp_pool {
> >  
> >  /* Allocation */
> >  void *hyp_alloc_pages(struct hyp_pool *pool, unsigned short order);
> > +void hyp_split_page(struct hyp_page *page);
> >  void hyp_get_page(struct hyp_pool *pool, void *addr);
> >  void hyp_put_page(struct hyp_pool *pool, void *addr);
> >  
> > diff --git a/arch/arm64/kvm/hyp/nvhe/mem_protect.c b/arch/arm64/kvm/hyp/nvhe/mem_protect.c
> > index bacd493a4eac..93a79736c283 100644
> > --- a/arch/arm64/kvm/hyp/nvhe/mem_protect.c
> > +++ b/arch/arm64/kvm/hyp/nvhe/mem_protect.c
> > @@ -35,7 +35,11 @@ const u8 pkvm_hyp_id = 1;
> >  
> >  static void *host_s2_zalloc_pages_exact(size_t size)
> >  {
> > -	return hyp_alloc_pages(&host_s2_pool, get_order(size));
> > +	void *addr = hyp_alloc_pages(&host_s2_pool, get_order(size));
> > +
> > +	hyp_split_page(hyp_virt_to_page(addr));
> 
> The only reason this doesn't lead to a subsequent memory leak is that
> concatenated page tables are always a power of two, right?

Indeed, and also because the host stage-2 is _never_ freed, so that's
not memory we're going to reclaim anyway -- we don't have an
implementation of ->free_pages_exact() in the host stage-2 mm_ops.

> If so, that deserves a comment, because I don't think this works in
> the general case unless you actively free the pages that are between
> size and (1 << order).

Ack, that'll probably confuse me too in a few weeks, so a comment won't
hurt. I'll re-spin shortly.

Thanks,
Quentin
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