[PATCH] arm64/mm: Intercept pfn changes in set_pte_at()

[PATCH] arm64/mm: Intercept pfn changes in set_pte_at()

[Anshuman Khandual]

Changing pfn on a user page table mapped entry, without first going through
break-before-make (BBM) procedure is unsafe. This just updates set_pte_at()
to intercept such changes, via an updated pgattr_change_is_safe(). This new
check happens via __check_racy_pte_update(), which has now been renamed as
__check_safe_pte_update().

Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Will Deacon <will@kernel.org>
Cc: Mark Rutland <mark.rutland@arm.com> 
Cc: Andrew Morton <akpm@linux-foundation.org> 
Cc: linux-arm-kernel@lists.infradead.org
Cc: linux-kernel@vger.kernel.org
Signed-off-by: Anshuman Khandual <anshuman.khandual@arm.com>
---
This applies on v6.1-rc4

 arch/arm64/include/asm/pgtable.h | 8 ++++++--
 arch/arm64/mm/mmu.c              | 8 +++++++-
 2 files changed, 13 insertions(+), 3 deletions(-)

diff --git a/arch/arm64/include/asm/pgtable.h b/arch/arm64/include/asm/pgtable.h
index 71a1af42f0e8..6b8b24e6cd35 100644
--- a/arch/arm64/include/asm/pgtable.h
+++ b/arch/arm64/include/asm/pgtable.h
@@ -275,6 +275,7 @@ static inline void set_pte(pte_t *ptep, pte_t pte)
 }
 
 extern void __sync_icache_dcache(pte_t pteval);
+bool pgattr_change_is_safe(u64 old, u64 new);
 
 /*
  * PTE bits configuration in the presence of hardware Dirty Bit Management
@@ -292,7 +293,7 @@ extern void __sync_icache_dcache(pte_t pteval);
  *   PTE_DIRTY || (PTE_WRITE && !PTE_RDONLY)
  */
 
-static inline void __check_racy_pte_update(struct mm_struct *mm, pte_t *ptep,
+static inline void __check_safe_pte_update(struct mm_struct *mm, pte_t *ptep,
 					   pte_t pte)
 {
 	pte_t old_pte;
@@ -318,6 +319,9 @@ static inline void __check_racy_pte_update(struct mm_struct *mm, pte_t *ptep,
 	VM_WARN_ONCE(pte_write(old_pte) && !pte_dirty(pte),
 		     "%s: racy dirty state clearing: 0x%016llx -> 0x%016llx",
 		     __func__, pte_val(old_pte), pte_val(pte));
+	VM_WARN_ONCE(!pgattr_change_is_safe(pte_val(old_pte), pte_val(pte)),
+		     "%s: unsafe attribute change: 0x%016llx -> 0x%016llx",
+		     __func__, pte_val(old_pte), pte_val(pte));
 }
 
 static inline void __set_pte_at(struct mm_struct *mm, unsigned long addr,
@@ -346,7 +350,7 @@ static inline void __set_pte_at(struct mm_struct *mm, unsigned long addr,
 			mte_sync_tags(old_pte, pte);
 	}
 
-	__check_racy_pte_update(mm, ptep, pte);
+	__check_safe_pte_update(mm, ptep, pte);
 
 	set_pte(ptep, pte);
 }
diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c
index 9a7c38965154..6c928ea99ab3 100644
--- a/arch/arm64/mm/mmu.c
+++ b/arch/arm64/mm/mmu.c
@@ -133,7 +133,7 @@ static phys_addr_t __init early_pgtable_alloc(int shift)
 	return phys;
 }
 
-static bool pgattr_change_is_safe(u64 old, u64 new)
+bool pgattr_change_is_safe(u64 old, u64 new)
 {
 	/*
 	 * The following mapping attributes may be updated in live
@@ -145,6 +145,12 @@ static bool pgattr_change_is_safe(u64 old, u64 new)
 	if (old == 0 || new == 0)
 		return true;
 
+	/* If old and new ptes are valid, pfn should not change */
+	if (pte_valid(__pte(old)) && pte_valid(__pte(new))) {
+		if (pte_pfn(__pte(old)) != pte_pfn(__pte(new)))
+			return false;
+	}
+
 	/* live contiguous mappings may not be manipulated at all */
 	if ((old | new) & PTE_CONT)
 		return false;
-- 
2.25.1

[PATCH] arm64/mm: Intercept pfn changes in set_pte_at()

[Anshuman Khandual]

Changing pfn on a user page table mapped entry, without first going through
break-before-make (BBM) procedure is unsafe. This just updates set_pte_at()
to intercept such changes, via an updated pgattr_change_is_safe(). This new
check happens via __check_racy_pte_update(), which has now been renamed as
__check_safe_pte_update().

Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Will Deacon <will@kernel.org>
Cc: Mark Rutland <mark.rutland@arm.com> 
Cc: Andrew Morton <akpm@linux-foundation.org> 
Cc: linux-arm-kernel@lists.infradead.org
Cc: linux-kernel@vger.kernel.org
Signed-off-by: Anshuman Khandual <anshuman.khandual@arm.com>
---
This applies on v6.1-rc4

 arch/arm64/include/asm/pgtable.h | 8 ++++++--
 arch/arm64/mm/mmu.c              | 8 +++++++-
 2 files changed, 13 insertions(+), 3 deletions(-)

diff --git a/arch/arm64/include/asm/pgtable.h b/arch/arm64/include/asm/pgtable.h
index 71a1af42f0e8..6b8b24e6cd35 100644
--- a/arch/arm64/include/asm/pgtable.h
+++ b/arch/arm64/include/asm/pgtable.h
@@ -275,6 +275,7 @@ static inline void set_pte(pte_t *ptep, pte_t pte)
 }
 
 extern void __sync_icache_dcache(pte_t pteval);
+bool pgattr_change_is_safe(u64 old, u64 new);
 
 /*
  * PTE bits configuration in the presence of hardware Dirty Bit Management
@@ -292,7 +293,7 @@ extern void __sync_icache_dcache(pte_t pteval);
  *   PTE_DIRTY || (PTE_WRITE && !PTE_RDONLY)
  */
 
-static inline void __check_racy_pte_update(struct mm_struct *mm, pte_t *ptep,
+static inline void __check_safe_pte_update(struct mm_struct *mm, pte_t *ptep,
 					   pte_t pte)
 {
 	pte_t old_pte;
@@ -318,6 +319,9 @@ static inline void __check_racy_pte_update(struct mm_struct *mm, pte_t *ptep,
 	VM_WARN_ONCE(pte_write(old_pte) && !pte_dirty(pte),
 		     "%s: racy dirty state clearing: 0x%016llx -> 0x%016llx",
 		     __func__, pte_val(old_pte), pte_val(pte));
+	VM_WARN_ONCE(!pgattr_change_is_safe(pte_val(old_pte), pte_val(pte)),
+		     "%s: unsafe attribute change: 0x%016llx -> 0x%016llx",
+		     __func__, pte_val(old_pte), pte_val(pte));
 }
 
 static inline void __set_pte_at(struct mm_struct *mm, unsigned long addr,
@@ -346,7 +350,7 @@ static inline void __set_pte_at(struct mm_struct *mm, unsigned long addr,
 			mte_sync_tags(old_pte, pte);
 	}
 
-	__check_racy_pte_update(mm, ptep, pte);
+	__check_safe_pte_update(mm, ptep, pte);
 
 	set_pte(ptep, pte);
 }
diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c
index 9a7c38965154..6c928ea99ab3 100644
--- a/arch/arm64/mm/mmu.c
+++ b/arch/arm64/mm/mmu.c
@@ -133,7 +133,7 @@ static phys_addr_t __init early_pgtable_alloc(int shift)
 	return phys;
 }
 
-static bool pgattr_change_is_safe(u64 old, u64 new)
+bool pgattr_change_is_safe(u64 old, u64 new)
 {
 	/*
 	 * The following mapping attributes may be updated in live
@@ -145,6 +145,12 @@ static bool pgattr_change_is_safe(u64 old, u64 new)
 	if (old == 0 || new == 0)
 		return true;
 
+	/* If old and new ptes are valid, pfn should not change */
+	if (pte_valid(__pte(old)) && pte_valid(__pte(new))) {
+		if (pte_pfn(__pte(old)) != pte_pfn(__pte(new)))
+			return false;
+	}
+
 	/* live contiguous mappings may not be manipulated at all */
 	if ((old | new) & PTE_CONT)
 		return false;
-- 
2.25.1


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Re: [PATCH] arm64/mm: Intercept pfn changes in set_pte_at()

[Will Deacon]

On Wed, Nov 16, 2022 at 08:40:01AM +0530, Anshuman Khandual wrote:
> Changing pfn on a user page table mapped entry, without first going through
> break-before-make (BBM) procedure is unsafe. This just updates set_pte_at()
> to intercept such changes, via an updated pgattr_change_is_safe(). This new
> check happens via __check_racy_pte_update(), which has now been renamed as
> __check_safe_pte_update().
> 
> Cc: Catalin Marinas <catalin.marinas@arm.com>
> Cc: Will Deacon <will@kernel.org>
> Cc: Mark Rutland <mark.rutland@arm.com> 
> Cc: Andrew Morton <akpm@linux-foundation.org> 
> Cc: linux-arm-kernel@lists.infradead.org
> Cc: linux-kernel@vger.kernel.org
> Signed-off-by: Anshuman Khandual <anshuman.khandual@arm.com>
> ---
> This applies on v6.1-rc4
> 
>  arch/arm64/include/asm/pgtable.h | 8 ++++++--
>  arch/arm64/mm/mmu.c              | 8 +++++++-
>  2 files changed, 13 insertions(+), 3 deletions(-)

I remember Mark saying that BBM is sometimes violated by the core code in
cases where the pte isn't actually part of a live pgtable (e.g. if it's on
the stack or part of a newly allocated table). Won't that cause false
positives here?

Will

Re: [PATCH] arm64/mm: Intercept pfn changes in set_pte_at()

[Will Deacon]

On Wed, Nov 16, 2022 at 08:40:01AM +0530, Anshuman Khandual wrote:
> Changing pfn on a user page table mapped entry, without first going through
> break-before-make (BBM) procedure is unsafe. This just updates set_pte_at()
> to intercept such changes, via an updated pgattr_change_is_safe(). This new
> check happens via __check_racy_pte_update(), which has now been renamed as
> __check_safe_pte_update().
> 
> Cc: Catalin Marinas <catalin.marinas@arm.com>
> Cc: Will Deacon <will@kernel.org>
> Cc: Mark Rutland <mark.rutland@arm.com> 
> Cc: Andrew Morton <akpm@linux-foundation.org> 
> Cc: linux-arm-kernel@lists.infradead.org
> Cc: linux-kernel@vger.kernel.org
> Signed-off-by: Anshuman Khandual <anshuman.khandual@arm.com>
> ---
> This applies on v6.1-rc4
> 
>  arch/arm64/include/asm/pgtable.h | 8 ++++++--
>  arch/arm64/mm/mmu.c              | 8 +++++++-
>  2 files changed, 13 insertions(+), 3 deletions(-)

I remember Mark saying that BBM is sometimes violated by the core code in
cases where the pte isn't actually part of a live pgtable (e.g. if it's on
the stack or part of a newly allocated table). Won't that cause false
positives here?

Will

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Re: [PATCH] arm64/mm: Intercept pfn changes in set_pte_at()

[Anshuman Khandual]

On 11/18/22 19:43, Will Deacon wrote:
> On Wed, Nov 16, 2022 at 08:40:01AM +0530, Anshuman Khandual wrote:
>> Changing pfn on a user page table mapped entry, without first going through
>> break-before-make (BBM) procedure is unsafe. This just updates set_pte_at()
>> to intercept such changes, via an updated pgattr_change_is_safe(). This new
>> check happens via __check_racy_pte_update(), which has now been renamed as
>> __check_safe_pte_update().
>>
>> Cc: Catalin Marinas <catalin.marinas@arm.com>
>> Cc: Will Deacon <will@kernel.org>
>> Cc: Mark Rutland <mark.rutland@arm.com> 
>> Cc: Andrew Morton <akpm@linux-foundation.org> 
>> Cc: linux-arm-kernel@lists.infradead.org
>> Cc: linux-kernel@vger.kernel.org
>> Signed-off-by: Anshuman Khandual <anshuman.khandual@arm.com>
>> ---
>> This applies on v6.1-rc4
>>
>>  arch/arm64/include/asm/pgtable.h | 8 ++++++--
>>  arch/arm64/mm/mmu.c              | 8 +++++++-
>>  2 files changed, 13 insertions(+), 3 deletions(-)
> 
> I remember Mark saying that BBM is sometimes violated by the core code in
> cases where the pte isn't actually part of a live pgtable (e.g. if it's on
> the stack or part of a newly allocated table). Won't that cause false
> positives here?

Could you please elaborate ? If the pte is not on a live page table, then
pte_valid() will return negative on such entries. So any update there will
be safe. I am wondering, how this change will cause false positives which
would not have been possible earlier.

Re: [PATCH] arm64/mm: Intercept pfn changes in set_pte_at()

[Anshuman Khandual]

On 11/18/22 19:43, Will Deacon wrote:
> On Wed, Nov 16, 2022 at 08:40:01AM +0530, Anshuman Khandual wrote:
>> Changing pfn on a user page table mapped entry, without first going through
>> break-before-make (BBM) procedure is unsafe. This just updates set_pte_at()
>> to intercept such changes, via an updated pgattr_change_is_safe(). This new
>> check happens via __check_racy_pte_update(), which has now been renamed as
>> __check_safe_pte_update().
>>
>> Cc: Catalin Marinas <catalin.marinas@arm.com>
>> Cc: Will Deacon <will@kernel.org>
>> Cc: Mark Rutland <mark.rutland@arm.com> 
>> Cc: Andrew Morton <akpm@linux-foundation.org> 
>> Cc: linux-arm-kernel@lists.infradead.org
>> Cc: linux-kernel@vger.kernel.org
>> Signed-off-by: Anshuman Khandual <anshuman.khandual@arm.com>
>> ---
>> This applies on v6.1-rc4
>>
>>  arch/arm64/include/asm/pgtable.h | 8 ++++++--
>>  arch/arm64/mm/mmu.c              | 8 +++++++-
>>  2 files changed, 13 insertions(+), 3 deletions(-)
> 
> I remember Mark saying that BBM is sometimes violated by the core code in
> cases where the pte isn't actually part of a live pgtable (e.g. if it's on
> the stack or part of a newly allocated table). Won't that cause false
> positives here?

Could you please elaborate ? If the pte is not on a live page table, then
pte_valid() will return negative on such entries. So any update there will
be safe. I am wondering, how this change will cause false positives which
would not have been possible earlier.

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Re: [PATCH] arm64/mm: Intercept pfn changes in set_pte_at()

[Will Deacon]

On Tue, Nov 22, 2022 at 01:43:17PM +0530, Anshuman Khandual wrote:
> 
> 
> On 11/18/22 19:43, Will Deacon wrote:
> > On Wed, Nov 16, 2022 at 08:40:01AM +0530, Anshuman Khandual wrote:
> >> Changing pfn on a user page table mapped entry, without first going through
> >> break-before-make (BBM) procedure is unsafe. This just updates set_pte_at()
> >> to intercept such changes, via an updated pgattr_change_is_safe(). This new
> >> check happens via __check_racy_pte_update(), which has now been renamed as
> >> __check_safe_pte_update().
> >>
> >> Cc: Catalin Marinas <catalin.marinas@arm.com>
> >> Cc: Will Deacon <will@kernel.org>
> >> Cc: Mark Rutland <mark.rutland@arm.com> 
> >> Cc: Andrew Morton <akpm@linux-foundation.org> 
> >> Cc: linux-arm-kernel@lists.infradead.org
> >> Cc: linux-kernel@vger.kernel.org
> >> Signed-off-by: Anshuman Khandual <anshuman.khandual@arm.com>
> >> ---
> >> This applies on v6.1-rc4
> >>
> >>  arch/arm64/include/asm/pgtable.h | 8 ++++++--
> >>  arch/arm64/mm/mmu.c              | 8 +++++++-
> >>  2 files changed, 13 insertions(+), 3 deletions(-)
> > 
> > I remember Mark saying that BBM is sometimes violated by the core code in
> > cases where the pte isn't actually part of a live pgtable (e.g. if it's on
> > the stack or part of a newly allocated table). Won't that cause false
> > positives here?
> 
> Could you please elaborate ? If the pte is not on a live page table, then
> pte_valid() will return negative on such entries. So any update there will
> be safe. I am wondering, how this change will cause false positives which
> would not have been possible earlier.

I don't think pte_valid() will always return false for these entries.
Consider, for example, ptes which are valid but which live in a table that
is not reachable by the MMU. I think this is what Mark had in mind, but it
would be helpful if he could chime in with the specific example he ran into.

Will

Re: [PATCH] arm64/mm: Intercept pfn changes in set_pte_at()

[Will Deacon]

On Tue, Nov 22, 2022 at 01:43:17PM +0530, Anshuman Khandual wrote:
> 
> 
> On 11/18/22 19:43, Will Deacon wrote:
> > On Wed, Nov 16, 2022 at 08:40:01AM +0530, Anshuman Khandual wrote:
> >> Changing pfn on a user page table mapped entry, without first going through
> >> break-before-make (BBM) procedure is unsafe. This just updates set_pte_at()
> >> to intercept such changes, via an updated pgattr_change_is_safe(). This new
> >> check happens via __check_racy_pte_update(), which has now been renamed as
> >> __check_safe_pte_update().
> >>
> >> Cc: Catalin Marinas <catalin.marinas@arm.com>
> >> Cc: Will Deacon <will@kernel.org>
> >> Cc: Mark Rutland <mark.rutland@arm.com> 
> >> Cc: Andrew Morton <akpm@linux-foundation.org> 
> >> Cc: linux-arm-kernel@lists.infradead.org
> >> Cc: linux-kernel@vger.kernel.org
> >> Signed-off-by: Anshuman Khandual <anshuman.khandual@arm.com>
> >> ---
> >> This applies on v6.1-rc4
> >>
> >>  arch/arm64/include/asm/pgtable.h | 8 ++++++--
> >>  arch/arm64/mm/mmu.c              | 8 +++++++-
> >>  2 files changed, 13 insertions(+), 3 deletions(-)
> > 
> > I remember Mark saying that BBM is sometimes violated by the core code in
> > cases where the pte isn't actually part of a live pgtable (e.g. if it's on
> > the stack or part of a newly allocated table). Won't that cause false
> > positives here?
> 
> Could you please elaborate ? If the pte is not on a live page table, then
> pte_valid() will return negative on such entries. So any update there will
> be safe. I am wondering, how this change will cause false positives which
> would not have been possible earlier.

I don't think pte_valid() will always return false for these entries.
Consider, for example, ptes which are valid but which live in a table that
is not reachable by the MMU. I think this is what Mark had in mind, but it
would be helpful if he could chime in with the specific example he ran into.

Will

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Re: [PATCH] arm64/mm: Intercept pfn changes in set_pte_at()

[Mark Rutland]

On Tue, Nov 22, 2022 at 09:57:49AM +0000, Will Deacon wrote:
> On Tue, Nov 22, 2022 at 01:43:17PM +0530, Anshuman Khandual wrote:
> > 
> > 
> > On 11/18/22 19:43, Will Deacon wrote:
> > > On Wed, Nov 16, 2022 at 08:40:01AM +0530, Anshuman Khandual wrote:
> > >> Changing pfn on a user page table mapped entry, without first going through
> > >> break-before-make (BBM) procedure is unsafe. This just updates set_pte_at()
> > >> to intercept such changes, via an updated pgattr_change_is_safe(). This new
> > >> check happens via __check_racy_pte_update(), which has now been renamed as
> > >> __check_safe_pte_update().
> > >>
> > >> Cc: Catalin Marinas <catalin.marinas@arm.com>
> > >> Cc: Will Deacon <will@kernel.org>
> > >> Cc: Mark Rutland <mark.rutland@arm.com> 
> > >> Cc: Andrew Morton <akpm@linux-foundation.org> 
> > >> Cc: linux-arm-kernel@lists.infradead.org
> > >> Cc: linux-kernel@vger.kernel.org
> > >> Signed-off-by: Anshuman Khandual <anshuman.khandual@arm.com>
> > >> ---
> > >> This applies on v6.1-rc4
> > >>
> > >>  arch/arm64/include/asm/pgtable.h | 8 ++++++--
> > >>  arch/arm64/mm/mmu.c              | 8 +++++++-
> > >>  2 files changed, 13 insertions(+), 3 deletions(-)
> > > 
> > > I remember Mark saying that BBM is sometimes violated by the core code in
> > > cases where the pte isn't actually part of a live pgtable (e.g. if it's on
> > > the stack or part of a newly allocated table). Won't that cause false
> > > positives here?
> > 
> > Could you please elaborate ? If the pte is not on a live page table, then
> > pte_valid() will return negative on such entries. So any update there will
> > be safe. I am wondering, how this change will cause false positives which
> > would not have been possible earlier.
> 
> I don't think pte_valid() will always return false for these entries.
> Consider, for example, ptes which are valid but which live in a table that
> is not reachable by the MMU. I think this is what Mark had in mind, but it
> would be helpful if he could chime in with the specific example he ran into.

Yup -- that was the case I had in mind. IIRC I hit that in the past when trying
to do something similar, but I can't recall exactly where that was. I suspect
that was probably to do with page migration or huge page splitting/merging.

Looking around, at least __split_huge_zero_page_pmd() and
__split_huge_pmd_locked() do something like that, creating a temporary pmd
entry on the stack, populating a table of non-live but valid ptes, then
plumbing it into the real pmd.

We'd need to check that there aren't other cases like that.

Thanks,
Mark.

Re: [PATCH] arm64/mm: Intercept pfn changes in set_pte_at()

[Mark Rutland]

On Tue, Nov 22, 2022 at 09:57:49AM +0000, Will Deacon wrote:
> On Tue, Nov 22, 2022 at 01:43:17PM +0530, Anshuman Khandual wrote:
> > 
> > 
> > On 11/18/22 19:43, Will Deacon wrote:
> > > On Wed, Nov 16, 2022 at 08:40:01AM +0530, Anshuman Khandual wrote:
> > >> Changing pfn on a user page table mapped entry, without first going through
> > >> break-before-make (BBM) procedure is unsafe. This just updates set_pte_at()
> > >> to intercept such changes, via an updated pgattr_change_is_safe(). This new
> > >> check happens via __check_racy_pte_update(), which has now been renamed as
> > >> __check_safe_pte_update().
> > >>
> > >> Cc: Catalin Marinas <catalin.marinas@arm.com>
> > >> Cc: Will Deacon <will@kernel.org>
> > >> Cc: Mark Rutland <mark.rutland@arm.com> 
> > >> Cc: Andrew Morton <akpm@linux-foundation.org> 
> > >> Cc: linux-arm-kernel@lists.infradead.org
> > >> Cc: linux-kernel@vger.kernel.org
> > >> Signed-off-by: Anshuman Khandual <anshuman.khandual@arm.com>
> > >> ---
> > >> This applies on v6.1-rc4
> > >>
> > >>  arch/arm64/include/asm/pgtable.h | 8 ++++++--
> > >>  arch/arm64/mm/mmu.c              | 8 +++++++-
> > >>  2 files changed, 13 insertions(+), 3 deletions(-)
> > > 
> > > I remember Mark saying that BBM is sometimes violated by the core code in
> > > cases where the pte isn't actually part of a live pgtable (e.g. if it's on
> > > the stack or part of a newly allocated table). Won't that cause false
> > > positives here?
> > 
> > Could you please elaborate ? If the pte is not on a live page table, then
> > pte_valid() will return negative on such entries. So any update there will
> > be safe. I am wondering, how this change will cause false positives which
> > would not have been possible earlier.
> 
> I don't think pte_valid() will always return false for these entries.
> Consider, for example, ptes which are valid but which live in a table that
> is not reachable by the MMU. I think this is what Mark had in mind, but it
> would be helpful if he could chime in with the specific example he ran into.

Yup -- that was the case I had in mind. IIRC I hit that in the past when trying
to do something similar, but I can't recall exactly where that was. I suspect
that was probably to do with page migration or huge page splitting/merging.

Looking around, at least __split_huge_zero_page_pmd() and
__split_huge_pmd_locked() do something like that, creating a temporary pmd
entry on the stack, populating a table of non-live but valid ptes, then
plumbing it into the real pmd.

We'd need to check that there aren't other cases like that.

Thanks,
Mark.

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Re: [PATCH] arm64/mm: Intercept pfn changes in set_pte_at()

[Anshuman Khandual]

On 11/22/22 16:41, Mark Rutland wrote:
> On Tue, Nov 22, 2022 at 09:57:49AM +0000, Will Deacon wrote:
>> On Tue, Nov 22, 2022 at 01:43:17PM +0530, Anshuman Khandual wrote:
>>>
>>>
>>> On 11/18/22 19:43, Will Deacon wrote:
>>>> On Wed, Nov 16, 2022 at 08:40:01AM +0530, Anshuman Khandual wrote:
>>>>> Changing pfn on a user page table mapped entry, without first going through
>>>>> break-before-make (BBM) procedure is unsafe. This just updates set_pte_at()
>>>>> to intercept such changes, via an updated pgattr_change_is_safe(). This new
>>>>> check happens via __check_racy_pte_update(), which has now been renamed as
>>>>> __check_safe_pte_update().
>>>>>
>>>>> Cc: Catalin Marinas <catalin.marinas@arm.com>
>>>>> Cc: Will Deacon <will@kernel.org>
>>>>> Cc: Mark Rutland <mark.rutland@arm.com> 
>>>>> Cc: Andrew Morton <akpm@linux-foundation.org> 
>>>>> Cc: linux-arm-kernel@lists.infradead.org
>>>>> Cc: linux-kernel@vger.kernel.org
>>>>> Signed-off-by: Anshuman Khandual <anshuman.khandual@arm.com>
>>>>> ---
>>>>> This applies on v6.1-rc4
>>>>>
>>>>>  arch/arm64/include/asm/pgtable.h | 8 ++++++--
>>>>>  arch/arm64/mm/mmu.c              | 8 +++++++-
>>>>>  2 files changed, 13 insertions(+), 3 deletions(-)
>>>>
>>>> I remember Mark saying that BBM is sometimes violated by the core code in
>>>> cases where the pte isn't actually part of a live pgtable (e.g. if it's on
>>>> the stack or part of a newly allocated table). Won't that cause false
>>>> positives here?
>>>
>>> Could you please elaborate ? If the pte is not on a live page table, then
>>> pte_valid() will return negative on such entries. So any update there will
>>> be safe. I am wondering, how this change will cause false positives which
>>> would not have been possible earlier.
>>
>> I don't think pte_valid() will always return false for these entries.
>> Consider, for example, ptes which are valid but which live in a table that
>> is not reachable by the MMU. I think this is what Mark had in mind, but it
>> would be helpful if he could chime in with the specific example he ran into.
> 
> Yup -- that was the case I had in mind. IIRC I hit that in the past when trying
> to do something similar, but I can't recall exactly where that was. I suspect
> that was probably to do with page migration or huge page splitting/merging.
> 
> Looking around, at least __split_huge_zero_page_pmd() and
> __split_huge_pmd_locked() do something like that, creating a temporary pmd
> entry on the stack, populating a table of non-live but valid ptes, then
> plumbing it into the real pmd.

In both cases i.e __split_huge_zero_page_pmd() and __split_huge_pmd_locked(), the
entry is first asserted to be empty via pte_none(), before writing a new value in
there. set_pte_at() would still consider such updates safe because pfn_valid(old)
will return negative on such entries.

	VM_BUG_ON(!pte_none(*pte));
	set_pte_at(mm, haddr, pte, entry);

But if these entries still get updated yet again (while still being inactive) with
new pte values, then set_pte_at() would complain for the pfn update on the entry,
while being "valid". But is this a viable scenario ?

> 
> We'd need to check that there aren't other cases like that.
> 
Sure, might be some what tricky but anything in particular to be looked into ? I
guess if this change gets into a CI system which runs all memory stress tests for
long enough with CONFIG_DEBUG_VM enabled, we might get some more clue if there 
are other similar scenarios possible. 

Re: [PATCH] arm64/mm: Intercept pfn changes in set_pte_at()

[Anshuman Khandual]

On 11/22/22 16:41, Mark Rutland wrote:
> On Tue, Nov 22, 2022 at 09:57:49AM +0000, Will Deacon wrote:
>> On Tue, Nov 22, 2022 at 01:43:17PM +0530, Anshuman Khandual wrote:
>>>
>>>
>>> On 11/18/22 19:43, Will Deacon wrote:
>>>> On Wed, Nov 16, 2022 at 08:40:01AM +0530, Anshuman Khandual wrote:
>>>>> Changing pfn on a user page table mapped entry, without first going through
>>>>> break-before-make (BBM) procedure is unsafe. This just updates set_pte_at()
>>>>> to intercept such changes, via an updated pgattr_change_is_safe(). This new
>>>>> check happens via __check_racy_pte_update(), which has now been renamed as
>>>>> __check_safe_pte_update().
>>>>>
>>>>> Cc: Catalin Marinas <catalin.marinas@arm.com>
>>>>> Cc: Will Deacon <will@kernel.org>
>>>>> Cc: Mark Rutland <mark.rutland@arm.com> 
>>>>> Cc: Andrew Morton <akpm@linux-foundation.org> 
>>>>> Cc: linux-arm-kernel@lists.infradead.org
>>>>> Cc: linux-kernel@vger.kernel.org
>>>>> Signed-off-by: Anshuman Khandual <anshuman.khandual@arm.com>
>>>>> ---
>>>>> This applies on v6.1-rc4
>>>>>
>>>>>  arch/arm64/include/asm/pgtable.h | 8 ++++++--
>>>>>  arch/arm64/mm/mmu.c              | 8 +++++++-
>>>>>  2 files changed, 13 insertions(+), 3 deletions(-)
>>>>
>>>> I remember Mark saying that BBM is sometimes violated by the core code in
>>>> cases where the pte isn't actually part of a live pgtable (e.g. if it's on
>>>> the stack or part of a newly allocated table). Won't that cause false
>>>> positives here?
>>>
>>> Could you please elaborate ? If the pte is not on a live page table, then
>>> pte_valid() will return negative on such entries. So any update there will
>>> be safe. I am wondering, how this change will cause false positives which
>>> would not have been possible earlier.
>>
>> I don't think pte_valid() will always return false for these entries.
>> Consider, for example, ptes which are valid but which live in a table that
>> is not reachable by the MMU. I think this is what Mark had in mind, but it
>> would be helpful if he could chime in with the specific example he ran into.
> 
> Yup -- that was the case I had in mind. IIRC I hit that in the past when trying
> to do something similar, but I can't recall exactly where that was. I suspect
> that was probably to do with page migration or huge page splitting/merging.
> 
> Looking around, at least __split_huge_zero_page_pmd() and
> __split_huge_pmd_locked() do something like that, creating a temporary pmd
> entry on the stack, populating a table of non-live but valid ptes, then
> plumbing it into the real pmd.

In both cases i.e __split_huge_zero_page_pmd() and __split_huge_pmd_locked(), the
entry is first asserted to be empty via pte_none(), before writing a new value in
there. set_pte_at() would still consider such updates safe because pfn_valid(old)
will return negative on such entries.

	VM_BUG_ON(!pte_none(*pte));
	set_pte_at(mm, haddr, pte, entry);

But if these entries still get updated yet again (while still being inactive) with
new pte values, then set_pte_at() would complain for the pfn update on the entry,
while being "valid". But is this a viable scenario ?

> 
> We'd need to check that there aren't other cases like that.
> 
Sure, might be some what tricky but anything in particular to be looked into ? I
guess if this change gets into a CI system which runs all memory stress tests for
long enough with CONFIG_DEBUG_VM enabled, we might get some more clue if there 
are other similar scenarios possible. 

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