x86: pkey-mprotect must allow pkey-0
Subsystems (3)
PATCH v2 1/1 x86: pkey-mprotect must allow pkey-0
Once an address range is associated with an allocated pkey, it cannot be reverted back to key-0. There is no valid reason for the above behavior. On the contrary applications need the ability to do so.
The patch relaxes the restriction.
Tested on x86_64.
--- arch/x86/include/asm/pkeys.h | 5 +++-- 1 file changed, 3 insertions(+), 2 deletions(-)
arch/x86/include/asm/pkeys.h +3 -2
@@ -52,7 +52,7 @@ bool mm_pkey_is_allocated(struct mm_struct *mm, int pkey)* from pkey_alloc(). pkey 0 is special, and never * returned from pkey_alloc(). */ -if (pkey <=0 ) +if (pkey <0 )return false;if (pkey >=arch_max_pkey ())return false;
@@ -92,7 +92,8 @@ int mm_pkey_alloc(struct mm_struct *mm)static inline int mm_pkey_free (struct mm_struct *mm,int pkey) { -if (!mm_pkey_is_allocated (mm, pkey)) +/* pkey 0 is special and can never be freed */ +if (!pkey || !mm_pkey_is_allocated (mm, pkey))return -EINVAL ;mm_set_pkey_free (mm, pkey);
Signature · 1 line
1.8.3.1
Quoted 3 lines · Ram Pai
Once an address range is associated with an allocated pkey, it cannot be reverted back to key-0. There is no valid reason for the above behavior. On the contrary applications need the ability to do so.
I'm trying to remember why we cared in the first place. :)
Could you add that to the changelog, please?
arch/x86/include/asm/pkeys.h int mm_pkey_alloc(struct mm_struct *mm) · line 97
Quoted 8 lines
arch/x86/include/asm/pkeys.h +2
-1@@ -92,7 +92,8 @@ int mm_pkey_alloc(struct mm_struct *mm)static inline int mm_pkey_free (struct mm_struct *mm,int pkey) { -if (!mm_pkey_is_allocated (mm, pkey)) +/* pkey 0 is special and can never be freed */ +if (!pkey || !mm_pkey_is_allocated (mm, pkey))return -EINVAL ;
If an app was being really careful, couldn't it free up all of the implicitly-pkey-0-assigned memory so that it is not in use at all? In that case, we might want to allow this.
On the other hand, nobody is likely to _ever_ actually do this so this is good shoot-yourself-in-the-foot protection.
Quoted 22 lines · Dave Hansen
Quoted 3 lines · Ram Pai
Once an address range is associated with an allocated pkey, it cannot be reverted back to key-0. There is no valid reason for the above behavior. On the contrary applications need the ability to do so.
I'm trying to remember why we cared in the first place. :)
Could you add that to the changelog, please?
arch/x86/include/asm/pkeys.h int mm_pkey_alloc(struct mm_struct *mm) · line 97
Quoted 8 lines
arch/x86/include/asm/pkeys.h +2
-1@@ -92,7 +92,8 @@ int mm_pkey_alloc(struct mm_struct *mm)static inline int mm_pkey_free (struct mm_struct *mm,int pkey) { -if (!mm_pkey_is_allocated (mm, pkey)) +/* pkey 0 is special and can never be freed */ +if (!pkey || !mm_pkey_is_allocated (mm, pkey))return -EINVAL ;If an app was being really careful, couldn't it free up all of the implicitly-pkey-0-assigned memory so that it is not in use at all? In that case, we might want to allow this.
Quoted 2 lines
On the other hand, nobody is likely to _ever_ actually do this so this is good shoot-yourself-in-the-foot protection.
I look at key-0 as 'the key'. It has special status.
(a) It always exist.
(b) it cannot be freed.
(c) it is assigned by default.
(d) its permissions cannot be modified.
(e) it bypasses key-permission checks when assigned.
An arch need not necessarily map 'the key-0' to its key-0. It could internally map it to any of its internal key of its choice, transparent to the application.
Do you see a problem to this view point?
RP
Quoted 2 lines · Ram Pai
I look at key-0 as 'the key'. It has special status.
(a) It always exist.
Do you mean "is always allocated"?
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(b) it cannot be freed.
This is the one I'm questioning.
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(c) it is assigned by default.
I agree on this totally. :)
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(d) its permissions cannot be modified.
Why not? You could pretty easily get a thread going that had its stack covered with another pkey and that was being very careful what it accesses. It could pretty easily set pkey-0's access or write-disable bits.
Quoted 1 line
(e) it bypasses key-permission checks when assigned.
I don't think this is necessary. I think the only rule we *need* is:
pkey-0 is allocated implicitly at execve() time. You do not need to call pkey_alloc() to allocate it.
Quoted 3 lines
An arch need not necessarily map 'the key-0' to its key-0. It could internally map it to any of its internal key of its choice, transparent to the application.
I don't understand what you are saying here.
Quoted 5 lines · Dave Hansen
Quoted 2 lines · Ram Pai
I look at key-0 as 'the key'. It has special status.
(a) It always exist.Do you mean "is always allocated"?
always allocated and cannot be freed. Hence always exists.
If we let it freed, than yes 'it is always allocated', but may not 'always exist'.
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(b) it cannot be freed.
This is the one I'm questioning.
this is a philosophical question. Should we allow the application shoot-its-own-feet or help it from doing so. I tend to gravitate towards the later.
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(c) it is assigned by default.
I agree on this totally. :)
good. we have some common ground :)
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(d) its permissions cannot be modified.
Why not? You could pretty easily get a thread going that had its stack covered with another pkey and that was being very careful what it accesses. It could pretty easily set pkey-0's access or write-disable bits.
ok. I see your point. Will not argue against it.
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(e) it bypasses key-permission checks when assigned.
I don't think this is necessary. I think the only rule we *need* is:
pkey-0 is allocated implicitly at
execve()time. You do not need to callpkey_alloc()to allocate it.
And can be explicitly associated with any address range ?
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An arch need not necessarily map 'the key-0' to its key-0. It could internally map it to any of its internal key of its choice, transparent to the application.
I don't understand what you are saying here.
I was trying to disassociate the notion that "application's key-0 means hardware's key-0". Nevermind. its not important for this discussion.
Signature · 1 line
Ram Pai
Quoted 6 lines · Ram Pai
Quoted 5 lines
Quoted 1 line
(e) it bypasses key-permission checks when assigned.
I don't think this is necessary. I think the only rule we *need* is:
pkey-0 is allocated implicitly at
execve()time. You do not need to callpkey_alloc()to allocate it.And can be explicitly associated with any address range ?
Yes, it should ideally be available for use just like any other key when allocated.