Messages in this thread Patch in this message | ![/](/images/icornerl.gif) | | Subject | [PATCH 22/37] x86, pkeys: optimize fault handling in access_error() | From | Dave Hansen <> | Date | Mon, 16 Nov 2015 19:35:42 -0800 |
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From: Dave Hansen <dave.hansen@linux.intel.com>
We might not strictly have to make modifictions to access_error() to check the VMA here.
If we do not, we will do this: 1. app sets VMA pkey to K 2. app touches a !present page 3. do_page_fault(), allocates and maps page, sets pte.pkey=K 4. return to userspace 5. touch instruction reexecutes, but triggers PF_PK 6. do PKEY signal
What happens with this patch applied: 1. app sets VMA pkey to K 2. app touches a !present page 3. do_page_fault() notices that K is inaccessible 4. do PKEY signal
We basically skip the fault that does an allocation.
So what this lets us do is protect areas from even being *populated* unless it is accessible according to protection keys. That seems handy to me and makes protection keys work more like an mprotect()'d mapping.
Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com> ---
b/arch/x86/mm/fault.c | 15 +++++++++++++++ 1 file changed, 15 insertions(+)
diff -puN arch/x86/mm/fault.c~pkeys-15-access_error arch/x86/mm/fault.c --- a/arch/x86/mm/fault.c~pkeys-15-access_error 2015-11-16 12:35:45.439630326 -0800 +++ b/arch/x86/mm/fault.c 2015-11-16 12:35:45.443630507 -0800 @@ -900,10 +900,16 @@ bad_area(struct pt_regs *regs, unsigned static inline bool bad_area_access_from_pkeys(unsigned long error_code, struct vm_area_struct *vma) { + /* This code is always called on the current mm */ + int foreign = 0; + if (!boot_cpu_has(X86_FEATURE_OSPKE)) return false; if (error_code & PF_PK) return true; + /* this checks permission keys on the VMA: */ + if (!arch_vma_access_permitted(vma, (error_code & PF_WRITE), foreign)) + return true; return false; } @@ -1086,6 +1092,8 @@ int show_unhandled_signals = 1; static inline int access_error(unsigned long error_code, struct vm_area_struct *vma) { + /* This is only called for the current mm, so: */ + int foreign = 0; /* * Access or read was blocked by protection keys. We do * this check before any others because we do not want @@ -1094,6 +1102,13 @@ access_error(unsigned long error_code, s */ if (error_code & PF_PK) return 1; + /* + * Make sure to check the VMA so that we do not perform + * faults just to hit a PF_PK as soon as we fill in a + * page. + */ + if (!arch_vma_access_permitted(vma, (error_code & PF_WRITE), foreign)) + return 1; if (error_code & PF_WRITE) { /* write, present and write, not present: */ _
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