From: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
We try to enforce protection keys in software the same way that we
do in hardware. (See long example below).
But, we only want to do this when accessing our *own* process's
memory. If GDB set PKRU[6].AD=1 (disable access to PKEY 6), then
tried to PTRACE_POKE a target process which just happened to have
some mprotect_pkey(pkey=6) memory, we do *not* want to deny the
debugger access to that memory. PKRU is fundamentally a
thread-local structure and we do not want to enforce it on access
to _another_ thread's data.
This gets especially tricky when we have workqueues or other
delayed-work mechanisms that might run in a random process's context.
We can check that we only enforce pkeys when operating on our *own* mm,
but delayed work gets performed when a random user context is active.
We might end up with a situation where a delayed-work gup fails when
running randomly under its "own" task but succeeds when running under
another process. We want to avoid that.
To avoid that, we add a GUP flag: FOLL_FOREIGN and a fault flag:
FAULT_FLAG_FOREIGN. They indicate that we are walking an mm
which is not guranteed to be the same as current->mm and should
not be subject to protection key enforcement.
Thanks to Jerome Glisse for pointing out this scenario.
*** Why do we enforce protection keys in software?? ***
Imagine that we disabled access to the memory pointer to by 'buf'.
The, we implemented sys_write() like this:
sys_read(fd, buf, len...)
{
struct page *page = follow_page(buf);
void *buf_mapped = kmap(page);
memcpy(buf_mapped, fd_data, len);
...
}
This writes to 'buf' via a *kernel* mapping, without a protection
key. While this implementation does the same thing:
sys_read(fd, buf, len...)
{
copy_to_user(buf, fd_data, len);
...
}
but would hit a protection key fault because the userspace 'buf'
mapping has a protection key set.
To provide consistency, and to make key-protected memory work
as much like mprotect()ed memory as possible, we try to enforce
the same protections as the hardware would when the *kernel* walks
the page tables (and other mm structures).
Signed-off-by: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
Cc: linux-arch@xxxxxxxxxxxxxxx
---
b/arch/powerpc/include/asm/mmu_context.h | 3 ++-
b/arch/s390/include/asm/mmu_context.h | 3 ++-
b/arch/unicore32/include/asm/mmu_context.h | 3 ++-
b/arch/x86/include/asm/mmu_context.h | 5 +++--
b/drivers/iommu/amd_iommu_v2.c | 8 +++++---
b/include/asm-generic/mm_hooks.h | 3 ++-
b/include/linux/mm.h | 2 ++
b/mm/gup.c | 13 +++++++++----
b/mm/ksm.c | 10 ++++++++--
b/mm/memory.c | 3 ++-
10 files changed, 37 insertions(+), 16 deletions(-)
diff -puN arch/powerpc/include/asm/mmu_context.h~pkeys-12-gup-fault-foreign-flag arch/powerpc/include/asm/mmu_context.h
--- a/arch/powerpc/include/asm/mmu_context.h~pkeys-12-gup-fault-foreign-flag 2015-11-16 12:35:44.769599914 -0800
+++ b/arch/powerpc/include/asm/mmu_context.h 2015-11-16 12:35:44.788600776 -0800
@@ -148,7 +148,8 @@ static inline void arch_bprm_mm_init(str
{
}
-static inline bool arch_vma_access_permitted(struct vm_area_struct *vma, bool write)
+static inline bool arch_vma_access_permitted(struct vm_area_struct *vma,
+ bool write, bool foreign)
{
/* by default, allow everything */
return true;
diff -puN arch/s390/include/asm/mmu_context.h~pkeys-12-gup-fault-foreign-flag arch/s390/include/asm/mmu_context.h
--- a/arch/s390/include/asm/mmu_context.h~pkeys-12-gup-fault-foreign-flag 2015-11-16 12:35:44.771600005 -0800
+++ b/arch/s390/include/asm/mmu_context.h 2015-11-16 12:35:44.789600822 -0800
@@ -130,7 +130,8 @@ static inline void arch_bprm_mm_init(str
{
}
-static inline bool arch_vma_access_permitted(struct vm_area_struct *vma, bool write)
+static inline bool arch_vma_access_permitted(struct vm_area_struct *vma,
+ bool write, bool foreign)
{
/* by default, allow everything */
return true;
diff -puN arch/unicore32/include/asm/mmu_context.h~pkeys-12-gup-fault-foreign-flag arch/unicore32/include/asm/mmu_context.h
--- a/arch/unicore32/include/asm/mmu_context.h~pkeys-12-gup-fault-foreign-flag 2015-11-16 12:35:44.773600095 -0800
+++ b/arch/unicore32/include/asm/mmu_context.h 2015-11-16 12:35:44.789600822 -0800
@@ -97,7 +97,8 @@ static inline void arch_bprm_mm_init(str
{
}
-static inline bool arch_vma_access_permitted(struct vm_area_struct *vma, bool write)
+static inline bool arch_vma_access_permitted(struct vm_area_struct *vma,
+ bool write, bool foreign)
{
/* by default, allow everything */
return true;
diff -puN arch/x86/include/asm/mmu_context.h~pkeys-12-gup-fault-foreign-flag arch/x86/include/asm/mmu_context.h
--- a/arch/x86/include/asm/mmu_context.h~pkeys-12-gup-fault-foreign-flag 2015-11-16 12:35:44.774600141 -0800
+++ b/arch/x86/include/asm/mmu_context.h 2015-11-16 12:35:44.789600822 -0800
@@ -299,10 +299,11 @@ static inline bool vma_is_foreign(struct
return false;
}
-static inline bool arch_vma_access_permitted(struct vm_area_struct *vma, bool write)
+static inline bool arch_vma_access_permitted(struct vm_area_struct *vma,
+ bool write, bool foreign)
{
/* allow access if the VMA is not one from this process */
- if (vma_is_foreign(vma))
+ if (foreign || vma_is_foreign(vma))
return true;
return __pkru_allows_pkey(vma_pkey(vma), write);
}
diff -puN drivers/iommu/amd_iommu_v2.c~pkeys-12-gup-fault-foreign-flag drivers/iommu/amd_iommu_v2.c
--- a/drivers/iommu/amd_iommu_v2.c~pkeys-12-gup-fault-foreign-flag 2015-11-16 12:35:44.776600231 -0800
+++ b/drivers/iommu/amd_iommu_v2.c 2015-11-16 12:35:44.790600867 -0800
@@ -500,9 +500,11 @@ static void do_fault(struct work_struct
struct mm_struct *mm;
struct vm_area_struct *vma;
u64 address;
- int ret, write;
+ int ret, flags;
- write = !!(fault->flags & PPR_FAULT_WRITE);
+ if (fault->flags & PPR_FAULT_WRITE)
+ flags = FAULT_FLAG_WRITE;
+ flags |= FAULT_FLAG_FOREIGN;
mm = fault->state->mm;
address = fault->address;
@@ -523,7 +525,7 @@ static void do_fault(struct work_struct
goto out;
}
- ret = handle_mm_fault(mm, vma, address, write);
+ ret = handle_mm_fault(mm, vma, address, flags);
if (ret & VM_FAULT_ERROR) {
/* failed to service fault */
up_read(&mm->mmap_sem);
diff -puN include/asm-generic/mm_hooks.h~pkeys-12-gup-fault-foreign-flag include/asm-generic/mm_hooks.h
--- a/include/asm-generic/mm_hooks.h~pkeys-12-gup-fault-foreign-flag 2015-11-16 12:35:44.778600322 -0800
+++ b/include/asm-generic/mm_hooks.h 2015-11-16 12:35:44.790600867 -0800
@@ -26,7 +26,8 @@ static inline void arch_bprm_mm_init(str
{
}
-static inline bool arch_vma_access_permitted(struct vm_area_struct *vma, bool write)
+static inline bool arch_vma_access_permitted(struct vm_area_struct *vma,
+ bool write, bool foreign)
{
/* by default, allow everything */
return true;
diff -puN include/linux/mm.h~pkeys-12-gup-fault-foreign-flag include/linux/mm.h
--- a/include/linux/mm.h~pkeys-12-gup-fault-foreign-flag 2015-11-16 12:35:44.779600368 -0800
+++ b/include/linux/mm.h 2015-11-16 12:35:44.791600912 -0800
@@ -232,6 +232,7 @@ extern pgprot_t protection_map[16];
#define FAULT_FLAG_KILLABLE 0x10 /* The fault task is in SIGKILL killable region */
#define FAULT_FLAG_TRIED 0x20 /* Second try */
#define FAULT_FLAG_USER 0x40 /* The fault originated in userspace */
+#define FAULT_FLAG_FOREIGN 0x80 /* faulting for non current tsk/mm */
/*
* vm_fault is filled by the the pagefault handler and passed to the vma's
@@ -2139,6 +2140,7 @@ static inline struct page *follow_page(s
#define FOLL_MIGRATION 0x400 /* wait for page to replace migration entry */
#define FOLL_TRIED 0x800 /* a retry, previous pass started an IO */
#define FOLL_MLOCK 0x1000 /* lock present pages */
+#define FOLL_FOREIGN 0x2000 /* we are working on non-current tsk/mm */
typedef int (*pte_fn_t)(pte_t *pte, pgtable_t token, unsigned long addr,
void *data);
diff -puN mm/gup.c~pkeys-12-gup-fault-foreign-flag mm/gup.c
--- a/mm/gup.c~pkeys-12-gup-fault-foreign-flag 2015-11-16 12:35:44.781600458 -0800
+++ b/mm/gup.c 2015-11-16 12:35:44.792600958 -0800
@@ -310,6 +310,8 @@ static int faultin_page(struct task_stru
return -ENOENT;
if (*flags & FOLL_WRITE)
fault_flags |= FAULT_FLAG_WRITE;
+ if (*flags & FOLL_FOREIGN)
+ fault_flags |= FAULT_FLAG_FOREIGN;
if (nonblocking)
fault_flags |= FAULT_FLAG_ALLOW_RETRY;
if (*flags & FOLL_NOWAIT)
@@ -360,11 +362,13 @@ static int faultin_page(struct task_stru
static int check_vma_flags(struct vm_area_struct *vma, unsigned long gup_flags)
{
vm_flags_t vm_flags = vma->vm_flags;
+ int write = (gup_flags & FOLL_WRITE);
+ int foreign = (gup_flags & FOLL_FOREIGN);
if (vm_flags & (VM_IO | VM_PFNMAP))
return -EFAULT;
- if (gup_flags & FOLL_WRITE) {
+ if (write) {
if (!(vm_flags & VM_WRITE)) {
if (!(gup_flags & FOLL_FORCE))
return -EFAULT;
@@ -392,7 +396,7 @@ static int check_vma_flags(struct vm_are
if (!(vm_flags & VM_MAYREAD))
return -EFAULT;
}
- if (!arch_vma_access_permitted(vma, (gup_flags & FOLL_WRITE)))
+ if (!arch_vma_access_permitted(vma, write, foreign))
return -EFAULT;
return 0;
}
@@ -563,6 +567,7 @@ EXPORT_SYMBOL(__get_user_pages);
bool vma_permits_fault(struct vm_area_struct *vma, unsigned int fault_flags)
{
int write = (fault_flags & FAULT_FLAG_WRITE);
+ int foreign = (fault_flags & FAULT_FLAG_FOREIGN);
vm_flags_t vm_flags = write ? VM_WRITE : VM_READ;
if (!(vm_flags & vma->vm_flags))
@@ -570,9 +575,9 @@ bool vma_permits_fault(struct vm_area_st
/*
* The architecture might have a hardware protection
- * mechanism other than read/write that can deny access
+ * mechanism other than read/write that can deny access.
*/
- if (!arch_vma_access_permitted(vma, write))
+ if (!arch_vma_access_permitted(vma, write, foreign))
return false;
return true;
diff -puN mm/ksm.c~pkeys-12-gup-fault-foreign-flag mm/ksm.c
--- a/mm/ksm.c~pkeys-12-gup-fault-foreign-flag 2015-11-16 12:35:44.783600549 -0800
+++ b/mm/ksm.c 2015-11-16 12:35:44.793601003 -0800
@@ -359,6 +359,10 @@ static inline bool ksm_test_exit(struct
* in case the application has unmapped and remapped mm,addr meanwhile.
* Could a ksm page appear anywhere else? Actually yes, in a VM_PFNMAP
* mmap of /dev/mem or /dev/kmem, where we would not want to touch it.
+ *
+ * FAULT_FLAG/FOLL_FOREIGN are because we do this outside the context
+ * of the process that owns 'vma'. We also do not want to enforce
+ * protection keys here anyway.
*/
static int break_ksm(struct vm_area_struct *vma, unsigned long addr)
{
@@ -367,12 +371,14 @@ static int break_ksm(struct vm_area_stru
do {
cond_resched();
- page = follow_page(vma, addr, FOLL_GET | FOLL_MIGRATION);
+ page = follow_page(vma, addr,
+ FOLL_GET | FOLL_MIGRATION | FOLL_FOREIGN);
if (IS_ERR_OR_NULL(page))
break;
if (PageKsm(page))
ret = handle_mm_fault(vma->vm_mm, vma, addr,
- FAULT_FLAG_WRITE);
+ FAULT_FLAG_WRITE |
+ FAULT_FLAG_FOREIGN);
else
ret = VM_FAULT_WRITE;
put_page(page);
diff -puN mm/memory.c~pkeys-12-gup-fault-foreign-flag mm/memory.c
--- a/mm/memory.c~pkeys-12-gup-fault-foreign-flag 2015-11-16 12:35:44.785600640 -0800
+++ b/mm/memory.c 2015-11-16 12:35:44.794601049 -0800
@@ -3345,7 +3345,8 @@ static int __handle_mm_fault(struct mm_s
pmd_t *pmd;
pte_t *pte;
- if (!arch_vma_access_permitted(vma, flags & FAULT_FLAG_WRITE))
+ if (!arch_vma_access_permitted(vma, flags & FAULT_FLAG_WRITE,
+ flags & FAULT_FLAG_FOREIGN))
return VM_FAULT_SIGSEGV;
if (unlikely(is_vm_hugetlb_page(vma)))
_
--
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