[PATCH 1/5] powerpc/64s: Guarded Userspace Access Prevention
Russell Currey
ruscur at russell.cc
Fri Oct 26 17:35:09 AEDT 2018
Guarded Userspace Access Prevention (GUAP) utilises a feature of
the Radix MMU which disallows read and write access to userspace
addresses. By utilising this, the kernel is prevented from accessing
user data from outside of trusted paths that perform proper safety checks,
such as copy_{to/from}_user() and friends.
Userspace access is disabled from early boot and is only enabled when:
- exiting the kernel and entering userspace
- performing an operation like copy_{to/from}_user()
- context switching to a process that has access enabled
and similarly, access is disabled again when exiting userspace and entering
the kernel.
This feature has a slight performance impact which I roughly measured to be
3% slower in the worst case (performing 1GB of 1 byte read()/write()
syscalls), and is gated behind the CONFIG_PPC_RADIX_GUAP option for
performance-critical builds.
This feature can be tested by using the lkdtm driver (CONFIG_LKDTM=y) and
performing the following:
echo ACCESS_USERSPACE > [debugfs]/provoke-crash/DIRECT
if enabled, this should send SIGSEGV to the thread.
Signed-off-by: Russell Currey <ruscur at russell.cc>
---
Since the previous version of this patchset (named KHRAP) there have been
several changes, some of which include:
- macro naming, suggested by Nick
- builds should be fixed outside of 64s
- no longer unlock heading out to userspace
- removal of unnecessary isyncs
- more config option testing
- removal of save/restore
- use pr_crit() and reword message on fault
arch/powerpc/include/asm/exception-64e.h | 3 ++
arch/powerpc/include/asm/exception-64s.h | 19 +++++++-
arch/powerpc/include/asm/mmu.h | 7 +++
arch/powerpc/include/asm/paca.h | 3 ++
arch/powerpc/include/asm/reg.h | 1 +
arch/powerpc/include/asm/uaccess.h | 57 ++++++++++++++++++++----
arch/powerpc/kernel/asm-offsets.c | 1 +
arch/powerpc/kernel/dt_cpu_ftrs.c | 4 ++
arch/powerpc/kernel/entry_64.S | 17 ++++++-
arch/powerpc/mm/fault.c | 12 +++++
arch/powerpc/mm/pgtable-radix.c | 2 +
arch/powerpc/mm/pkeys.c | 7 ++-
arch/powerpc/platforms/Kconfig.cputype | 15 +++++++
13 files changed, 135 insertions(+), 13 deletions(-)
diff --git a/arch/powerpc/include/asm/exception-64e.h b/arch/powerpc/include/asm/exception-64e.h
index 555e22d5e07f..bf25015834ee 100644
--- a/arch/powerpc/include/asm/exception-64e.h
+++ b/arch/powerpc/include/asm/exception-64e.h
@@ -215,5 +215,8 @@ exc_##label##_book3e:
#define RFI_TO_USER \
rfi
+#define UNLOCK_USER_ACCESS(reg)
+#define LOCK_USER_ACCESS(reg)
+
#endif /* _ASM_POWERPC_EXCEPTION_64E_H */
diff --git a/arch/powerpc/include/asm/exception-64s.h b/arch/powerpc/include/asm/exception-64s.h
index 3b4767ed3ec5..0cac5bd380ca 100644
--- a/arch/powerpc/include/asm/exception-64s.h
+++ b/arch/powerpc/include/asm/exception-64s.h
@@ -264,6 +264,19 @@ BEGIN_FTR_SECTION_NESTED(943) \
std ra,offset(r13); \
END_FTR_SECTION_NESTED(ftr,ftr,943)
+#define LOCK_USER_ACCESS(reg) \
+BEGIN_MMU_FTR_SECTION_NESTED(944) \
+ LOAD_REG_IMMEDIATE(reg,AMR_LOCKED); \
+ mtspr SPRN_AMR,reg; \
+END_MMU_FTR_SECTION_NESTED(MMU_FTR_RADIX_GUAP,MMU_FTR_RADIX_GUAP,944)
+
+#define UNLOCK_USER_ACCESS(reg) \
+BEGIN_MMU_FTR_SECTION_NESTED(945) \
+ li reg,0; \
+ mtspr SPRN_AMR,reg; \
+ isync \
+END_MMU_FTR_SECTION_NESTED(MMU_FTR_RADIX_GUAP,MMU_FTR_RADIX_GUAP,945)
+
#define EXCEPTION_PROLOG_0(area) \
GET_PACA(r13); \
std r9,area+EX_R9(r13); /* save r9 */ \
@@ -500,7 +513,11 @@ END_FTR_SECTION_NESTED(ftr,ftr,943)
beq 4f; /* if from kernel mode */ \
ACCOUNT_CPU_USER_ENTRY(r13, r9, r10); \
SAVE_PPR(area, r9); \
-4: EXCEPTION_PROLOG_COMMON_2(area) \
+4: lbz r9,PACA_USER_ACCESS_ALLOWED(r13); \
+ cmpwi cr1,r9,0; \
+ beq 5f; \
+ LOCK_USER_ACCESS(r9); \
+5: EXCEPTION_PROLOG_COMMON_2(area) \
EXCEPTION_PROLOG_COMMON_3(n) \
ACCOUNT_STOLEN_TIME
diff --git a/arch/powerpc/include/asm/mmu.h b/arch/powerpc/include/asm/mmu.h
index eb20eb3b8fb0..3b31ed702785 100644
--- a/arch/powerpc/include/asm/mmu.h
+++ b/arch/powerpc/include/asm/mmu.h
@@ -107,6 +107,10 @@
*/
#define MMU_FTR_1T_SEGMENT ASM_CONST(0x40000000)
+/* Supports GUAP (key 0 controlling userspace addresses) on radix
+ */
+#define MMU_FTR_RADIX_GUAP ASM_CONST(0x80000000)
+
/* MMU feature bit sets for various CPUs */
#define MMU_FTRS_DEFAULT_HPTE_ARCH_V2 \
MMU_FTR_HPTE_TABLE | MMU_FTR_PPCAS_ARCH_V2
@@ -143,6 +147,9 @@ enum {
MMU_FTR_KERNEL_RO | MMU_FTR_68_BIT_VA |
#ifdef CONFIG_PPC_RADIX_MMU
MMU_FTR_TYPE_RADIX |
+#endif
+#ifdef CONFIG_PPC_RADIX_GUAP
+ MMU_FTR_RADIX_GUAP |
#endif
0,
};
diff --git a/arch/powerpc/include/asm/paca.h b/arch/powerpc/include/asm/paca.h
index e843bc5d1a0f..e905f09b2d38 100644
--- a/arch/powerpc/include/asm/paca.h
+++ b/arch/powerpc/include/asm/paca.h
@@ -169,6 +169,9 @@ struct paca_struct {
u64 saved_r1; /* r1 save for RTAS calls or PM or EE=0 */
u64 saved_msr; /* MSR saved here by enter_rtas */
u16 trap_save; /* Used when bad stack is encountered */
+#ifdef CONFIG_PPC_RADIX_GUAP
+ u8 user_access_allowed; /* set when AMR allows user accesses */
+#endif
u8 irq_soft_mask; /* mask for irq soft masking */
u8 irq_happened; /* irq happened while soft-disabled */
u8 io_sync; /* writel() needs spin_unlock sync */
diff --git a/arch/powerpc/include/asm/reg.h b/arch/powerpc/include/asm/reg.h
index 640a4d818772..b994099a906b 100644
--- a/arch/powerpc/include/asm/reg.h
+++ b/arch/powerpc/include/asm/reg.h
@@ -246,6 +246,7 @@
#define SPRN_DSCR 0x11
#define SPRN_CFAR 0x1c /* Come From Address Register */
#define SPRN_AMR 0x1d /* Authority Mask Register */
+#define AMR_LOCKED 0xC000000000000000ULL /* Read & Write disabled */
#define SPRN_UAMOR 0x9d /* User Authority Mask Override Register */
#define SPRN_AMOR 0x15d /* Authority Mask Override Register */
#define SPRN_ACOP 0x1F /* Available Coprocessor Register */
diff --git a/arch/powerpc/include/asm/uaccess.h b/arch/powerpc/include/asm/uaccess.h
index 15bea9a0f260..209bfc47c340 100644
--- a/arch/powerpc/include/asm/uaccess.h
+++ b/arch/powerpc/include/asm/uaccess.h
@@ -62,6 +62,27 @@ static inline int __access_ok(unsigned long addr, unsigned long size,
#endif
+static inline void unlock_user_access(void)
+{
+#ifdef CONFIG_PPC_RADIX_GUAP
+ if (mmu_has_feature(MMU_FTR_RADIX_GUAP)) {
+ mtspr(SPRN_AMR, 0);
+ isync();
+ get_paca()->user_access_allowed = 1;
+ }
+#endif
+}
+
+static inline void lock_user_access(void)
+{
+#ifdef CONFIG_PPC_RADIX_GUAP
+ if (mmu_has_feature(MMU_FTR_RADIX_GUAP)) {
+ mtspr(SPRN_AMR, AMR_LOCKED);
+ get_paca()->user_access_allowed = 0;
+ }
+#endif
+}
+
#define access_ok(type, addr, size) \
(__chk_user_ptr(addr), \
__access_ok((__force unsigned long)(addr), (size), get_fs()))
@@ -141,6 +162,7 @@ extern long __put_user_bad(void);
#define __put_user_size(x, ptr, size, retval) \
do { \
retval = 0; \
+ unlock_user_access(); \
switch (size) { \
case 1: __put_user_asm(x, ptr, retval, "stb"); break; \
case 2: __put_user_asm(x, ptr, retval, "sth"); break; \
@@ -148,6 +170,7 @@ do { \
case 8: __put_user_asm2(x, ptr, retval); break; \
default: __put_user_bad(); \
} \
+ lock_user_access(); \
} while (0)
#define __put_user_nocheck(x, ptr, size) \
@@ -240,6 +263,7 @@ do { \
__chk_user_ptr(ptr); \
if (size > sizeof(x)) \
(x) = __get_user_bad(); \
+ unlock_user_access(); \
switch (size) { \
case 1: __get_user_asm(x, ptr, retval, "lbz"); break; \
case 2: __get_user_asm(x, ptr, retval, "lhz"); break; \
@@ -247,6 +271,7 @@ do { \
case 8: __get_user_asm2(x, ptr, retval); break; \
default: (x) = __get_user_bad(); \
} \
+ lock_user_access(); \
} while (0)
/*
@@ -306,15 +331,20 @@ extern unsigned long __copy_tofrom_user(void __user *to,
static inline unsigned long
raw_copy_in_user(void __user *to, const void __user *from, unsigned long n)
{
- return __copy_tofrom_user(to, from, n);
+ unsigned long ret;
+ unlock_user_access(); \
+ ret = __copy_tofrom_user(to, from, n); \
+ lock_user_access(); \
+ return ret; \
}
#endif /* __powerpc64__ */
static inline unsigned long raw_copy_from_user(void *to,
const void __user *from, unsigned long n)
{
+ unsigned long ret;
if (__builtin_constant_p(n) && (n <= 8)) {
- unsigned long ret = 1;
+ ret = 1;
switch (n) {
case 1:
@@ -339,14 +369,18 @@ static inline unsigned long raw_copy_from_user(void *to,
}
barrier_nospec();
- return __copy_tofrom_user((__force void __user *)to, from, n);
+ unlock_user_access();
+ ret = __copy_tofrom_user((__force void __user *)to, from, n);
+ lock_user_access();
+ return ret;
}
static inline unsigned long raw_copy_to_user(void __user *to,
const void *from, unsigned long n)
{
+ unsigned long ret;
if (__builtin_constant_p(n) && (n <= 8)) {
- unsigned long ret = 1;
+ ret = 1;
switch (n) {
case 1:
@@ -366,17 +400,24 @@ static inline unsigned long raw_copy_to_user(void __user *to,
return 0;
}
- return __copy_tofrom_user(to, (__force const void __user *)from, n);
+ unlock_user_access();
+ ret = __copy_tofrom_user(to, (__force const void __user *)from, n);
+ lock_user_access();
+ return ret;
}
extern unsigned long __clear_user(void __user *addr, unsigned long size);
static inline unsigned long clear_user(void __user *addr, unsigned long size)
{
+ unsigned long ret = size;
might_fault();
- if (likely(access_ok(VERIFY_WRITE, addr, size)))
- return __clear_user(addr, size);
- return size;
+ if (likely(access_ok(VERIFY_WRITE, addr, size))) {
+ unlock_user_access();
+ ret = __clear_user(addr, size);
+ lock_user_access();
+ }
+ return ret;
}
extern long strncpy_from_user(char *dst, const char __user *src, long count);
diff --git a/arch/powerpc/kernel/asm-offsets.c b/arch/powerpc/kernel/asm-offsets.c
index 10ef2e4db2fd..5050f15ad2f5 100644
--- a/arch/powerpc/kernel/asm-offsets.c
+++ b/arch/powerpc/kernel/asm-offsets.c
@@ -260,6 +260,7 @@ int main(void)
OFFSET(ACCOUNT_STARTTIME_USER, paca_struct, accounting.starttime_user);
OFFSET(ACCOUNT_USER_TIME, paca_struct, accounting.utime);
OFFSET(ACCOUNT_SYSTEM_TIME, paca_struct, accounting.stime);
+ OFFSET(PACA_USER_ACCESS_ALLOWED, paca_struct, user_access_allowed);
OFFSET(PACA_TRAP_SAVE, paca_struct, trap_save);
OFFSET(PACA_NAPSTATELOST, paca_struct, nap_state_lost);
OFFSET(PACA_SPRG_VDSO, paca_struct, sprg_vdso);
diff --git a/arch/powerpc/kernel/dt_cpu_ftrs.c b/arch/powerpc/kernel/dt_cpu_ftrs.c
index f432054234a4..df4716624840 100644
--- a/arch/powerpc/kernel/dt_cpu_ftrs.c
+++ b/arch/powerpc/kernel/dt_cpu_ftrs.c
@@ -337,6 +337,10 @@ static int __init feat_enable_mmu_radix(struct dt_cpu_feature *f)
cur_cpu_spec->mmu_features |= MMU_FTRS_HASH_BASE;
cur_cpu_spec->cpu_user_features |= PPC_FEATURE_HAS_MMU;
+#ifdef CONFIG_PPC_RADIX_GUAP
+ cur_cpu_spec->mmu_features |= MMU_FTR_RADIX_GUAP;
+#endif
+
return 1;
#endif
return 0;
diff --git a/arch/powerpc/kernel/entry_64.S b/arch/powerpc/kernel/entry_64.S
index 7b1693adff2a..23f0944185d3 100644
--- a/arch/powerpc/kernel/entry_64.S
+++ b/arch/powerpc/kernel/entry_64.S
@@ -297,7 +297,12 @@ END_FTR_SECTION_IFSET(CPU_FTR_HAS_PPR)
b . /* prevent speculative execution */
/* exit to kernel */
-1: ld r2,GPR2(r1)
+1: /* if the AMR was unlocked before, unlock it again */
+ lbz r2,PACA_USER_ACCESS_ALLOWED(r13)
+ cmpwi cr1,0
+ bne 2f
+ UNLOCK_USER_ACCESS(r2)
+2: ld r2,GPR2(r1)
ld r1,GPR1(r1)
mtlr r4
mtcr r5
@@ -965,6 +970,7 @@ BEGIN_FTR_SECTION
ld r2,_PPR(r1)
mtspr SPRN_PPR,r2
END_FTR_SECTION_IFSET(CPU_FTR_HAS_PPR)
+
ACCOUNT_CPU_USER_EXIT(r13, r2, r4)
REST_GPR(13, r1)
@@ -983,7 +989,14 @@ END_FTR_SECTION_IFSET(CPU_FTR_HAS_PPR)
RFI_TO_USER
b . /* prevent speculative execution */
-1: mtspr SPRN_SRR1,r3
+1: /* exit to kernel */
+ /* if the AMR was unlocked before, unlock it again */
+ lbz r2,PACA_USER_ACCESS_ALLOWED(r13)
+ cmpwi cr1,0
+ bne 2f
+ UNLOCK_USER_ACCESS(r2)
+
+2: mtspr SPRN_SRR1,r3
ld r2,_CCR(r1)
mtcrf 0xFF,r2
diff --git a/arch/powerpc/mm/fault.c b/arch/powerpc/mm/fault.c
index d51cf5f4e45e..17fd8c6b055b 100644
--- a/arch/powerpc/mm/fault.c
+++ b/arch/powerpc/mm/fault.c
@@ -462,6 +462,18 @@ static int __do_page_fault(struct pt_regs *regs, unsigned long address,
return bad_key_fault_exception(regs, address,
get_mm_addr_key(mm, address));
+#ifdef CONFIG_PPC_RADIX_SMAP
+ if (mmu_has_feature(MMU_FTR_RADIX_GUAP)) {
+ if (unlikely(!is_user &&
+ (error_code & DSISR_PROTFAULT) &&
+ (mfspr(SPRN_AMR) & AMR_LOCKED))) {
+ pr_crit("Kernel attempted to access user data"
+ " unsafely, possible exploit attempt\n");
+ return bad_area_nosemaphore(regs, address);
+ }
+ }
+#endif
+
/*
* We want to do this outside mmap_sem, because reading code around nip
* can result in fault, which will cause a deadlock when called with
diff --git a/arch/powerpc/mm/pgtable-radix.c b/arch/powerpc/mm/pgtable-radix.c
index c879979faa73..9e5b98887a05 100644
--- a/arch/powerpc/mm/pgtable-radix.c
+++ b/arch/powerpc/mm/pgtable-radix.c
@@ -29,6 +29,7 @@
#include <asm/powernv.h>
#include <asm/sections.h>
#include <asm/trace.h>
+#include <asm/uaccess.h>
#include <trace/events/thp.h>
@@ -608,6 +609,7 @@ void __init radix__early_init_mmu(void)
mtspr(SPRN_LPCR, lpcr | LPCR_UPRT | LPCR_HR);
radix_init_partition_table();
radix_init_amor();
+ lock_user_access();
} else {
radix_init_pseries();
}
diff --git a/arch/powerpc/mm/pkeys.c b/arch/powerpc/mm/pkeys.c
index b271b283c785..0b9bc320138c 100644
--- a/arch/powerpc/mm/pkeys.c
+++ b/arch/powerpc/mm/pkeys.c
@@ -7,6 +7,7 @@
#include <asm/mman.h>
#include <asm/setup.h>
+#include <asm/uaccess.h>
#include <linux/pkeys.h>
#include <linux/of_device.h>
@@ -266,7 +267,8 @@ int __arch_set_user_pkey_access(struct task_struct *tsk, int pkey,
void thread_pkey_regs_save(struct thread_struct *thread)
{
- if (static_branch_likely(&pkey_disabled))
+ if (static_branch_likely(&pkey_disabled) &&
+ !mmu_has_feature(MMU_FTR_RADIX_GUAP))
return;
/*
@@ -280,7 +282,8 @@ void thread_pkey_regs_save(struct thread_struct *thread)
void thread_pkey_regs_restore(struct thread_struct *new_thread,
struct thread_struct *old_thread)
{
- if (static_branch_likely(&pkey_disabled))
+ if (static_branch_likely(&pkey_disabled) &&
+ !mmu_has_feature(MMU_FTR_RADIX_GUAP))
return;
if (old_thread->amr != new_thread->amr)
diff --git a/arch/powerpc/platforms/Kconfig.cputype b/arch/powerpc/platforms/Kconfig.cputype
index f4e2c5729374..6617d3e415a7 100644
--- a/arch/powerpc/platforms/Kconfig.cputype
+++ b/arch/powerpc/platforms/Kconfig.cputype
@@ -351,6 +351,21 @@ config PPC_RADIX_MMU_DEFAULT
If you're unsure, say Y.
+config PPC_RADIX_GUAP
+ bool "Guarded Userspace Access Prevention on Radix"
+ depends on PPC_RADIX_MMU
+ default y
+ help
+ Enable support for Guarded Userspace Access Prevention (GUAP)
+ when using the Radix MMU. GUAP is a security feature
+ preventing the kernel from directly accessing userspace data
+ without going through the proper checks.
+
+ GUAP has a minor performance impact on context switching and can be
+ disabled at boot time using the "nosmap" kernel command line option.
+
+ If you're unsure, say Y.
+
config ARCH_ENABLE_HUGEPAGE_MIGRATION
def_bool y
depends on PPC_BOOK3S_64 && HUGETLB_PAGE && MIGRATION
--
2.19.1
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