[PATCH 7/7] powerpc/book3s32: Implement Kernel Userspace Access Protection
From: Christophe Leroy
Date: Tue Mar 05 2019 - 16:18:36 EST
This patch implements Kernel Userspace Access Protection for
book3s/32.
Due to limitations of the processor page protection capabilities,
the protection is only against writing. read protection cannot be
achieved using page protection.
The previous patch modifies the page protection so that RW user
pages are RW for Key 0 and RO for Key 1, and it sets Key 0 for
both user and kernel.
This patch changes userspace segment registers are set to Ku 0
and Ks 1. When kernel needs to write to RW pages, the associated
segment register is then changed to Ks 0 in order to allow write
access to the kernel.
In order to avoid having the read all segment registers when
locking/unlocking the access, some data is kept in the thread_struct
and saved on stack on exceptions. The field identifies both the
first unlocked segment and the first segment following the last
unlocked one. When no segment is unlocked, it contains value 0.
Signed-off-by: Christophe Leroy <christophe.leroy@xxxxxx>
---
arch/powerpc/include/asm/book3s/32/kup.h | 96 ++++++++++++++++++++++++++++++++
arch/powerpc/include/asm/kup.h | 3 +
arch/powerpc/include/asm/processor.h | 3 +
arch/powerpc/kernel/asm-offsets.c | 3 +
arch/powerpc/kernel/head_32.S | 7 +++
arch/powerpc/mm/ppc_mmu_32.c | 10 ++++
arch/powerpc/platforms/Kconfig.cputype | 1 +
7 files changed, 123 insertions(+)
create mode 100644 arch/powerpc/include/asm/book3s/32/kup.h
diff --git a/arch/powerpc/include/asm/book3s/32/kup.h b/arch/powerpc/include/asm/book3s/32/kup.h
new file mode 100644
index 000000000000..3f02db633849
--- /dev/null
+++ b/arch/powerpc/include/asm/book3s/32/kup.h
@@ -0,0 +1,96 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+#ifndef _ASM_POWERPC_BOOK3S_32_KUP_H
+#define _ASM_POWERPC_BOOK3S_32_KUP_H
+
+#ifdef CONFIG_PPC_KUAP
+
+#include <asm/book3s/32/mmu-hash.h>
+
+#ifdef __ASSEMBLY__
+
+.macro kuap_update_sr gpr1, gpr2, gpr3 /* NEVER use r0 as gpr2 due to addis */
+101: mtsrin \gpr1, \gpr2
+ addi \gpr1, \gpr1, 0x111 /* next VSID */
+ rlwinm \gpr1, \gpr1, 0, 0xf0ffffff /* clear VSID overflow */
+ addis \gpr2, \gpr2, 0x1000 /* address of next segment */
+ cmplw \gpr2, \gpr3
+ blt- 101b
+.endm
+
+.macro kuap_save_and_lock sp, thread, gpr1, gpr2, gpr3
+ lwz \gpr2, KUAP(\thread)
+ rlwinm. \gpr3, \gpr2, 28, 0xf0000000
+ stw \gpr2, STACK_REGS_KUAP(\sp)
+ beq+ 102f
+ li \gpr1, 0
+ stw \gpr1, KUAP(\thread)
+ mfsrin \gpr1, \gpr2
+ oris \gpr1, \gpr1, SR_KS@h /* set Ks */
+ kuap_update_sr \gpr1, \gpr2, \gpr3
+102:
+.endm
+
+.macro kuap_restore sp, current, gpr1, gpr2, gpr3
+ lwz \gpr2, STACK_REGS_KUAP(\sp)
+ rlwinm. \gpr3, \gpr2, 28, 0xf0000000
+ stw \gpr2, THREAD + KUAP(\current)
+ beq+ 102f
+ mfsrin \gpr1, \gpr2
+ rlwinm \gpr1, \gpr1, 0, ~SR_KS /* Clear Ks */
+ kuap_update_sr \gpr1, \gpr2, \gpr3
+102:
+.endm
+
+.macro kuap_check current, gpr
+#ifdef CONFIG_PPC_KUAP_DEBUG
+ lwz \gpr2, KUAP(thread)
+999: twnei \gpr, 0
+ EMIT_BUG_ENTRY 999b, __FILE__, __LINE__, (BUGFLAG_WARNING | BUGFLAG_ONCE)
+#endif
+.endm
+
+#else /* !__ASSEMBLY__ */
+
+#include <linux/sched.h>
+
+static inline void kuap_update_sr(u32 sr, u32 addr, u32 end)
+{
+ barrier(); /* make sure thread.kuap is updated before playing with SRs */
+ while (addr < end) {
+ mtsrin(sr, addr);
+ sr += 0x111; /* next VSID */
+ sr &= 0xf0ffffff; /* clear VSID overflow */
+ addr += 0x10000000; /* address of next segment */
+ }
+ isync(); /* Context sync required after mtsrin() */
+}
+
+static inline void allow_user_access(void __user *to, const void __user *from, u32 size)
+{
+ u32 addr = (u32)to;
+ u32 end = min(addr + size, TASK_SIZE);
+
+ if (!addr || addr >= TASK_SIZE || !size)
+ return;
+
+ current->thread.kuap = (addr & 0xf0000000) | ((((end - 1) >> 28) + 1) & 0xf);
+ kuap_update_sr(mfsrin(addr) & ~SR_KS, addr, end); /* Clear Ks */
+}
+
+static inline void prevent_user_access(void __user *to, const void __user *from, u32 size)
+{
+ u32 addr = (u32)to;
+ u32 end = min(addr + size, TASK_SIZE);
+
+ if (!addr || addr >= TASK_SIZE || !size)
+ return;
+
+ current->thread.kuap = 0;
+ kuap_update_sr(mfsrin(addr) | SR_KS, addr, end); /* set Ks */
+}
+
+#endif /* !__ASSEMBLY__ */
+
+#endif /* CONFIG_PPC_KUAP */
+
+#endif /* _ASM_POWERPC_BOOK3S_32_KUP_H */
diff --git a/arch/powerpc/include/asm/kup.h b/arch/powerpc/include/asm/kup.h
index 2ab9e904c22c..2f42e8c19506 100644
--- a/arch/powerpc/include/asm/kup.h
+++ b/arch/powerpc/include/asm/kup.h
@@ -8,6 +8,9 @@
#ifdef CONFIG_PPC_8xx
#include <asm/nohash/32/kup-8xx.h>
#endif
+#ifdef CONFIG_PPC_BOOK3S_32
+#include <asm/book3s/32/kup.h>
+#endif
#ifdef __ASSEMBLY__
#ifndef CONFIG_PPC_KUAP
diff --git a/arch/powerpc/include/asm/processor.h b/arch/powerpc/include/asm/processor.h
index 3351bcf42f2d..540949b397d4 100644
--- a/arch/powerpc/include/asm/processor.h
+++ b/arch/powerpc/include/asm/processor.h
@@ -164,6 +164,9 @@ struct thread_struct {
unsigned long rtas_sp; /* stack pointer for when in RTAS */
#endif
#endif
+#if defined(CONFIG_PPC_BOOK3S_32) && defined(CONFIG_PPC_KUAP)
+ unsigned long kuap; /* opened segments for user access */
+#endif
/* Debug Registers */
struct debug_reg debug;
struct thread_fp_state fp_state;
diff --git a/arch/powerpc/kernel/asm-offsets.c b/arch/powerpc/kernel/asm-offsets.c
index 66202e02fee2..60b82198de7c 100644
--- a/arch/powerpc/kernel/asm-offsets.c
+++ b/arch/powerpc/kernel/asm-offsets.c
@@ -147,6 +147,9 @@ int main(void)
#if defined(CONFIG_KVM) && defined(CONFIG_BOOKE)
OFFSET(THREAD_KVM_VCPU, thread_struct, kvm_vcpu);
#endif
+#if defined(CONFIG_PPC_BOOK3S_32) && defined(CONFIG_PPC_KUAP)
+ OFFSET(KUAP, thread_struct, kuap);
+#endif
#ifdef CONFIG_PPC_TRANSACTIONAL_MEM
OFFSET(PACATMSCRATCH, paca_struct, tm_scratch);
diff --git a/arch/powerpc/kernel/head_32.S b/arch/powerpc/kernel/head_32.S
index 46b633514422..d5dc62997267 100644
--- a/arch/powerpc/kernel/head_32.S
+++ b/arch/powerpc/kernel/head_32.S
@@ -900,6 +900,9 @@ load_up_mmu:
li r0, NUM_USER_SEGMENTS /* load up user segment register values */
mtctr r0 /* for context 0 */
li r3, 0 /* Kp = 0, Ks = 0, VSID = 0 */
+#ifdef CONFIG_PPC_KUAP
+ oris r3, r3, SR_KS@h /* Set Ks */
+#endif
li r4,0
3: mtsrin r3,r4
addi r3,r3,0x111 /* increment VSID */
@@ -907,6 +910,7 @@ load_up_mmu:
bdnz 3b
li r0, 16 - NUM_USER_SEGMENTS /* load up kernel segment registers */
mtctr r0 /* for context 0 */
+ rlwinm r3, r3, 0, ~SR_KS /* Ks = 0 */
oris r3, r3, SR_KP@h /* Kp = 1 */
3: mtsrin r3, r4
addi r3, r3, 0x111 /* increment VSID */
@@ -1015,6 +1019,9 @@ _ENTRY(switch_mmu_context)
blt- 4f
mulli r3,r3,897 /* multiply context by skew factor */
rlwinm r3,r3,4,8,27 /* VSID = (context & 0xfffff) << 4 */
+#ifdef CONFIG_PPC_KUAP
+ oris r3, r3, SR_KS@h /* Set Ks */
+#endif
li r0,NUM_USER_SEGMENTS
mtctr r0
diff --git a/arch/powerpc/mm/ppc_mmu_32.c b/arch/powerpc/mm/ppc_mmu_32.c
index 2d5b0d50fb31..417cf33aa5ba 100644
--- a/arch/powerpc/mm/ppc_mmu_32.c
+++ b/arch/powerpc/mm/ppc_mmu_32.c
@@ -392,3 +392,13 @@ void setup_initial_memory_limit(phys_addr_t first_memblock_base,
else /* Anything else has 256M mapped */
memblock_set_current_limit(min_t(u64, first_memblock_size, 0x10000000));
}
+
+#ifdef CONFIG_PPC_KUAP
+void __init setup_kuap(bool disabled)
+{
+ pr_info("Activating Kernel Userspace Access Protection\n");
+
+ if (disabled)
+ pr_warn("KUAP cannot be disabled yet on 6xx when compiled in\n");
+}
+#endif
diff --git a/arch/powerpc/platforms/Kconfig.cputype b/arch/powerpc/platforms/Kconfig.cputype
index ab586963893a..e3a5cbcc7700 100644
--- a/arch/powerpc/platforms/Kconfig.cputype
+++ b/arch/powerpc/platforms/Kconfig.cputype
@@ -25,6 +25,7 @@ config PPC_BOOK3S_32
bool "512x/52xx/6xx/7xx/74xx/82xx/83xx/86xx"
select PPC_FPU
select PPC_HAVE_PMU_SUPPORT
+ select PPC_HAVE_KUAP
config PPC_85xx
bool "Freescale 85xx"
--
2.13.3