[PATCH 3/5] powerpc/kuap: Rework Kernel Userspace Access Protection on book3s/32

From: Christophe Leroy
Date: Wed May 12 2021 - 04:35:22 EST

On book3s/32, KUAP is provided by toggling Ks bit in segment registers.
One segment registers addresses 256M of virtual memory.

At the time being, KUAP implements a complex logic to apply the
unlock/lock on the exact number of segments covering the user range
to access, with saving the boundaries of the range of segments in
a member of thread struct.

But most if not all user accesses are within a single segment.

Rework KUAP with a different approach:
- Open only one segment, the one corresponding to the starting
address of the range to be accessed.
- If a second segment is involved, it will generate a page fault. The
segment will then be open by the page fault handler.

The kuap member of thread struct will now contain:
- The start address of the current on going user access, that will be
used to know which segment to lock at the end of the user access.
- ~0 when no user access is open
- ~1 when additionnal segments are opened by a page fault.

Then, at lock time
- When only one segment is open, close it.
- When several segments are open, close all user segments.

Almost 100% of the time, only one segment will be involved.

In interrupts, inline the function that unlock/lock all segments,
because not inlining them implies a lot of register save/restore.

With the patch, writing value 128 in userspace in perf_copy_attr() is
done with 16 instructions:

3890: 93 82 04 dc stw r28,1244(r2)
3894: 7d 20 e5 26 mfsrin r9,r28
3898: 55 29 00 80 rlwinm r9,r9,0,2,0
389c: 7d 20 e1 e4 mtsrin r9,r28
38a0: 4c 00 01 2c isync

38a4: 39 20 00 80 li r9,128
38a8: 91 3c 00 00 stw r9,0(r28)

38ac: 81 42 04 dc lwz r10,1244(r2)
38b0: 39 00 ff ff li r8,-1
38b4: 91 02 04 dc stw r8,1244(r2)
38b8: 2c 0a ff fe cmpwi r10,-2
38bc: 41 82 00 88 beq 3944 <perf_copy_attr+0x36c>
38c0: 7d 20 55 26 mfsrin r9,r10
38c4: 65 29 40 00 oris r9,r9,16384
38c8: 7d 20 51 e4 mtsrin r9,r10
38cc: 4c 00 01 2c isync
...
3944: 48 00 00 01 bl 3944 <perf_copy_attr+0x36c>
3944: R_PPC_REL24 kuap_lock_all_ool

Before the patch it was 118 instructions. In reality only 42 are
executed in most cases, but GCC is not able to see that a properly
aligned user access cannot involve more than one segment.

5060: 39 1d 00 04 addi r8,r29,4
5064: 3d 20 b0 00 lis r9,-20480
5068: 7c 08 48 40 cmplw r8,r9
506c: 40 81 00 08 ble 5074 <perf_copy_attr+0x2cc>
5070: 3d 00 b0 00 lis r8,-20480
5074: 39 28 ff ff addi r9,r8,-1
5078: 57 aa 00 06 rlwinm r10,r29,0,0,3
507c: 55 29 27 3e rlwinm r9,r9,4,28,31
5080: 39 29 00 01 addi r9,r9,1
5084: 7d 29 53 78 or r9,r9,r10
5088: 91 22 04 dc stw r9,1244(r2)
508c: 7d 20 ed 26 mfsrin r9,r29
5090: 55 29 00 80 rlwinm r9,r9,0,2,0
5094: 7c 08 50 40 cmplw r8,r10
5098: 40 81 00 c0 ble 5158 <perf_copy_attr+0x3b0>
509c: 7d 46 50 f8 not r6,r10
50a0: 7c c6 42 14 add r6,r6,r8
50a4: 54 c6 27 be rlwinm r6,r6,4,30,31
50a8: 7d 20 51 e4 mtsrin r9,r10
50ac: 3c ea 10 00 addis r7,r10,4096
50b0: 39 29 01 11 addi r9,r9,273
50b4: 7f 88 38 40 cmplw cr7,r8,r7
50b8: 55 29 02 06 rlwinm r9,r9,0,8,3
50bc: 40 9d 00 9c ble cr7,5158 <perf_copy_attr+0x3b0>

50c0: 2f 86 00 00 cmpwi cr7,r6,0
50c4: 41 9e 00 4c beq cr7,5110 <perf_copy_attr+0x368>
50c8: 2f 86 00 01 cmpwi cr7,r6,1
50cc: 41 9e 00 2c beq cr7,50f8 <perf_copy_attr+0x350>
50d0: 2f 86 00 02 cmpwi cr7,r6,2
50d4: 41 9e 00 14 beq cr7,50e8 <perf_copy_attr+0x340>
50d8: 7d 20 39 e4 mtsrin r9,r7
50dc: 39 29 01 11 addi r9,r9,273
50e0: 3c e7 10 00 addis r7,r7,4096
50e4: 55 29 02 06 rlwinm r9,r9,0,8,3
50e8: 7d 20 39 e4 mtsrin r9,r7
50ec: 39 29 01 11 addi r9,r9,273
50f0: 3c e7 10 00 addis r7,r7,4096
50f4: 55 29 02 06 rlwinm r9,r9,0,8,3
50f8: 7d 20 39 e4 mtsrin r9,r7
50fc: 3c e7 10 00 addis r7,r7,4096
5100: 39 29 01 11 addi r9,r9,273
5104: 7f 88 38 40 cmplw cr7,r8,r7
5108: 55 29 02 06 rlwinm r9,r9,0,8,3
510c: 40 9d 00 4c ble cr7,5158 <perf_copy_attr+0x3b0>
5110: 7d 20 39 e4 mtsrin r9,r7
5114: 39 29 01 11 addi r9,r9,273
5118: 3c c7 10 00 addis r6,r7,4096
511c: 55 29 02 06 rlwinm r9,r9,0,8,3
5120: 7d 20 31 e4 mtsrin r9,r6
5124: 39 29 01 11 addi r9,r9,273
5128: 3c c6 10 00 addis r6,r6,4096
512c: 55 29 02 06 rlwinm r9,r9,0,8,3
5130: 7d 20 31 e4 mtsrin r9,r6
5134: 39 29 01 11 addi r9,r9,273
5138: 3c c7 30 00 addis r6,r7,12288
513c: 55 29 02 06 rlwinm r9,r9,0,8,3
5140: 7d 20 31 e4 mtsrin r9,r6
5144: 3c e7 40 00 addis r7,r7,16384
5148: 39 29 01 11 addi r9,r9,273
514c: 7f 88 38 40 cmplw cr7,r8,r7
5150: 55 29 02 06 rlwinm r9,r9,0,8,3
5154: 41 9d ff bc bgt cr7,5110 <perf_copy_attr+0x368>
5158: 4c 00 01 2c isync

515c: 39 20 00 80 li r9,128
5160: 91 3d 00 00 stw r9,0(r29)

5164: 38 e0 00 00 li r7,0
5168: 90 e2 04 dc stw r7,1244(r2)
516c: 7d 20 ed 26 mfsrin r9,r29
5170: 65 29 40 00 oris r9,r9,16384
5174: 40 81 00 c0 ble 5234 <perf_copy_attr+0x48c>
5178: 7d 47 50 f8 not r7,r10
517c: 7c e7 42 14 add r7,r7,r8
5180: 54 e7 27 be rlwinm r7,r7,4,30,31
5184: 7d 20 51 e4 mtsrin r9,r10
5188: 3d 4a 10 00 addis r10,r10,4096
518c: 39 29 01 11 addi r9,r9,273
5190: 7c 08 50 40 cmplw r8,r10
5194: 55 29 02 06 rlwinm r9,r9,0,8,3
5198: 40 81 00 9c ble 5234 <perf_copy_attr+0x48c>

519c: 2c 07 00 00 cmpwi r7,0
51a0: 41 82 00 4c beq 51ec <perf_copy_attr+0x444>
51a4: 2c 07 00 01 cmpwi r7,1
51a8: 41 82 00 2c beq 51d4 <perf_copy_attr+0x42c>
51ac: 2c 07 00 02 cmpwi r7,2
51b0: 41 82 00 14 beq 51c4 <perf_copy_attr+0x41c>
51b4: 7d 20 51 e4 mtsrin r9,r10
51b8: 39 29 01 11 addi r9,r9,273
51bc: 3d 4a 10 00 addis r10,r10,4096
51c0: 55 29 02 06 rlwinm r9,r9,0,8,3
51c4: 7d 20 51 e4 mtsrin r9,r10
51c8: 39 29 01 11 addi r9,r9,273
51cc: 3d 4a 10 00 addis r10,r10,4096
51d0: 55 29 02 06 rlwinm r9,r9,0,8,3
51d4: 7d 20 51 e4 mtsrin r9,r10
51d8: 3d 4a 10 00 addis r10,r10,4096
51dc: 39 29 01 11 addi r9,r9,273
51e0: 7c 08 50 40 cmplw r8,r10
51e4: 55 29 02 06 rlwinm r9,r9,0,8,3
51e8: 40 81 00 4c ble 5234 <perf_copy_attr+0x48c>
51ec: 7d 20 51 e4 mtsrin r9,r10
51f0: 39 29 01 11 addi r9,r9,273
51f4: 3c ea 10 00 addis r7,r10,4096
51f8: 55 29 02 06 rlwinm r9,r9,0,8,3
51fc: 7d 20 39 e4 mtsrin r9,r7
5200: 39 29 01 11 addi r9,r9,273
5204: 3c e7 10 00 addis r7,r7,4096
5208: 55 29 02 06 rlwinm r9,r9,0,8,3
520c: 7d 20 39 e4 mtsrin r9,r7
5210: 39 29 01 11 addi r9,r9,273
5214: 3c ea 30 00 addis r7,r10,12288
5218: 55 29 02 06 rlwinm r9,r9,0,8,3
521c: 7d 20 39 e4 mtsrin r9,r7
5220: 3d 4a 40 00 addis r10,r10,16384
5224: 39 29 01 11 addi r9,r9,273
5228: 7c 08 50 40 cmplw r8,r10
522c: 55 29 02 06 rlwinm r9,r9,0,8,3
5230: 41 81 ff bc bgt 51ec <perf_copy_attr+0x444>
5234: 4c 00 01 2c isync

Signed-off-by: Christophe Leroy <christophe.leroy@xxxxxxxxxx>
---
arch/powerpc/include/asm/book3s/32/kup.h | 148 ++++++++++++-----------
arch/powerpc/include/asm/processor.h | 10 +-
arch/powerpc/kernel/process.c | 3 +
arch/powerpc/mm/book3s32/Makefile | 1 +
arch/powerpc/mm/book3s32/kuap.c | 14 +++
5 files changed, 106 insertions(+), 70 deletions(-)
create mode 100644 arch/powerpc/mm/book3s32/kuap.c

diff --git a/arch/powerpc/include/asm/book3s/32/kup.h b/arch/powerpc/include/asm/book3s/32/kup.h
index 1670dfe9d4f1..c67b91c1ca80 100644
--- a/arch/powerpc/include/asm/book3s/32/kup.h
+++ b/arch/powerpc/include/asm/book3s/32/kup.h
@@ -11,31 +11,66 @@

#include <linux/sched.h>

-static inline void kuap_update_sr(u32 sr, u32 addr, u32 end)
-{
- addr &= 0xf0000000; /* align addr to start of segment */
- barrier(); /* make sure thread.kuap is updated before playing with SRs */
- while (addr < end) {
- mtsr(sr, addr);
- sr += 0x111; /* next VSID */
- sr &= 0xf0ffffff; /* clear VSID overflow */
- addr += 0x10000000; /* address of next segment */
- }
+#define KUAP_NONE (~0UL)
+#define KUAP_ALL (~1UL)
+
+static inline void kuap_lock_one(unsigned long addr)
+{
+ mtsr(mfsr(addr) | SR_KS, addr);
+ isync(); /* Context sync required after mtsr() */
+}
+
+static inline void kuap_unlock_one(unsigned long addr)
+{
+ mtsr(mfsr(addr) & ~SR_KS, addr);
+ isync(); /* Context sync required after mtsr() */
+}
+
+static inline void kuap_lock_all(void)
+{
+ update_user_segments(mfsr(0) | SR_KS);
isync(); /* Context sync required after mtsr() */
}

+static inline void kuap_unlock_all(void)
+{
+ update_user_segments(mfsr(0) & ~SR_KS);
+ isync(); /* Context sync required after mtsr() */
+}
+
+void kuap_lock_all_ool(void);
+void kuap_unlock_all_ool(void);
+
+static inline void kuap_lock(unsigned long addr, bool ool)
+{
+ if (likely(addr != KUAP_ALL))
+ kuap_lock_one(addr);
+ else if (!ool)
+ kuap_lock_all();
+ else
+ kuap_lock_all_ool();
+}
+
+static inline void kuap_unlock(unsigned long addr, bool ool)
+{
+ if (likely(addr != KUAP_ALL))
+ kuap_unlock_one(addr);
+ else if (!ool)
+ kuap_unlock_all();
+ else
+ kuap_unlock_all_ool();
+}
+
static inline void kuap_save_and_lock(struct pt_regs *regs)
{
unsigned long kuap = current->thread.kuap;
- u32 addr = kuap & 0xf0000000;
- u32 end = kuap << 28;

regs->kuap = kuap;
- if (unlikely(!kuap))
+ if (unlikely(kuap == KUAP_NONE))
return;

- current->thread.kuap = 0;
- kuap_update_sr(mfsr(addr) | SR_KS, addr, end); /* Set Ks */
+ current->thread.kuap = KUAP_NONE;
+ kuap_lock(kuap, false);
}

static inline void kuap_user_restore(struct pt_regs *regs)
@@ -44,22 +79,16 @@ static inline void kuap_user_restore(struct pt_regs *regs)

static inline void kuap_kernel_restore(struct pt_regs *regs, unsigned long kuap)
{
- u32 addr = regs->kuap & 0xf0000000;
- u32 end = regs->kuap << 28;
-
current->thread.kuap = regs->kuap;

- if (unlikely(regs->kuap == kuap))
- return;
-
- kuap_update_sr(mfsr(addr) & ~SR_KS, addr, end); /* Clear Ks */
+ kuap_unlock(regs->kuap, false);
}

static inline unsigned long kuap_get_and_assert_locked(void)
{
unsigned long kuap = current->thread.kuap;

- WARN_ON_ONCE(IS_ENABLED(CONFIG_PPC_KUAP_DEBUG) && kuap != 0);
+ WARN_ON_ONCE(IS_ENABLED(CONFIG_PPC_KUAP_DEBUG) && kuap != KUAP_NONE);

return kuap;
}
@@ -72,84 +101,65 @@ static inline void kuap_assert_locked(void)
static __always_inline void allow_user_access(void __user *to, const void __user *from,
u32 size, unsigned long dir)
{
- u32 addr, end;
-
BUILD_BUG_ON(!__builtin_constant_p(dir));
BUILD_BUG_ON(dir & ~KUAP_READ_WRITE);
-
if (!(dir & KUAP_WRITE))
return;

- addr = (__force u32)to;
-
- if (unlikely(addr >= TASK_SIZE || !size))
- return;
-
- end = min(addr + size, TASK_SIZE);
-
- current->thread.kuap = (addr & 0xf0000000) | ((((end - 1) >> 28) + 1) & 0xf);
- kuap_update_sr(mfsr(addr) & ~SR_KS, addr, end); /* Clear Ks */
+ current->thread.kuap = (__force u32)to;
+ kuap_unlock_one((__force u32)to);
}

static __always_inline void prevent_user_access(void __user *to, const void __user *from,
u32 size, unsigned long dir)
{
- u32 addr, end;
+ u32 kuap = current->thread.kuap;

BUILD_BUG_ON(!__builtin_constant_p(dir));
-
- if (dir & KUAP_CURRENT_WRITE) {
- u32 kuap = current->thread.kuap;
-
- if (unlikely(!kuap))
- return;
-
- addr = kuap & 0xf0000000;
- end = kuap << 28;
- } else if (dir & KUAP_WRITE) {
- addr = (__force u32)to;
- end = min(addr + size, TASK_SIZE);
-
- if (unlikely(addr >= TASK_SIZE || !size))
- return;
- } else {
+ if (!(dir & KUAP_WRITE))
return;
- }

- current->thread.kuap = 0;
- kuap_update_sr(mfsr(addr) | SR_KS, addr, end); /* set Ks */
+ current->thread.kuap = KUAP_NONE;
+ kuap_lock(kuap, true);
}

static inline unsigned long prevent_user_access_return(void)
{
unsigned long flags = current->thread.kuap;
- unsigned long addr = flags & 0xf0000000;
- unsigned long end = flags << 28;
- void __user *to = (__force void __user *)addr;

- if (flags)
- prevent_user_access(to, to, end - addr, KUAP_READ_WRITE);
+ if (flags != KUAP_NONE) {
+ current->thread.kuap = KUAP_NONE;
+ kuap_lock(flags, true);
+ }

return flags;
}

static inline void restore_user_access(unsigned long flags)
{
- unsigned long addr = flags & 0xf0000000;
- unsigned long end = flags << 28;
- void __user *to = (__force void __user *)addr;
-
- if (flags)
- allow_user_access(to, to, end - addr, KUAP_READ_WRITE);
+ if (flags != KUAP_NONE) {
+ current->thread.kuap = flags;
+ kuap_unlock(flags, true);
+ }
}

static inline bool
bad_kuap_fault(struct pt_regs *regs, unsigned long address, bool is_write)
{
- unsigned long begin = regs->kuap & 0xf0000000;
- unsigned long end = regs->kuap << 28;
+ unsigned long kuap = regs->kuap;
+
+ if (!is_write)
+ return false;
+ if (kuap == KUAP_NONE)
+ return true;
+ if (kuap == KUAP_ALL)
+ return false;
+
+ /* If faulting address doesn't match unlocked segment, unlock all */
+ if ((kuap ^ address) & 0xf0000000)
+ regs->kuap = KUAP_ALL;

- return is_write && (address < begin || address >= end);
+ return false;
}

#endif /* CONFIG_PPC_KUAP */
diff --git a/arch/powerpc/include/asm/processor.h b/arch/powerpc/include/asm/processor.h
index 7bf8a15af224..aeb1a35163d1 100644
--- a/arch/powerpc/include/asm/processor.h
+++ b/arch/powerpc/include/asm/processor.h
@@ -276,7 +276,15 @@ struct thread_struct {
#define SPEFSCR_INIT
#endif

-#ifdef CONFIG_PPC32
+#if defined(CONFIG_PPC_BOOK3S_32) && defined(CONFIG_PPC_KUAP)
+#define INIT_THREAD { \
+ .ksp = INIT_SP, \
+ .pgdir = swapper_pg_dir, \
+ .kuap = ~0UL, /* KUAP_NONE */ \
+ .fpexc_mode = MSR_FE0 | MSR_FE1, \
+ SPEFSCR_INIT \
+}
+#elif defined(CONFIG_PPC32)
#define INIT_THREAD { \
.ksp = INIT_SP, \
.pgdir = swapper_pg_dir, \
diff --git a/arch/powerpc/kernel/process.c b/arch/powerpc/kernel/process.c
index 89e34aa273e2..c6d21604457d 100644
--- a/arch/powerpc/kernel/process.c
+++ b/arch/powerpc/kernel/process.c
@@ -1736,6 +1736,9 @@ int copy_thread(unsigned long clone_flags, unsigned long usp,
#ifdef CONFIG_ALTIVEC
p->thread.vr_save_area = NULL;
#endif
+#if defined(CONFIG_PPC_BOOK3S_32) && defined(CONFIG_PPC_KUAP)
+ p->thread.kuap = KUAP_NONE;
+#endif

setup_ksp_vsid(p, sp);

diff --git a/arch/powerpc/mm/book3s32/Makefile b/arch/powerpc/mm/book3s32/Makefile
index 446d9de88ce4..50dd8f6bdf46 100644
--- a/arch/powerpc/mm/book3s32/Makefile
+++ b/arch/powerpc/mm/book3s32/Makefile
@@ -9,3 +9,4 @@ endif
obj-y += mmu.o mmu_context.o
obj-$(CONFIG_PPC_BOOK3S_603) += nohash_low.o
obj-$(CONFIG_PPC_BOOK3S_604) += hash_low.o tlb.o
+obj-$(CONFIG_PPC_KUAP) += kuap.o
diff --git a/arch/powerpc/mm/book3s32/kuap.c b/arch/powerpc/mm/book3s32/kuap.c
new file mode 100644
index 000000000000..5b01e746a90f
--- /dev/null
+++ b/arch/powerpc/mm/book3s32/kuap.c
@@ -0,0 +1,14 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
+
+#include <asm/kup.h>
+
+void kuap_lock_all_ool(void)
+{
+ kuap_lock_all();
+}
+
+void kuap_unlock_all_ool(void)
+{
+ kuap_unlock_all();
+}
+
--
2.25.0