[PATCH -next v3 0/2] powerpc: add support for syscall stack randomization

[PATCH -next v3 0/2] powerpc: add support for syscall stack randomization

[Xiu Jianfeng]

Hi,

This patchset adds support for syscall stack randomization for
powerpc, which can make harder the various stack-based attacks that
rely on deterministic stack structure.

Changes in v3:
  -add a lead-up patch to move system_call_exception() to syscall.c to
  avoid disabling stack protector for all functions in interrupt.c
Changes in v2:
  -move choose choose_random_kstack_offset() to the end of system_call_exception
  -allow full 6 (10) bits of entropy
  -disable stack-protector for interrupt.c

Xiu Jianfeng (2):
  powerpc: Move system_call_exception() to syscall.c
  powerpc: add support for syscall stack randomization

 arch/powerpc/Kconfig            |   1 +
 arch/powerpc/kernel/Makefile    |   9 +-
 arch/powerpc/kernel/interrupt.c | 161 ---------------------------
 arch/powerpc/kernel/syscall.c   | 190 ++++++++++++++++++++++++++++++++
 4 files changed, 199 insertions(+), 162 deletions(-)
 create mode 100644 arch/powerpc/kernel/syscall.c

-- 
2.17.1

[PATCH -next v3 1/2] powerpc: Move system_call_exception() to syscall.c

[Xiu Jianfeng]

This is a lead-up patch to enable syscall stack randomization, which
uses alloca() and makes the compiler add unconditional stack canaries
on syscall entry. In order to avoid triggering needless checks and
slowing down the entry path, the feature needs to disable stack
protector at the compilation unit level as there is no general way to
control stack protector coverage with a function attribute.

So move system_call_exception() to syscall.c to avoid affecting other
functions in interrupt.c.

Suggested-by: Michael Ellerman <mpe@ellerman.id.au>
Signed-off-by: Xiu Jianfeng <xiujianfeng@huawei.com>
---
 arch/powerpc/kernel/Makefile    |   2 +-
 arch/powerpc/kernel/interrupt.c | 161 -----------------------------
 arch/powerpc/kernel/syscall.c   | 173 ++++++++++++++++++++++++++++++++
 3 files changed, 174 insertions(+), 162 deletions(-)
 create mode 100644 arch/powerpc/kernel/syscall.c

diff --git a/arch/powerpc/kernel/Makefile b/arch/powerpc/kernel/Makefile
index f91f0f29a566..ecfd333b95d1 100644
--- a/arch/powerpc/kernel/Makefile
+++ b/arch/powerpc/kernel/Makefile
@@ -61,7 +61,7 @@ obj-y				:= cputable.o syscalls.o \
 				   udbg.o misc.o io.o misc_$(BITS).o \
 				   of_platform.o prom_parse.o firmware.o \
 				   hw_breakpoint_constraints.o interrupt.o \
-				   kdebugfs.o stacktrace.o
+				   kdebugfs.o stacktrace.o syscall.o
 obj-y				+= ptrace/
 obj-$(CONFIG_PPC64)		+= setup_64.o \
 				   paca.o nvram_64.o note.o
diff --git a/arch/powerpc/kernel/interrupt.c b/arch/powerpc/kernel/interrupt.c
index 784ea3289c84..0e75cb03244a 100644
--- a/arch/powerpc/kernel/interrupt.c
+++ b/arch/powerpc/kernel/interrupt.c
@@ -24,8 +24,6 @@
 unsigned long global_dbcr0[NR_CPUS];
 #endif
 
-typedef long (*syscall_fn)(long, long, long, long, long, long);
-
 #ifdef CONFIG_PPC_BOOK3S_64
 DEFINE_STATIC_KEY_FALSE(interrupt_exit_not_reentrant);
 static inline bool exit_must_hard_disable(void)
@@ -73,165 +71,6 @@ static notrace __always_inline bool prep_irq_for_enabled_exit(bool restartable)
 	return true;
 }
 
-/* Has to run notrace because it is entered not completely "reconciled" */
-notrace long system_call_exception(long r3, long r4, long r5,
-				   long r6, long r7, long r8,
-				   unsigned long r0, struct pt_regs *regs)
-{
-	syscall_fn f;
-
-	kuap_lock();
-
-	regs->orig_gpr3 = r3;
-
-	if (IS_ENABLED(CONFIG_PPC_IRQ_SOFT_MASK_DEBUG))
-		BUG_ON(irq_soft_mask_return() != IRQS_ALL_DISABLED);
-
-	trace_hardirqs_off(); /* finish reconciling */
-
-	CT_WARN_ON(ct_state() == CONTEXT_KERNEL);
-	user_exit_irqoff();
-
-	BUG_ON(regs_is_unrecoverable(regs));
-	BUG_ON(!(regs->msr & MSR_PR));
-	BUG_ON(arch_irq_disabled_regs(regs));
-
-#ifdef CONFIG_PPC_PKEY
-	if (mmu_has_feature(MMU_FTR_PKEY)) {
-		unsigned long amr, iamr;
-		bool flush_needed = false;
-		/*
-		 * When entering from userspace we mostly have the AMR/IAMR
-		 * different from kernel default values. Hence don't compare.
-		 */
-		amr = mfspr(SPRN_AMR);
-		iamr = mfspr(SPRN_IAMR);
-		regs->amr  = amr;
-		regs->iamr = iamr;
-		if (mmu_has_feature(MMU_FTR_BOOK3S_KUAP)) {
-			mtspr(SPRN_AMR, AMR_KUAP_BLOCKED);
-			flush_needed = true;
-		}
-		if (mmu_has_feature(MMU_FTR_BOOK3S_KUEP)) {
-			mtspr(SPRN_IAMR, AMR_KUEP_BLOCKED);
-			flush_needed = true;
-		}
-		if (flush_needed)
-			isync();
-	} else
-#endif
-		kuap_assert_locked();
-
-	booke_restore_dbcr0();
-
-	account_cpu_user_entry();
-
-	account_stolen_time();
-
-	/*
-	 * This is not required for the syscall exit path, but makes the
-	 * stack frame look nicer. If this was initialised in the first stack
-	 * frame, or if the unwinder was taught the first stack frame always
-	 * returns to user with IRQS_ENABLED, this store could be avoided!
-	 */
-	irq_soft_mask_regs_set_state(regs, IRQS_ENABLED);
-
-	/*
-	 * If system call is called with TM active, set _TIF_RESTOREALL to
-	 * prevent RFSCV being used to return to userspace, because POWER9
-	 * TM implementation has problems with this instruction returning to
-	 * transactional state. Final register values are not relevant because
-	 * the transaction will be aborted upon return anyway. Or in the case
-	 * of unsupported_scv SIGILL fault, the return state does not much
-	 * matter because it's an edge case.
-	 */
-	if (IS_ENABLED(CONFIG_PPC_TRANSACTIONAL_MEM) &&
-			unlikely(MSR_TM_TRANSACTIONAL(regs->msr)))
-		set_bits(_TIF_RESTOREALL, &current_thread_info()->flags);
-
-	/*
-	 * If the system call was made with a transaction active, doom it and
-	 * return without performing the system call. Unless it was an
-	 * unsupported scv vector, in which case it's treated like an illegal
-	 * instruction.
-	 */
-#ifdef CONFIG_PPC_TRANSACTIONAL_MEM
-	if (unlikely(MSR_TM_TRANSACTIONAL(regs->msr)) &&
-	    !trap_is_unsupported_scv(regs)) {
-		/* Enable TM in the kernel, and disable EE (for scv) */
-		hard_irq_disable();
-		mtmsr(mfmsr() | MSR_TM);
-
-		/* tabort, this dooms the transaction, nothing else */
-		asm volatile(".long 0x7c00071d | ((%0) << 16)"
-				:: "r"(TM_CAUSE_SYSCALL|TM_CAUSE_PERSISTENT));
-
-		/*
-		 * Userspace will never see the return value. Execution will
-		 * resume after the tbegin. of the aborted transaction with the
-		 * checkpointed register state. A context switch could occur
-		 * or signal delivered to the process before resuming the
-		 * doomed transaction context, but that should all be handled
-		 * as expected.
-		 */
-		return -ENOSYS;
-	}
-#endif // CONFIG_PPC_TRANSACTIONAL_MEM
-
-	local_irq_enable();
-
-	if (unlikely(read_thread_flags() & _TIF_SYSCALL_DOTRACE)) {
-		if (unlikely(trap_is_unsupported_scv(regs))) {
-			/* Unsupported scv vector */
-			_exception(SIGILL, regs, ILL_ILLOPC, regs->nip);
-			return regs->gpr[3];
-		}
-		/*
-		 * We use the return value of do_syscall_trace_enter() as the
-		 * syscall number. If the syscall was rejected for any reason
-		 * do_syscall_trace_enter() returns an invalid syscall number
-		 * and the test against NR_syscalls will fail and the return
-		 * value to be used is in regs->gpr[3].
-		 */
-		r0 = do_syscall_trace_enter(regs);
-		if (unlikely(r0 >= NR_syscalls))
-			return regs->gpr[3];
-		r3 = regs->gpr[3];
-		r4 = regs->gpr[4];
-		r5 = regs->gpr[5];
-		r6 = regs->gpr[6];
-		r7 = regs->gpr[7];
-		r8 = regs->gpr[8];
-
-	} else if (unlikely(r0 >= NR_syscalls)) {
-		if (unlikely(trap_is_unsupported_scv(regs))) {
-			/* Unsupported scv vector */
-			_exception(SIGILL, regs, ILL_ILLOPC, regs->nip);
-			return regs->gpr[3];
-		}
-		return -ENOSYS;
-	}
-
-	/* May be faster to do array_index_nospec? */
-	barrier_nospec();
-
-	if (unlikely(is_compat_task())) {
-		f = (void *)compat_sys_call_table[r0];
-
-		r3 &= 0x00000000ffffffffULL;
-		r4 &= 0x00000000ffffffffULL;
-		r5 &= 0x00000000ffffffffULL;
-		r6 &= 0x00000000ffffffffULL;
-		r7 &= 0x00000000ffffffffULL;
-		r8 &= 0x00000000ffffffffULL;
-
-	} else {
-		f = (void *)sys_call_table[r0];
-	}
-
-	return f(r3, r4, r5, r6, r7, r8);
-}
-
 static notrace void booke_load_dbcr0(void)
 {
 #ifdef CONFIG_PPC_ADV_DEBUG_REGS
diff --git a/arch/powerpc/kernel/syscall.c b/arch/powerpc/kernel/syscall.c
new file mode 100644
index 000000000000..4d5689eeaf25
--- /dev/null
+++ b/arch/powerpc/kernel/syscall.c
@@ -0,0 +1,173 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
+
+#include <linux/compat.h>
+#include <linux/context_tracking.h>
+
+#include <asm/interrupt.h>
+#include <asm/kup.h>
+#include <asm/syscall.h>
+#include <asm/time.h>
+#include <asm/tm.h>
+#include <asm/unistd.h>
+
+
+typedef long (*syscall_fn)(long, long, long, long, long, long);
+
+/* Has to run notrace because it is entered not completely "reconciled" */
+notrace long system_call_exception(long r3, long r4, long r5,
+				   long r6, long r7, long r8,
+				   unsigned long r0, struct pt_regs *regs)
+{
+	syscall_fn f;
+
+	kuap_lock();
+
+	regs->orig_gpr3 = r3;
+
+	if (IS_ENABLED(CONFIG_PPC_IRQ_SOFT_MASK_DEBUG))
+		BUG_ON(irq_soft_mask_return() != IRQS_ALL_DISABLED);
+
+	trace_hardirqs_off(); /* finish reconciling */
+
+	CT_WARN_ON(ct_state() == CONTEXT_KERNEL);
+	user_exit_irqoff();
+
+	BUG_ON(regs_is_unrecoverable(regs));
+	BUG_ON(!(regs->msr & MSR_PR));
+	BUG_ON(arch_irq_disabled_regs(regs));
+
+#ifdef CONFIG_PPC_PKEY
+	if (mmu_has_feature(MMU_FTR_PKEY)) {
+		unsigned long amr, iamr;
+		bool flush_needed = false;
+		/*
+		 * When entering from userspace we mostly have the AMR/IAMR
+		 * different from kernel default values. Hence don't compare.
+		 */
+		amr = mfspr(SPRN_AMR);
+		iamr = mfspr(SPRN_IAMR);
+		regs->amr  = amr;
+		regs->iamr = iamr;
+		if (mmu_has_feature(MMU_FTR_BOOK3S_KUAP)) {
+			mtspr(SPRN_AMR, AMR_KUAP_BLOCKED);
+			flush_needed = true;
+		}
+		if (mmu_has_feature(MMU_FTR_BOOK3S_KUEP)) {
+			mtspr(SPRN_IAMR, AMR_KUEP_BLOCKED);
+			flush_needed = true;
+		}
+		if (flush_needed)
+			isync();
+	} else
+#endif
+		kuap_assert_locked();
+
+	booke_restore_dbcr0();
+
+	account_cpu_user_entry();
+
+	account_stolen_time();
+
+	/*
+	 * This is not required for the syscall exit path, but makes the
+	 * stack frame look nicer. If this was initialised in the first stack
+	 * frame, or if the unwinder was taught the first stack frame always
+	 * returns to user with IRQS_ENABLED, this store could be avoided!
+	 */
+	irq_soft_mask_regs_set_state(regs, IRQS_ENABLED);
+
+	/*
+	 * If system call is called with TM active, set _TIF_RESTOREALL to
+	 * prevent RFSCV being used to return to userspace, because POWER9
+	 * TM implementation has problems with this instruction returning to
+	 * transactional state. Final register values are not relevant because
+	 * the transaction will be aborted upon return anyway. Or in the case
+	 * of unsupported_scv SIGILL fault, the return state does not much
+	 * matter because it's an edge case.
+	 */
+	if (IS_ENABLED(CONFIG_PPC_TRANSACTIONAL_MEM) &&
+			unlikely(MSR_TM_TRANSACTIONAL(regs->msr)))
+		set_bits(_TIF_RESTOREALL, &current_thread_info()->flags);
+
+	/*
+	 * If the system call was made with a transaction active, doom it and
+	 * return without performing the system call. Unless it was an
+	 * unsupported scv vector, in which case it's treated like an illegal
+	 * instruction.
+	 */
+#ifdef CONFIG_PPC_TRANSACTIONAL_MEM
+	if (unlikely(MSR_TM_TRANSACTIONAL(regs->msr)) &&
+	    !trap_is_unsupported_scv(regs)) {
+		/* Enable TM in the kernel, and disable EE (for scv) */
+		hard_irq_disable();
+		mtmsr(mfmsr() | MSR_TM);
+
+		/* tabort, this dooms the transaction, nothing else */
+		asm volatile(".long 0x7c00071d | ((%0) << 16)"
+				:: "r"(TM_CAUSE_SYSCALL|TM_CAUSE_PERSISTENT));
+
+		/*
+		 * Userspace will never see the return value. Execution will
+		 * resume after the tbegin. of the aborted transaction with the
+		 * checkpointed register state. A context switch could occur
+		 * or signal delivered to the process before resuming the
+		 * doomed transaction context, but that should all be handled
+		 * as expected.
+		 */
+		return -ENOSYS;
+	}
+#endif // CONFIG_PPC_TRANSACTIONAL_MEM
+
+	local_irq_enable();
+
+	if (unlikely(read_thread_flags() & _TIF_SYSCALL_DOTRACE)) {
+		if (unlikely(trap_is_unsupported_scv(regs))) {
+			/* Unsupported scv vector */
+			_exception(SIGILL, regs, ILL_ILLOPC, regs->nip);
+			return regs->gpr[3];
+		}
+		/*
+		 * We use the return value of do_syscall_trace_enter() as the
+		 * syscall number. If the syscall was rejected for any reason
+		 * do_syscall_trace_enter() returns an invalid syscall number
+		 * and the test against NR_syscalls will fail and the return
+		 * value to be used is in regs->gpr[3].
+		 */
+		r0 = do_syscall_trace_enter(regs);
+		if (unlikely(r0 >= NR_syscalls))
+			return regs->gpr[3];
+		r3 = regs->gpr[3];
+		r4 = regs->gpr[4];
+		r5 = regs->gpr[5];
+		r6 = regs->gpr[6];
+		r7 = regs->gpr[7];
+		r8 = regs->gpr[8];
+
+	} else if (unlikely(r0 >= NR_syscalls)) {
+		if (unlikely(trap_is_unsupported_scv(regs))) {
+			/* Unsupported scv vector */
+			_exception(SIGILL, regs, ILL_ILLOPC, regs->nip);
+			return regs->gpr[3];
+		}
+		return -ENOSYS;
+	}
+
+	/* May be faster to do array_index_nospec? */
+	barrier_nospec();
+
+	if (unlikely(is_compat_task())) {
+		f = (void *)compat_sys_call_table[r0];
+
+		r3 &= 0x00000000ffffffffULL;
+		r4 &= 0x00000000ffffffffULL;
+		r5 &= 0x00000000ffffffffULL;
+		r6 &= 0x00000000ffffffffULL;
+		r7 &= 0x00000000ffffffffULL;
+		r8 &= 0x00000000ffffffffULL;
+
+	} else {
+		f = (void *)sys_call_table[r0];
+	}
+
+	return f(r3, r4, r5, r6, r7, r8);
+}
-- 
2.17.1

[PATCH -next v3 2/2] powerpc: add support for syscall stack randomization

[Xiu Jianfeng]

Add support for adding a random offset to the stack while handling
syscalls. This patch uses mftb() instead of get_random_int() for better
performance.

In order to avoid unconditional stack canaries on syscall entry (due to
the use of alloca()), also disable stack protector to avoid triggering
needless checks and slowing down the entry path. As there is no general
way to control stack protector coverage with a function attribute, this
must be disabled at the compilation unit level.

Signed-off-by: Xiu Jianfeng <xiujianfeng@huawei.com>
---
 arch/powerpc/Kconfig          |  1 +
 arch/powerpc/kernel/Makefile  |  7 +++++++
 arch/powerpc/kernel/syscall.c | 19 ++++++++++++++++++-
 3 files changed, 26 insertions(+), 1 deletion(-)

diff --git a/arch/powerpc/Kconfig b/arch/powerpc/Kconfig
index c2ce2e60c8f0..a402b97abc9c 100644
--- a/arch/powerpc/Kconfig
+++ b/arch/powerpc/Kconfig
@@ -195,6 +195,7 @@ config PPC
 	select HAVE_ARCH_KASAN			if PPC_RADIX_MMU
 	select HAVE_ARCH_KASAN_VMALLOC		if HAVE_ARCH_KASAN
 	select HAVE_ARCH_KFENCE			if PPC_BOOK3S_32 || PPC_8xx || 40x
+	select HAVE_ARCH_RANDOMIZE_KSTACK_OFFSET
 	select HAVE_ARCH_KGDB
 	select HAVE_ARCH_MMAP_RND_BITS
 	select HAVE_ARCH_MMAP_RND_COMPAT_BITS	if COMPAT
diff --git a/arch/powerpc/kernel/Makefile b/arch/powerpc/kernel/Makefile
index ecfd333b95d1..c29a58ea6e31 100644
--- a/arch/powerpc/kernel/Makefile
+++ b/arch/powerpc/kernel/Makefile
@@ -53,6 +53,13 @@ CFLAGS_cputable.o += -DDISABLE_BRANCH_PROFILING
 CFLAGS_btext.o += -DDISABLE_BRANCH_PROFILING
 endif
 
+#ifdef CONFIG_RANDOMIZE_KSTACK_OFFSET
+# Remove stack protector to avoid triggering unneeded stack canary
+# checks due to randomize_kstack_offset.
+CFLAGS_REMOVE_syscall.o = -fstack-protector -fstack-protector-strong
+CFLAGS_syscall.o += -fno-stack-protector
+#endif
+
 obj-y				:= cputable.o syscalls.o \
 				   irq.o align.o signal_$(BITS).o pmc.o vdso.o \
 				   process.o systbl.o idle.o \
diff --git a/arch/powerpc/kernel/syscall.c b/arch/powerpc/kernel/syscall.c
index 4d5689eeaf25..9a30fe0d3a93 100644
--- a/arch/powerpc/kernel/syscall.c
+++ b/arch/powerpc/kernel/syscall.c
@@ -2,6 +2,7 @@
 
 #include <linux/compat.h>
 #include <linux/context_tracking.h>
+#include <linux/randomize_kstack.h>
 
 #include <asm/interrupt.h>
 #include <asm/kup.h>
@@ -18,10 +19,12 @@ notrace long system_call_exception(long r3, long r4, long r5,
 				   long r6, long r7, long r8,
 				   unsigned long r0, struct pt_regs *regs)
 {
+	long ret;
 	syscall_fn f;
 
 	kuap_lock();
 
+	add_random_kstack_offset();
 	regs->orig_gpr3 = r3;
 
 	if (IS_ENABLED(CONFIG_PPC_IRQ_SOFT_MASK_DEBUG))
@@ -169,5 +172,19 @@ notrace long system_call_exception(long r3, long r4, long r5,
 		f = (void *)sys_call_table[r0];
 	}
 
-	return f(r3, r4, r5, r6, r7, r8);
+	ret = f(r3, r4, r5, r6, r7, r8);
+	/*
+	 * Ultimately, this value will get limited by KSTACK_OFFSET_MAX(),
+	 * so the maximum stack offset is 1k bytes(10 bits).
+	 *
+	 * The actual entropy will be further reduced by the compiler when
+	 * applying stack alignment constraints: the powerpc architecture
+	 * may have two kinds of stack alignment(16-bytes and 8-bytes).
+	 *
+	 * So the resulting 6 or 7 bits of entropy is seen in SP[9:4] or SP[9:3].
+	 *
+	 */
+	choose_random_kstack_offset(mftb());
+
+	return ret;
 }
-- 
2.17.1

Re: [PATCH -next v3 2/2] powerpc: add support for syscall stack randomization

[Kees Cook]

On Fri, Jul 01, 2022 at 04:24:35PM +0800, Xiu Jianfeng wrote:
> Add support for adding a random offset to the stack while handling
> syscalls. This patch uses mftb() instead of get_random_int() for better
> performance.
> 
> In order to avoid unconditional stack canaries on syscall entry (due to
> the use of alloca()), also disable stack protector to avoid triggering
> needless checks and slowing down the entry path. As there is no general
> way to control stack protector coverage with a function attribute, this
> must be disabled at the compilation unit level.
> 
> Signed-off-by: Xiu Jianfeng <xiujianfeng@huawei.com>

Reviewed-by: Kees Cook <keescook@chromium.org>

-Kees

-- 
Kees Cook

Re: [PATCH -next v3 0/2] powerpc: add support for syscall stack randomization

[Michael Ellerman]

On Fri, 1 Jul 2022 16:24:33 +0800, Xiu Jianfeng wrote:
> This patchset adds support for syscall stack randomization for
> powerpc, which can make harder the various stack-based attacks that
> rely on deterministic stack structure.
> 
> Changes in v3:
>   -add a lead-up patch to move system_call_exception() to syscall.c to
>   avoid disabling stack protector for all functions in interrupt.c
> Changes in v2:
>   -move choose choose_random_kstack_offset() to the end of system_call_exception
>   -allow full 6 (10) bits of entropy
>   -disable stack-protector for interrupt.c
> 
> [...]

Applied to powerpc/next.

[1/2] powerpc: Move system_call_exception() to syscall.c
      https://git.kernel.org/powerpc/c/1547db7d1f4481c1f3ec731f3edc724ef3026ede
[2/2] powerpc: add support for syscall stack randomization
      https://git.kernel.org/powerpc/c/f4a0318f278d98d9492916722e85f258c2221f88

cheers