[PATCH v6 0/2] riscv: kprobes: reject probes inside LR/SC sequences

[PATCH v6 0/2] riscv: kprobes: reject probes inside LR/SC sequences

[Xiaofeng Yuan]

A breakpoint trap taken in the middle of an LR/SC sequence clears the
load reservation, so an SC following the probed instruction always fails
and the enclosing retry loop re-enters the breakpoint, livelocking the
CPU.  This series makes the RISC-V kprobes implementation reject probes
placed on an LR/SC instruction or between them, following arm64's
approach for its equivalent exclusive load/store instructions.

Changes since v5:
- Fix a dangling stack pointer in the KUnit test: a probe that was
  expected to be rejected but was unexpectedly registered stayed on
  a stack-allocated kprobe.
- Add Acked-by: Masami Hiramatsu to the implementation patch.
- Annotate that test_kprobes_lrsc() is never executed and that
  reject[] must match the length of test_kprobes_lrsc_offsets[].

Verified by cross-compiling for RISC-V and running the kprobes KUnit
test in QEMU (ok 2 kprobes_riscv).

Xiaofeng Yuan (2):
  riscv: kprobes: reject probes inside LR/SC sequences
  riscv: kprobes: add KUnit test for LR/SC sequence rejection

 arch/riscv/include/asm/insn.h                 | 33 ++++++++++
 arch/riscv/kernel/probes/decode-insn.c        |  2 +
 arch/riscv/kernel/probes/kprobes.c            | 63 +++++++++++++++----
 .../kernel/tests/kprobes/test-kprobes-asm.S   | 30 +++++++++
 .../riscv/kernel/tests/kprobes/test-kprobes.c | 31 +++++++++
 .../riscv/kernel/tests/kprobes/test-kprobes.h |  3 +
 6 files changed, 150 insertions(+), 12 deletions(-)

-- 
2.34.1


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[PATCH v6 1/2] riscv: kprobes: reject probes inside LR/SC sequences

[Xiaofeng Yuan]

A trap taken in the middle of an LR/SC sequence clears the load
reservation, so an SC following the probed instruction always fails and
the enclosing retry loop re-enters the breakpoint, livelocking the CPU.
Reject probes of the LR/SC instructions themselves and of any address
between them.

arm64's kprobes rejects its equivalent exclusive load/store instructions
and, in arm_kprobe_decode_insn(), walks backward from the probe address
(bounded by the enclosing symbol) to reject any address inside an
exclusive sequence.  RISC-V cannot walk backward because
variable-length instructions make instruction boundaries ambiguous, so
walk forward from the enclosing symbol, a known instruction boundary, up
to the probe address and track whether an LR is still outstanding.

A Zalrsc-constrained LR/SC retry loop holds at most 16 instructions, so
an LR more than MAX_LRSC_GAP instructions ahead of the probe cannot
start a sequence containing it.  Count instructions, not bytes:
compressed instructions would make a byte window ambiguous.

The LR/SC instructions themselves are rejected in
riscv_probe_decode_insn(), and an address inside a sequence is rejected
in arch_check_kprobe().

Signed-off-by: Xiaofeng Yuan <yuanxiaofeng@eswincomputing.com>
Acked-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
---
 arch/riscv/include/asm/insn.h          | 33 ++++++++++++++
 arch/riscv/kernel/probes/decode-insn.c |  2 +
 arch/riscv/kernel/probes/kprobes.c     | 63 +++++++++++++++++++++-----
 3 files changed, 86 insertions(+), 12 deletions(-)

diff --git a/arch/riscv/include/asm/insn.h b/arch/riscv/include/asm/insn.h
index ad348d8b8..46a1666d2 100644
--- a/arch/riscv/include/asm/insn.h
+++ b/arch/riscv/include/asm/insn.h
@@ -141,6 +141,7 @@
 #define RVG_OPCODE_JALR		0x67
 #define RVG_OPCODE_JAL		0x6f
 #define RVG_OPCODE_SYSTEM	0x73
+#define RVG_OPCODE_AMO		0x2f
 #define RVG_SYSTEM_CSR_OFF	20
 #define RVG_SYSTEM_CSR_MASK	GENMASK(11, 0)
 
@@ -175,6 +176,8 @@
 #define RVG_FUNCT3_BGE		0x5
 #define RVG_FUNCT3_BLTU		0x6
 #define RVG_FUNCT3_BGEU		0x7
+#define RVG_FUNCT3_AMO_W	0x2
+#define RVG_FUNCT3_AMO_D	0x3
 
 /* parts of funct3 code for C extension*/
 #define RVC_FUNCT3_C_BEQZ	0x6
@@ -263,6 +266,36 @@ __RISCV_INSN_FUNCS(ebreak, RVG_MASK_EBREAK, RVG_MATCH_EBREAK)
 __RISCV_INSN_FUNCS(sret, RVG_MASK_SRET, RVG_MATCH_SRET)
 __RISCV_INSN_FUNCS(fence, RVG_MASK_FENCE, RVG_MATCH_FENCE);
 
+/*
+ * LR/SC are identified by the AMO opcode plus the funct3 width field
+ * (.w/.d) and the funct5 field (funct7[6:2]) distinguishing LR from SC.
+ * The aq/rl bits (funct7[1:0]) are ignored.  For LR the rs2 field must
+ * encode x0.
+ */
+#define RVG_AMO_FUNCT5_LR	0x02
+#define RVG_AMO_FUNCT5_SC	0x03
+
+static __always_inline bool riscv_insn_is_lr(u32 code)
+{
+	u32 funct5 = (code >> 27) & 0x1f;
+	u32 funct3 = (code >> 12) & 0x7;
+
+	return (code & RV_INSN_OPCODE_MASK) == RVG_OPCODE_AMO &&
+	       funct5 == RVG_AMO_FUNCT5_LR &&
+	       (funct3 == RVG_FUNCT3_AMO_W || funct3 == RVG_FUNCT3_AMO_D) &&
+	       !(code & GENMASK(24, 20));
+}
+
+static __always_inline bool riscv_insn_is_sc(u32 code)
+{
+	u32 funct5 = (code >> 27) & 0x1f;
+	u32 funct3 = (code >> 12) & 0x7;
+
+	return (code & RV_INSN_OPCODE_MASK) == RVG_OPCODE_AMO &&
+	       funct5 == RVG_AMO_FUNCT5_SC &&
+	       (funct3 == RVG_FUNCT3_AMO_W || funct3 == RVG_FUNCT3_AMO_D);
+}
+
 /* special case to catch _any_ system instruction */
 static __always_inline bool riscv_insn_is_system(u32 code)
 {
diff --git a/arch/riscv/kernel/probes/decode-insn.c b/arch/riscv/kernel/probes/decode-insn.c
index 433d9035b..e6a889467 100644
--- a/arch/riscv/kernel/probes/decode-insn.c
+++ b/arch/riscv/kernel/probes/decode-insn.c
@@ -23,6 +23,8 @@ riscv_probe_decode_insn(probe_opcode_t *addr, struct arch_probe_insn *api)
 	 */
 	RISCV_INSN_REJECTED(system,		insn);
 	RISCV_INSN_REJECTED(fence,		insn);
+	RISCV_INSN_REJECTED(lr,			insn);
+	RISCV_INSN_REJECTED(sc,			insn);
 
 	/*
 	 * Simulate instructions list:
diff --git a/arch/riscv/kernel/probes/kprobes.c b/arch/riscv/kernel/probes/kprobes.c
index a76eab297..8f20f1535 100644
--- a/arch/riscv/kernel/probes/kprobes.c
+++ b/arch/riscv/kernel/probes/kprobes.c
@@ -7,11 +7,13 @@
 #include <linux/slab.h>
 #include <linux/stop_machine.h>
 #include <linux/vmalloc.h>
+#include <linux/unaligned.h>
 #include <asm/ptrace.h>
 #include <linux/uaccess.h>
 #include <asm/sections.h>
 #include <asm/cacheflush.h>
 #include <asm/bug.h>
+#include <asm/insn.h>
 #include <asm/text-patching.h>
 
 #include "decode-insn.h"
@@ -49,35 +51,72 @@ static void __kprobes arch_simulate_insn(struct kprobe *p, struct pt_regs *regs)
 	post_kprobe_handler(p, kcb, regs);
 }
 
-static bool __kprobes arch_check_kprobe(unsigned long addr)
+/*
+ * A trap taken in the middle of an LR/SC sequence clears the load
+ * reservation, so an SC following the probed instruction would always
+ * fail and the enclosing retry loop would re-enter the breakpoint.
+ * Reject probes inside such a sequence.
+ *
+ * RISC-V instruction boundaries cannot be recovered by walking
+ * backwards - a 32-bit instruction whose upper halfword looks like a
+ * compressed instruction is ambiguous - so walk forward from the
+ * enclosing symbol, a known instruction boundary, up to the probe address
+ * and track whether an LR is still outstanding.  A Zalrsc-constrained
+ * LR/SC retry loop holds at most 16 instructions, so an LR more than
+ * that many instructions ahead of the probe cannot start a sequence
+ * containing it.  Count instructions, not bytes: compressed
+ * instructions would make a byte window ambiguous.
+ */
+#define MAX_LRSC_GAP	16
+
+/*
+ * Verify that @addr points at the start of an instruction and is not
+ * inside an LR/SC sequence.  Returns 0 on success, -EILSEQ if @addr is
+ * not an instruction boundary, or -EINVAL if the probe would land
+ * inside an LR/SC sequence.
+ */
+static int __kprobes arch_check_kprobe(unsigned long addr)
 {
-	unsigned long tmp, offset;
+	unsigned long pc, offset;
+	int outstanding_lr = -1;
 
-	/* start iterating at the closest preceding symbol */
 	if (!kallsyms_lookup_size_offset(addr, NULL, &offset))
-		return false;
+		return -EILSEQ;
 
-	tmp = addr - offset;
+	/* pc starts at the closest preceding symbol, a known instruction boundary */
+	pc = addr - offset;
 
-	while (tmp <= addr) {
-		if (tmp == addr)
-			return true;
+	while (pc < addr) {
+		u32 insn = get_unaligned((u32 *)pc);
+		unsigned int len = GET_INSN_LENGTH(insn);
 
-		tmp += GET_INSN_LENGTH(*(u16 *)tmp);
+		if (riscv_insn_is_lr(insn))
+			outstanding_lr = 0;
+		else if (riscv_insn_is_sc(insn))
+			outstanding_lr = -1;
+		else if (outstanding_lr >= 0 && ++outstanding_lr > MAX_LRSC_GAP)
+			outstanding_lr = -1;
+
+		pc += len;
 	}
 
-	return false;
+	if (pc != addr)
+		return -EILSEQ;
+
+	return outstanding_lr >= 0 ? -EINVAL : 0;
 }
 
 int __kprobes arch_prepare_kprobe(struct kprobe *p)
 {
 	u16 *insn = (u16 *)p->addr;
+	int ret;
 
 	if ((unsigned long)insn & 0x1)
 		return -EILSEQ;
 
-	if (!arch_check_kprobe((unsigned long)p->addr))
-		return -EILSEQ;
+	ret = arch_check_kprobe((unsigned long)p->addr);
+	if (ret)
+		return ret;
 
 	/* copy instruction */
 	p->opcode = (kprobe_opcode_t)(*insn++);
-- 
2.34.1


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[PATCH v6 2/2] riscv: kprobes: add KUnit test for LR/SC sequence rejection

[Xiaofeng Yuan]

Extend the kprobes KUnit test with a function containing an LR/SC
sequence and assert that register_kprobe() accepts a probe placed right
before or right after the sequence, and rejects a probe placed on the
LR, on an instruction inside the sequence, or on the SC.

Signed-off-by: Xiaofeng Yuan <yuanxiaofeng@eswincomputing.com>
---
 .../kernel/tests/kprobes/test-kprobes-asm.S   | 30 ++++++++++++++++++
 .../riscv/kernel/tests/kprobes/test-kprobes.c | 31 +++++++++++++++++++
 .../riscv/kernel/tests/kprobes/test-kprobes.h |  3 ++
 3 files changed, 64 insertions(+)

diff --git a/arch/riscv/kernel/tests/kprobes/test-kprobes-asm.S b/arch/riscv/kernel/tests/kprobes/test-kprobes-asm.S
index 15e50934b..76826bf7e 100644
--- a/arch/riscv/kernel/tests/kprobes/test-kprobes-asm.S
+++ b/arch/riscv/kernel/tests/kprobes/test-kprobes-asm.S
@@ -100,6 +100,27 @@ test_kprobes_branch_addr8:
 	.option pop
 SYM_FUNC_END(test_kprobes_branch)
 
+/*
+ * This function is never executed; it only provides the probe addresses
+ * in test_kprobes_lrsc_offsets[] for test_kprobe_lrsc().  The lr.w/sc.w
+ * use the invalid address 0(x0) and would fault if executed, so do not
+ * add this function to test_kprobes_functions[].
+ */
+SYM_FUNC_START(test_kprobes_lrsc)
+.Llrsc_before:
+	nop
+1:
+.Llrsc_lr:
+	lr.w t0, 0(x0)
+.Llrsc_inside:
+	addi t0, t0, 1
+.Llrsc_sc:
+	sc.w t0, t0, 0(x0)
+	bnez t0, 1b
+.Llrsc_after:
+	ret
+SYM_FUNC_END(test_kprobes_lrsc)
+
 #ifdef CONFIG_RISCV_ISA_C
 
 SYM_FUNC_START(test_kprobes_c_j)
@@ -253,3 +274,12 @@ SYM_DATA_START(test_kprobes_functions)
 #endif /* CONFIG_RISCV_ISA_C */
 	RISCV_PTR 0
 SYM_DATA_END(test_kprobes_functions)
+
+.section .rodata
+SYM_DATA_START(test_kprobes_lrsc_offsets)
+	RISCV_PTR .Llrsc_before
+	RISCV_PTR .Llrsc_lr
+	RISCV_PTR .Llrsc_inside
+	RISCV_PTR .Llrsc_sc
+	RISCV_PTR .Llrsc_after
+SYM_DATA_END(test_kprobes_lrsc_offsets)
diff --git a/arch/riscv/kernel/tests/kprobes/test-kprobes.c b/arch/riscv/kernel/tests/kprobes/test-kprobes.c
index 027424a3f..c2d9c87f2 100644
--- a/arch/riscv/kernel/tests/kprobes/test-kprobes.c
+++ b/arch/riscv/kernel/tests/kprobes/test-kprobes.c
@@ -43,8 +43,39 @@ static void test_kprobe_riscv(struct kunit *test)
 	kfree(kp);
 }
 
+static void test_kprobe_lrsc(struct kunit *test)
+{
+	/*
+	 * test_kprobes_lrsc_offsets[] points, in order, at an instruction
+	 * right before an LR/SC sequence, the LR itself, an instruction
+	 * inside the sequence, the SC, and an instruction right after it.
+	 * reject[] must have the same number of entries.
+	 */
+	bool reject[] = { false, true, true, true, false };
+	int i;
+
+	for (i = 0; i < ARRAY_SIZE(reject); i++) {
+		struct kprobe kp = {};
+		int ret;
+
+		kp.pre_handler = kprobe_dummy_handler;
+		kp.addr = (kprobe_opcode_t *)test_kprobes_lrsc_offsets[i];
+
+		ret = register_kprobe(&kp);
+
+		if (reject[i])
+			KUNIT_EXPECT_LT(test, ret, 0);
+		else
+			KUNIT_EXPECT_EQ(test, ret, 0);
+
+		if (!ret)
+			unregister_kprobe(&kp);
+	}
+}
+
 static struct kunit_case kprobes_testcases[] = {
 	KUNIT_CASE(test_kprobe_riscv),
+	KUNIT_CASE(test_kprobe_lrsc),
 	{}
 };
 
diff --git a/arch/riscv/kernel/tests/kprobes/test-kprobes.h b/arch/riscv/kernel/tests/kprobes/test-kprobes.h
index 537f44aa9..65a8e7217 100644
--- a/arch/riscv/kernel/tests/kprobes/test-kprobes.h
+++ b/arch/riscv/kernel/tests/kprobes/test-kprobes.h
@@ -19,6 +19,9 @@ extern void *test_kprobes_addresses[];
 /* array of functions that return KPROBE_TEST_MAGIC */
 extern long (*test_kprobes_functions[])(void);
 
+/* offsets of the probe points inside test_kprobes_lrsc() */
+extern void *test_kprobes_lrsc_offsets[];
+
 #endif /* __ASSEMBLER__ */
 
 #endif /* TEST_KPROBES_H */
-- 
2.34.1


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Re: [PATCH v6 1/2] riscv: kprobes: reject probes inside LR/SC sequences

[Nam Cao]

Xiaofeng Yuan <yuanxiaofeng@eswincomputing.com> writes:
> +static __always_inline bool riscv_insn_is_lr(u32 code)
> +{
> +	u32 funct5 = (code >> 27) & 0x1f;
> +	u32 funct3 = (code >> 12) & 0x7;
> +
> +	return (code & RV_INSN_OPCODE_MASK) == RVG_OPCODE_AMO &&
> +	       funct5 == RVG_AMO_FUNCT5_LR &&
> +	       (funct3 == RVG_FUNCT3_AMO_W || funct3 == RVG_FUNCT3_AMO_D) &&
> +	       !(code & GENMASK(24, 20));
> +}
> +
> +static __always_inline bool riscv_insn_is_sc(u32 code)
> +{
> +	u32 funct5 = (code >> 27) & 0x1f;
> +	u32 funct3 = (code >> 12) & 0x7;
> +
> +	return (code & RV_INSN_OPCODE_MASK) == RVG_OPCODE_AMO &&
> +	       funct5 == RVG_AMO_FUNCT5_SC &&
> +	       (funct3 == RVG_FUNCT3_AMO_W || funct3 == RVG_FUNCT3_AMO_D);
> +}

There are already RV_EXTRACT_FUNCT3() and RV_EXTRACT_RS2_REG(), we
should use them to avoid code duplications.

And I think we should also have RV_EXTRACT_FUNCT5() to stay consistent.

> +static int __kprobes arch_check_kprobe(unsigned long addr)
>  {
> -	unsigned long tmp, offset;
> +	unsigned long pc, offset;
> +	int outstanding_lr = -1;
>  
> -	/* start iterating at the closest preceding symbol */
>  	if (!kallsyms_lookup_size_offset(addr, NULL, &offset))
> -		return false;
> +		return -EILSEQ;
>  
> -	tmp = addr - offset;
> +	/* pc starts at the closest preceding symbol, a known instruction boundary */
> +	pc = addr - offset;
>  
> -	while (tmp <= addr) {
> -		if (tmp == addr)
> -			return true;
> +	while (pc < addr) {
> +		u32 insn = get_unaligned((u32 *)pc);
> +		unsigned int len = GET_INSN_LENGTH(insn);
>  
> -		tmp += GET_INSN_LENGTH(*(u16 *)tmp);
> +		if (riscv_insn_is_lr(insn))
> +			outstanding_lr = 0;
> +		else if (riscv_insn_is_sc(insn))
> +			outstanding_lr = -1;
> +		else if (outstanding_lr >= 0 && ++outstanding_lr > MAX_LRSC_GAP)
> +			outstanding_lr = -1;
> +
> +		pc += len;
>  	}

The spec says "The loop comprises only an LR/SC sequence and code to
retry the sequence in the case of failure, and must comprise at most 16
instructions placed sequentially in memory". The 16 instructions limit
includes LR. So I think this implementation is off by one:

    lr     (outstanding_lr=0)
    inst1  (outstanding_lr=1)
    inst2  (outstanding_lr=2)
    ...
    inst16 (outstanding_lr=16)
    inst17 (outstanding_lr=-1)

As it is considering instruction from LR to inst16 to be a loop, so it
allows 17 instructions.

Nam

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Re: [PATCH v6 2/2] riscv: kprobes: add KUnit test for LR/SC sequence rejection

[Nam Cao]

Xiaofeng Yuan <yuanxiaofeng@eswincomputing.com> writes:
> +.section .rodata
> +SYM_DATA_START(test_kprobes_lrsc_offsets)
> +	RISCV_PTR .Llrsc_before
> +	RISCV_PTR .Llrsc_lr
> +	RISCV_PTR .Llrsc_inside
> +	RISCV_PTR .Llrsc_sc
> +	RISCV_PTR .Llrsc_after
> +SYM_DATA_END(test_kprobes_lrsc_offsets)

"_offsets" is not precise, as this array has absolute addresses. Maybe
rename to "_addresses"?

> diff --git a/arch/riscv/kernel/tests/kprobes/test-kprobes.c b/arch/riscv/kernel/tests/kprobes/test-kprobes.c
> index 027424a3f..c2d9c87f2 100644
> --- a/arch/riscv/kernel/tests/kprobes/test-kprobes.c
> +++ b/arch/riscv/kernel/tests/kprobes/test-kprobes.c
> @@ -43,8 +43,39 @@ static void test_kprobe_riscv(struct kunit *test)
>  	kfree(kp);
>  }
>  
> +static void test_kprobe_lrsc(struct kunit *test)
> +{
> +	/*
> +	 * test_kprobes_lrsc_offsets[] points, in order, at an instruction
> +	 * right before an LR/SC sequence, the LR itself, an instruction
> +	 * inside the sequence, the SC, and an instruction right after it.
> +	 * reject[] must have the same number of entries.
> +	 */
> +	bool reject[] = { false, true, true, true, false };

I am not a fan of coupled code. A change at test_kprobes_lrsc_offsets
mandates a change here.

Instead of this, how about we split test_kprobes_lrsc_offsets into
test_kprobes_lrsc_allowed_addresses and test_kprobes_lrsc_rejected_addresses?

Nam

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