[PATCH v5 3/3] riscv: probes: reject probes inside an LR/SC sequence
Vladimir Kondratiev
vladimir.kondratiev at mobileye.com
Thu Nov 5 08:37:38 PST 2026
A kprobe or uprobe replaces an instruction with a breakpoint. The trap
clears the hart's reservation, so if the breakpoint sits on the lr, on
the sc, or on any instruction between them, the sc fails, the loop
branches back to the lr and the probe fires again. The sequence never
completes.
This has always been possible on RISC-V, because cmpxchg and the
conditional atomics are LR/SC loops. Once Zaamo is disabled every
kernel atomic is such a loop, and so is every atomic in userspace built
for the same hart, so the chance of a probe landing in one grows
accordingly. arm64 has the same problem and rejects probes inside an
exclusive-load/store-exclusive region.
Teach the shared instruction decoder to reject lr and sc themselves.
This covers both kprobes and uprobes.
For kprobes extend arch_check_kprobe(), which already walks the
instructions from the enclosing symbol up to the probe address, to
track whether the address lies between an lr and the following sc, and
reject it if so. The walk goes forward, so compressed instructions do
not make it ambiguous.
uprobes have no symbol to start from and only the probed instruction
is copied. Use the rules for a constrained LR/SC sequence instead: at
most 16 instructions between lr and sc, all base integer instructions
other than loads, stores, JALR, FENCE, SYSTEM and backward jumps or
branches, or their compressed forms. Read the text that follows the
probe from the task, which the caller's mmap_lock allows, and walk
forward. Reaching an sc through permitted instructions only means the
probe sits inside a sequence; any other instruction, an lr, the end of
the window or unreadable text means it does not. Only a sequence that
follows these rules is guaranteed to make progress, so a probe inside
any other one is already on unsupported ground.
Signed-off-by: Vladimir Kondratiev <vladimir.kondratiev at mobileye.com>
---
arch/riscv/include/asm/insn.h | 28 ++++++++++
arch/riscv/kernel/probes/decode-insn.c | 94 ++++++++++++++++++++++++++++++++++
arch/riscv/kernel/probes/decode-insn.h | 7 +++
arch/riscv/kernel/probes/kprobes.c | 22 +++++++-
arch/riscv/kernel/probes/uprobes.c | 31 +++++++++++
5 files changed, 180 insertions(+), 2 deletions(-)
diff --git a/arch/riscv/include/asm/insn.h b/arch/riscv/include/asm/insn.h
index c3005573e8c9..f5de81753bec 100644
--- a/arch/riscv/include/asm/insn.h
+++ b/arch/riscv/include/asm/insn.h
@@ -10,11 +10,16 @@
#define RV_INSN_FUNCT3_MASK GENMASK(14, 12)
#define RV_INSN_FUNCT3_OPOFF 12
+#define RV_INSN_FUNCT5_MASK GENMASK(31, 27)
+#define RV_INSN_FUNCT5_OPOFF 27
+#define RV_INSN_FUNCT7_MASK GENMASK(31, 25)
+#define RV_INSN_FUNCT7_OPOFF 25
#define RV_INSN_OPCODE_MASK GENMASK(6, 0)
#define RV_INSN_OPCODE_OPOFF 0
#define RV_INSN_FUNCT12_OPOFF 20
#define RV_ENCODE_FUNCT3(f_) (RVG_FUNCT3_##f_ << RV_INSN_FUNCT3_OPOFF)
+#define RV_ENCODE_FUNCT5(f_) (RVG_FUNCT5_##f_ << RV_INSN_FUNCT5_OPOFF)
#define RV_ENCODE_FUNCT12(f_) (RVG_FUNCT12_##f_ << RV_INSN_FUNCT12_OPOFF)
/* The bit field of immediate value in I-type instruction */
@@ -136,7 +141,13 @@
/* parts of opcode for RVG*/
#define RVG_OPCODE_FENCE 0x0f
+#define RVG_OPCODE_OP_IMM 0x13
#define RVG_OPCODE_AUIPC 0x17
+#define RVG_OPCODE_OP_IMM_32 0x1b
+#define RVG_OPCODE_AMO 0x2f
+#define RVG_OPCODE_OP 0x33
+#define RVG_OPCODE_LUI 0x37
+#define RVG_OPCODE_OP_32 0x3b
#define RVG_OPCODE_BRANCH 0x63
#define RVG_OPCODE_JALR 0x67
#define RVG_OPCODE_JAL 0x6f
@@ -188,6 +199,17 @@
#define RVG_FUNCT12_EBREAK 0x1
#define RVG_FUNCT12_SRET 0x102
+/* parts of funct5 code for A extension */
+#define RVG_FUNCT5_LR 0x02
+#define RVG_FUNCT5_SC 0x03
+
+/*
+ * The AMO funct3 field holds the access width: 010 for .w, 011 for .d.
+ * Matching bits 14:13 alone accepts both and nothing else.
+ */
+#define RVG_AMO_WIDTH_MASK GENMASK(14, 13)
+#define RVG_AMO_WIDTH_WD BIT(13)
+
#define RVG_MATCH_AUIPC (RVG_OPCODE_AUIPC)
#define RVG_MATCH_JALR (RV_ENCODE_FUNCT3(JALR) | RVG_OPCODE_JALR)
#define RVG_MATCH_JAL (RVG_OPCODE_JAL)
@@ -200,6 +222,8 @@
#define RVG_MATCH_BGEU (RV_ENCODE_FUNCT3(BGEU) | RVG_OPCODE_BRANCH)
#define RVG_MATCH_EBREAK (RV_ENCODE_FUNCT12(EBREAK) | RVG_OPCODE_SYSTEM)
#define RVG_MATCH_SRET (RV_ENCODE_FUNCT12(SRET) | RVG_OPCODE_SYSTEM)
+#define RVG_MATCH_LR (RV_ENCODE_FUNCT5(LR) | RVG_AMO_WIDTH_WD | RVG_OPCODE_AMO)
+#define RVG_MATCH_SC (RV_ENCODE_FUNCT5(SC) | RVG_AMO_WIDTH_WD | RVG_OPCODE_AMO)
#define RVC_MATCH_C_BEQZ (RVC_ENCODE_FUNCT3(C_BEQZ) | RVC_OPCODE_C1)
#define RVC_MATCH_C_BNEZ (RVC_ENCODE_FUNCT3(C_BNEZ) | RVC_OPCODE_C1)
#define RVC_MATCH_C_J (RVC_ENCODE_FUNCT3(C_J) | RVC_OPCODE_C1)
@@ -227,6 +251,8 @@
#define RVC_MASK_C_EBREAK 0xffff
#define RVG_MASK_EBREAK 0xffffffff
#define RVG_MASK_SRET 0xffffffff
+#define RVG_MASK_LR (RV_INSN_FUNCT5_MASK | RVG_AMO_WIDTH_MASK | RV_INSN_OPCODE_MASK)
+#define RVG_MASK_SC (RV_INSN_FUNCT5_MASK | RVG_AMO_WIDTH_MASK | RV_INSN_OPCODE_MASK)
#define __INSN_LENGTH_MASK _UL(0x3)
#define __INSN_LENGTH_GE_32 _UL(0x3)
@@ -262,6 +288,8 @@ __RISCV_INSN_FUNCS(c_ebreak, RVC_MASK_C_EBREAK, RVC_MATCH_C_EBREAK)
__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);
+__RISCV_INSN_FUNCS(lr, RVG_MASK_LR, RVG_MATCH_LR)
+__RISCV_INSN_FUNCS(sc, RVG_MASK_SC, RVG_MATCH_SC)
/* 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 433d9035b888..6a4f55b07d04 100644
--- a/arch/riscv/kernel/probes/decode-insn.c
+++ b/arch/riscv/kernel/probes/decode-insn.c
@@ -4,11 +4,99 @@
#include <linux/kprobes.h>
#include <linux/module.h>
#include <linux/kallsyms.h>
+#include <linux/bug.h>
#include <asm/sections.h>
#include "decode-insn.h"
#include "simulate-insn.h"
+/*
+ * Can insn sit between the lr and the sc of a constrained LR/SC sequence?
+ * The ISA allows base integer instructions there, except loads, stores,
+ * JALR, FENCE, SYSTEM, backward jumps and taken backward branches, plus
+ * their compressed forms. Backward branches are treated as not allowed:
+ * the retry branch follows the sc, so one inside the body is not a
+ * constrained sequence.
+ */
+static bool __kprobes riscv_insn_is_lrsc_body(u32 insn)
+{
+ u32 funct3;
+
+ if (GET_INSN_LENGTH(insn) == 2) {
+ switch (insn & RVC_INSN_OPCODE_MASK) {
+ case RVC_OPCODE_C0:
+ /* c.addi4spn; the rest of the quadrant is loads and stores */
+ return (insn & RVC_INSN_FUNCT3_MASK) == 0;
+ case RVC_OPCODE_C1:
+ /* Arithmetic, c.j, c.jal and branches, bit 12 is the offset sign */
+ if (riscv_insn_is_c_j(insn) || riscv_insn_is_c_jal(insn) ||
+ riscv_insn_is_c_beqz(insn) || riscv_insn_is_c_bnez(insn))
+ return !(insn & BIT(12));
+ return true;
+ case RVC_OPCODE_C2:
+ funct3 = (insn & RVC_INSN_FUNCT3_MASK) >> RVC_INSN_FUNCT3_OPOFF;
+ if (funct3 == 0) /* c.slli */
+ return true;
+ if (funct3 == 4) /* c.mv, c.add, but not c.jr, c.jalr, c.ebreak */
+ return !riscv_insn_is_c_jr(insn) &&
+ !riscv_insn_is_c_jalr(insn) &&
+ !riscv_insn_is_c_ebreak(insn);
+ return false; /* stack loads and stores */
+ }
+ return false;
+ }
+
+ switch (insn & RV_INSN_OPCODE_MASK) {
+ case RVG_OPCODE_OP_IMM:
+ case RVG_OPCODE_OP_IMM_32:
+ case RVG_OPCODE_LUI:
+ case RVG_OPCODE_AUIPC:
+ return true;
+ case RVG_OPCODE_OP:
+ case RVG_OPCODE_OP_32:
+ /* funct7 0 or 0x20 is base I; anything else is M, B or another extension */
+ return !((insn & RV_INSN_FUNCT7_MASK) & ~(0x20U << RV_INSN_FUNCT7_OPOFF));
+ case RVG_OPCODE_BRANCH:
+ case RVG_OPCODE_JAL:
+ /* forward only, bit 31 is the offset sign */
+ return !(insn & BIT(31));
+ }
+ return false;
+}
+
+/*
+ * Does the instruction at the start of buf lie inside a constrained LR/SC
+ * sequence? Such a sequence holds at most RISCV_LRSC_MAX_BODY instructions
+ * between the lr and the sc, all from the set above. Walk forward from the
+ * probed instruction: reaching an sc through permitted instructions only
+ * means the probe sits in the body; anything else, including an lr that
+ * opens a new sequence, means it does not. The walk stops at the end of
+ * buf, so a short buffer only makes the answer more permissive.
+ */
+bool __kprobes riscv_probe_in_lrsc_body(const u16 *buf, size_t len)
+{
+ size_t off = 0;
+ u32 insn;
+ int n;
+
+ for (n = 0; n <= RISCV_LRSC_MAX_BODY; n++) {
+ if (off + 2 > len)
+ return false;
+ insn = buf[off / 2];
+ if (GET_INSN_LENGTH(insn) == 4) {
+ if (off + 4 > len)
+ return false;
+ insn |= (u32)buf[off / 2 + 1] << 16;
+ }
+ if (riscv_insn_is_sc(insn))
+ return true;
+ if (!riscv_insn_is_lrsc_body(insn))
+ return false;
+ off += GET_INSN_LENGTH(insn);
+ }
+ return false;
+}
+
/* Return:
* INSN_REJECTED If instruction is one not allowed to kprobe,
* INSN_GOOD_NO_SLOT If instruction is supported but doesn't use its slot.
@@ -23,6 +111,12 @@ riscv_probe_decode_insn(probe_opcode_t *addr, struct arch_probe_insn *api)
*/
RISCV_INSN_REJECTED(system, insn);
RISCV_INSN_REJECTED(fence, insn);
+ /*
+ * The breakpoint trap drops the reservation, so a probe on either
+ * end of an LR/SC sequence makes the SC fail on every retry.
+ */
+ RISCV_INSN_REJECTED(lr, insn);
+ RISCV_INSN_REJECTED(sc, insn);
/*
* Simulate instructions list:
diff --git a/arch/riscv/kernel/probes/decode-insn.h b/arch/riscv/kernel/probes/decode-insn.h
index 42269a7d676d..aa1b0d363545 100644
--- a/arch/riscv/kernel/probes/decode-insn.h
+++ b/arch/riscv/kernel/probes/decode-insn.h
@@ -15,4 +15,11 @@ enum probe_insn {
enum probe_insn __kprobes
riscv_probe_decode_insn(probe_opcode_t *addr, struct arch_probe_insn *asi);
+/* A constrained LR/SC sequence has at most this many instructions between lr and sc */
+#define RISCV_LRSC_MAX_BODY 16
+/* Bytes from the probed instruction through the farthest possible sc */
+#define RISCV_LRSC_SCAN_BYTES ((RISCV_LRSC_MAX_BODY + 1) * 4)
+
+bool __kprobes riscv_probe_in_lrsc_body(const u16 *buf, size_t len);
+
#endif /* _RISCV_KERNEL_KPROBES_DECODE_INSN_H */
diff --git a/arch/riscv/kernel/probes/kprobes.c b/arch/riscv/kernel/probes/kprobes.c
index 9e2afabf94e8..9c2284eafae9 100644
--- a/arch/riscv/kernel/probes/kprobes.c
+++ b/arch/riscv/kernel/probes/kprobes.c
@@ -12,6 +12,7 @@
#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"
@@ -49,9 +50,17 @@ static void __kprobes arch_simulate_insn(struct kprobe *p, struct pt_regs *regs)
post_kprobe_handler(p, kcb, regs);
}
+/*
+ * Walk the instructions from the enclosing symbol up to addr. This checks
+ * that addr is an instruction boundary and that it does not lie inside an
+ * LR/SC sequence: the breakpoint trap drops the reservation, so the SC
+ * would fail and the loop would hit the probe again on every retry.
+ */
static bool __kprobes arch_check_kprobe(unsigned long addr)
{
unsigned long tmp, offset;
+ bool in_lrsc = false;
+ u32 insn;
/* start iterating at the closest preceding symbol */
if (!kallsyms_lookup_size_offset(addr, NULL, &offset))
@@ -61,9 +70,18 @@ static bool __kprobes arch_check_kprobe(unsigned long addr)
while (tmp <= addr) {
if (tmp == addr)
- return true;
+ return !in_lrsc;
+
+ insn = *(u16 *)tmp;
+ if (GET_INSN_LENGTH(insn) == 4) {
+ insn |= (u32)*(u16 *)(tmp + 2) << 16;
+ if (riscv_insn_is_lr(insn))
+ in_lrsc = true;
+ else if (riscv_insn_is_sc(insn))
+ in_lrsc = false;
+ }
- tmp += GET_INSN_LENGTH(*(u16 *)tmp);
+ tmp += GET_INSN_LENGTH(insn);
}
return false;
diff --git a/arch/riscv/kernel/probes/uprobes.c b/arch/riscv/kernel/probes/uprobes.c
index eb177d0ce8ab..d97a10262166 100644
--- a/arch/riscv/kernel/probes/uprobes.c
+++ b/arch/riscv/kernel/probes/uprobes.c
@@ -28,6 +28,34 @@ unsigned long uprobe_get_swbp_addr(struct pt_regs *regs)
return instruction_pointer(regs);
}
+/*
+ * Is the instruction at addr inside a constrained LR/SC sequence? Only the
+ * probed instruction was copied, so read the text that follows it from the
+ * task. The caller holds mmap_lock, as uprobe_write_opcode() requires too.
+ * Text that cannot be read ends the scan early, which accepts the probe.
+ */
+static bool uprobe_in_lrsc_body(struct mm_struct *mm, unsigned long addr)
+{
+ u16 buf[RISCV_LRSC_SCAN_BYTES / sizeof(u16)];
+ struct vm_area_struct *vma;
+ struct page *page;
+ size_t got = 0;
+ int len;
+
+ while (got < sizeof(buf)) {
+ len = min_t(size_t, sizeof(buf) - got,
+ PAGE_SIZE - offset_in_page(addr + got));
+ page = get_user_page_vma_remote(mm, addr + got, FOLL_FORCE, &vma);
+ if (IS_ERR(page))
+ break;
+ uprobe_copy_from_page(page, addr + got, (void *)buf + got, len);
+ put_page(page);
+ got += len;
+ }
+
+ return riscv_probe_in_lrsc_body(buf, got);
+}
+
int arch_uprobe_analyze_insn(struct arch_uprobe *auprobe, struct mm_struct *mm,
unsigned long addr)
{
@@ -53,6 +81,9 @@ int arch_uprobe_analyze_insn(struct arch_uprobe *auprobe, struct mm_struct *mm,
return -EINVAL;
}
+ if (uprobe_in_lrsc_body(mm, addr))
+ return -EINVAL;
+
return 0;
}
--
2.43.0
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