[PATCH bpf-next v2 5/8] bpf, x86: emit KASAN checks into x86 JITed programs
Yonghong Song
yonghong.song at linux.dev
Fri Jun 5 07:54:10 PDT 2026
On 6/4/26 1:22 PM, Alexis Lothoré (eBPF Foundation) wrote:
> Insert KASAN shadow memory checks before memory load and store
> operations in JIT-compiled BPF programs. This helps detect memory safety
> bugs such as use-after-free and out-of-bounds accesses at runtime.
>
> The main instructions being targeted are BPF_ST, BPF_STX and BPF_LDX,
> but not all of them are being instrumented:
> - if the load/store instruction is in fact accessing the program stack,
> emit_kasan_check silently skips the instrumentation, as we already
> have page guards to monitor stack accesses.
> - if the load/store instruction is a BPF_PROBE_MEM or a BPF_PROBE_ATOMIC
> instruction, we do not instrument it, as the passed address can fault
> (hence the custom fault management with BPF_PROBE_XXX instructions),
> and so the corresponding kasan check could fault as well.
>
> Signed-off-by: Alexis Lothoré (eBPF Foundation) <alexis.lothore at bootlin.com>
> ---
> Changes in v2:
> - support BPF_ATOMICS
> - support BPF_ST
> - make sure to systematically pass correct instruction to kasan check
> ---
> arch/x86/net/bpf_jit_comp.c | 63 ++++++++++++++++++++++++++++++++++++++-------
> 1 file changed, 53 insertions(+), 10 deletions(-)
>
> diff --git a/arch/x86/net/bpf_jit_comp.c b/arch/x86/net/bpf_jit_comp.c
> index 943a0f315cf2..cb3c03edc4bd 100644
> --- a/arch/x86/net/bpf_jit_comp.c
> +++ b/arch/x86/net/bpf_jit_comp.c
> @@ -1516,17 +1516,30 @@ static int emit_atomic_rmw_index(u8 **pprog, u32 atomic_op, u32 size,
> return 0;
> }
>
> -static int emit_atomic_ld_st(u8 **pprog, u32 atomic_op, u32 dst_reg,
> - u32 src_reg, s16 off, u8 bpf_size)
> +static int emit_atomic_ld_st(u8 **pprog, struct bpf_insn *insn, u8 *ip,
> + u32 dst_reg, u32 src_reg, bool accesses_stack_only)
> {
> + u32 atomic_op = insn->imm;
> + int err;
> +
> switch (atomic_op) {
> case BPF_LOAD_ACQ:
> + err = emit_kasan_check(pprog, src_reg, insn, ip, false,
> + accesses_stack_only);
> + if (err)
> + return err;
> /* dst_reg = smp_load_acquire(src_reg + off16) */
> - emit_ldx(pprog, bpf_size, dst_reg, src_reg, off);
> + emit_ldx(pprog, BPF_SIZE(insn->code), dst_reg, src_reg,
> + insn->off);
> break;
> case BPF_STORE_REL:
> + err = emit_kasan_check(pprog, dst_reg, insn, ip, true,
> + accesses_stack_only);
> + if (err)
> + return err;
> /* smp_store_release(dst_reg + off16, src_reg) */
> - emit_stx(pprog, bpf_size, dst_reg, src_reg, off);
> + emit_stx(pprog, BPF_SIZE(insn->code), dst_reg, src_reg,
> + insn->off);
> break;
> default:
> pr_err("bpf_jit: unknown atomic load/store opcode %02x\n",
> @@ -1904,6 +1917,7 @@ static int do_jit(struct bpf_verifier_env *env, struct bpf_prog *bpf_prog, int *
> const s32 imm32 = insn->imm;
> u32 dst_reg = insn->dst_reg;
> u32 src_reg = insn->src_reg;
> + bool accesses_stack_only;
> u8 b2 = 0, b3 = 0;
> u8 *start_of_ldx;
> s64 jmp_offset;
> @@ -1924,6 +1938,8 @@ static int do_jit(struct bpf_verifier_env *env, struct bpf_prog *bpf_prog, int *
> EMIT_ENDBR();
>
> ip = image + addrs[i - 1] + (prog - temp);
> + accesses_stack_only =
> + bpf_insn_accesses_stack_only(env, bpf_prog, i - 1);
>
> switch (insn->code) {
> /* ALU */
> @@ -2304,6 +2320,10 @@ static int do_jit(struct bpf_verifier_env *env, struct bpf_prog *bpf_prog, int *
> case BPF_ST | BPF_MEM | BPF_H:
> case BPF_ST | BPF_MEM | BPF_W:
> case BPF_ST | BPF_MEM | BPF_DW:
> + err = emit_kasan_check(&prog, dst_reg, insn, ip, true,
> + accesses_stack_only);
> + if (err)
> + return err;
> switch (BPF_SIZE(insn->code)) {
> case BPF_B:
> if (is_ereg(dst_reg))
> @@ -2369,6 +2389,10 @@ static int do_jit(struct bpf_verifier_env *env, struct bpf_prog *bpf_prog, int *
> insn_off = outgoing_arg_base - outgoing_rsp - insn_off - 16;
> dst_reg = BPF_REG_FP;
> }
> + err = emit_kasan_check(&prog, dst_reg, insn, ip, true,
> + accesses_stack_only);
> + if (err)
> + return err;
> emit_stx(&prog, BPF_SIZE(insn->code), dst_reg, src_reg, insn_off);
> break;
>
> @@ -2530,6 +2554,12 @@ static int do_jit(struct bpf_verifier_env *env, struct bpf_prog *bpf_prog, int *
> /* populate jmp_offset for JAE above to jump to start_of_ldx */
> start_of_ldx = prog;
> end_of_jmp[-1] = start_of_ldx - end_of_jmp;
> + } else {
> + err = emit_kasan_check(&prog, src_reg, insn, ip,
> + false,
> + accesses_stack_only);
> + if (err)
> + return err;
> }
> if (BPF_MODE(insn->code) == BPF_PROBE_MEMSX ||
> BPF_MODE(insn->code) == BPF_MEMSX)
> @@ -2592,13 +2622,13 @@ static int do_jit(struct bpf_verifier_env *env, struct bpf_prog *bpf_prog, int *
> fallthrough;
> case BPF_STX | BPF_ATOMIC | BPF_W:
> case BPF_STX | BPF_ATOMIC | BPF_DW:
> + bool is64 = BPF_SIZE(insn->code) == BPF_DW;
> + u32 real_src_reg = src_reg;
> + u32 real_dst_reg = dst_reg;
With llvm23, I got the following build failure:
/home/yhs/work/bpf-next/arch/x86/net/bpf_jit_comp.c:2625:4: error:
label followed by a declaration is a C23 extension [-Werror,-Wc23-extensions]
2625 | bool is64 = BPF_SIZE(insn->code) == BPF_DW;
| ^
1 error generated.
The below is a fix:
diff --git a/arch/x86/net/bpf_jit_comp.c b/arch/x86/net/bpf_jit_comp.c
index cb3c03edc4bd..61faf8d8f157 100644
--- a/arch/x86/net/bpf_jit_comp.c
+++ b/arch/x86/net/bpf_jit_comp.c
@@ -2621,7 +2621,7 @@ static int do_jit(struct bpf_verifier_env *env, struct bpf_prog *bpf_prog, int *
}
fallthrough;
case BPF_STX | BPF_ATOMIC | BPF_W:
- case BPF_STX | BPF_ATOMIC | BPF_DW:
+ case BPF_STX | BPF_ATOMIC | BPF_DW: {
bool is64 = BPF_SIZE(insn->code) == BPF_DW;
u32 real_src_reg = src_reg;
u32 real_dst_reg = dst_reg;
@@ -2695,6 +2695,7 @@ static int do_jit(struct bpf_verifier_env *env, struct bpf_prog *bpf_prog, int *
if (err)
return err;
break;
+ }
case BPF_STX | BPF_PROBE_ATOMIC | BPF_B:
case BPF_STX | BPF_PROBE_ATOMIC | BPF_H:
> + u8 *branch_target;
> if (insn->imm == (BPF_AND | BPF_FETCH) ||
> insn->imm == (BPF_OR | BPF_FETCH) ||
> insn->imm == (BPF_XOR | BPF_FETCH)) {
> - bool is64 = BPF_SIZE(insn->code) == BPF_DW;
> - u32 real_src_reg = src_reg;
> - u32 real_dst_reg = dst_reg;
> - u8 *branch_target;
>
> /*
> * Can't be implemented with a single x86 insn.
> @@ -2612,7 +2642,19 @@ static int do_jit(struct bpf_verifier_env *env, struct bpf_prog *bpf_prog, int *
> if (dst_reg == BPF_REG_0)
> real_dst_reg = BPF_REG_AX;
>
> + ip += 3;
> + }
> + if (!bpf_atomic_is_load_store(insn)) {
> + err = emit_kasan_check(&prog, real_dst_reg,
> + insn, ip, false,
> + accesses_stack_only);
> + if (err)
> + return err;
> branch_target = prog;
> + }
> + if (insn->imm == (BPF_AND | BPF_FETCH) ||
> + insn->imm == (BPF_OR | BPF_FETCH) ||
> + insn->imm == (BPF_XOR | BPF_FETCH)) {
> /* Load old value */
> emit_ldx(&prog, BPF_SIZE(insn->code),
> BPF_REG_0, real_dst_reg, insn->off);
> @@ -2644,8 +2686,9 @@ static int do_jit(struct bpf_verifier_env *env, struct bpf_prog *bpf_prog, int *
> }
>
> if (bpf_atomic_is_load_store(insn))
> - err = emit_atomic_ld_st(&prog, insn->imm, dst_reg, src_reg,
> - insn->off, BPF_SIZE(insn->code));
> + err = emit_atomic_ld_st(&prog, insn, ip,
> + dst_reg, src_reg,
> + accesses_stack_only);
> else
> err = emit_atomic_rmw(&prog, insn->imm, dst_reg, src_reg,
> insn->off, BPF_SIZE(insn->code));
>
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