[PATCH bpf-next v2 5/8] bpf, x86: emit KASAN checks into x86 JITed programs
Alexis Lothoré (eBPF Foundation)
alexis.lothore at bootlin.com
Thu Jun 4 13:22:03 PDT 2026
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;
+ 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));
--
2.54.0
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