[PATCH RFC bpf-next 4/8] bpf, x86: add helper to emit kasan checks in x86 JITed programs

Alexis Lothoré (eBPF Foundation)
2026-04-13T18:29:15Z

Add the emit_kasan_check() function that emits KASAN shadow memory
checks before memory accesses in JIT-compiled BPF programs. The
implementation relies on the existing __asan_{load,store}X functions
from KASAN subsystem. The helper:
- ensures that the kasan instrumention is actually needed: if the
instruction being processed accesses the program stack, we skip the
instrumentation, as those accesses are already protected with page
guards
- saves registers. This includes caller-saved registers, but also
temporary registers, as those were possibly used by the
affected program
- computes the accessed address and stores it in %rdi
- calls the relevant function, depending on the instruction being a load
or a store, and the size of the access.
- restores registeres

The special care needed when inserting this instrumentation comes at the
cost of a non negligeable increase in JITed code size. For example, a
bare

mov 0x0(%si),rbx # Load in rbx content at address stored in rsi

becomes

push %rax
push %rcx
push %rdx
push %rsi
push %rdi
push %r8
push %r9
push %r10
push %r11
sub $0x8,%rsp
mov %rsi,%rdi
call 0xffffffff81da0a60 <__asan_load8>
add $0x8,%rsp
pop %r11
pop %r10
pop %r9
pop %r8
pop %rdi
pop %rsi
pop %rdx
pop %rcx
pop %rax
mov 0x0(%rsi),rbx

Signed-off-by: Alexis Lothoré (eBPF Foundation) <alexis.lothore@bootlin.com>
---
arch/x86/net/bpf_jit_comp.c | 93 +++++++++++++++++++++++++++++++++++++++++++++
1 file changed, 93 insertions(+)

diff --git a/arch/x86/net/bpf_jit_comp.c b/arch/x86/net/bpf_jit_comp.c
index ea9e707e8abf..b90103bd0080 100644
--- a/arch/x86/net/bpf_jit_comp.c
+++ b/arch/x86/net/bpf_jit_comp.c
@@ -20,6 +20,10 @@
#include <asm/unwind.h>
#include <asm/cfi.h>

+#ifdef CONFIG_BPF_JIT_KASAN
+#include <linux/kasan.h>
+#endif
+
static bool all_callee_regs_used[4] = {true, true, true, true};

static u8 *emit_code(u8 *ptr, u32 bytes, unsigned int len)
@@ -1301,6 +1305,95 @@ static void emit_store_stack_imm64(u8 **pprog, int reg, int stack_off, u64 imm64
emit_stx(pprog, BPF_DW, BPF_REG_FP, reg, stack_off);
}

+static int emit_kasan_check(u8 **pprog, u32 addr_reg, struct bpf_insn *insn,
+ u8 *ip, bool accesses_stack)
+{
+#ifdef CONFIG_BPF_JIT_KASAN
+ bool is_write = BPF_CLASS(insn->code) == BPF_STX;
+ u32 bpf_size = BPF_SIZE(insn->code);
+ s32 off = insn->off;
+ u8 *prog = *pprog;
+ void *kasan_func;
+
+ if (accesses_stack)
+ return 0;
+
+ /* Derive KASAN check function from access type and size */
+ switch (bpf_size) {
+ case BPF_B:
+ kasan_func = is_write ? __asan_store1 : __asan_load1;
+ break;
+ case BPF_H:
+ kasan_func = is_write ? __asan_store2 : __asan_load2;
+ break;
+ case BPF_W:
+ kasan_func = is_write ? __asan_store4 : __asan_load4;
+ break;
+ case BPF_DW:
+ kasan_func = is_write ? __asan_store8 : __asan_load8;
+ break;
+ default:
+ return -EINVAL;
+ }
+
+ /* Save rax */
+ EMIT1(0x50);
+ /* Save rcx */
+ EMIT1(0x51);
+ /* Save rdx */
+ EMIT1(0x52);
+ /* Save rsi */
+ EMIT1(0x56);
+ /* Save rdi */
+ EMIT1(0x57);
+ /* Save r8 */
+ EMIT2(0x41, 0x50);
+ /* Save r9 */
+ EMIT2(0x41, 0x51);
+ /* Save r10 */
+ EMIT2(0x41, 0x52);
+ /* Save r11 */
+ EMIT2(0x41, 0x53);
+ /* We have pushed 72 bytes, realign stack to 16 bytes: sub rsp, 8 */
+ EMIT4(0x48, 0x83, 0xEC, 8);
+
+ /* mov rdi, addr_reg */
+ EMIT_mov(BPF_REG_1, addr_reg);
+
+ /* add rdi, off (if offset is non-zero) */
+ if (off) {
+ if (is_imm8(off)) {
+ /* add rdi, imm8 */
+ EMIT4(0x48, 0x83, 0xC7, (u8)off);
+ } else {
+ /* add rdi, imm32 */
+ EMIT3_off32(0x48, 0x81, 0xC7, off);
+ }
+ }
+
+ /* Adjust ip to account for the instrumentation generated so far */
+ ip += (prog - *pprog);
+ /* call kasan_func */
+ if (emit_call(&prog, kasan_func, ip))
+ return -ERANGE;
+
+ /* Restore registers */
+ EMIT4(0x48, 0x83, 0xC4, 8);
+ EMIT2(0x41, 0x5B);
+ EMIT2(0x41, 0x5A);
+ EMIT2(0x41, 0x59);
+ EMIT2(0x41, 0x58);
+ EMIT1(0x5F);
+ EMIT1(0x5E);
+ EMIT1(0x5A);
+ EMIT1(0x59);
+ EMIT1(0x58);
+
+ *pprog = prog;
+#endif /* CONFIG_BPF_JIT_KASAN */
+ return 0;
+}
+
static int emit_atomic_rmw(u8 **pprog, u32 atomic_op,
u32 dst_reg, u32 src_reg, s16 off, u8 bpf_size)
{

--
2.53.0