bpf: mark a NULL call argument precise

check_func_arg() allows bpf_register_is_null() for nullable arguments
w/o marking the underlying scalar register precise. Hence a checkpoint
created on such a path would prune against arbitrary scalar value.

check_helper_call() enforces second parameter of the
bpf_get_local_storage() to be zero, w/o marking the underlying scalar
register precise. Hence a checkpoint created on such a path would
prune against arbitrary scalar value.

Grouping these two into one patch, as they share the same fixes tag.

Fixes: b5dc0163d8fd ("bpf: precise scalar_value tracking")
Signed-off-by: Eduard Zingerman <eddyz87@gmail.com>
Link: https://lore.kernel.org/r/20260904-register-is-null-precise-fixes-v1-1-0f5a360ff15d@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

2 files changed, 14 insertions, 7 deletions

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index c8699a8831df..b107f551a62d 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -8759,11 +8759,15 @@ static int check_func_arg(struct bpf_verifier_env *env, u32 arg,
 			return err;
 	}
 
-	if (bpf_register_is_null(reg) && type_may_be_null(arg_type))
+	if (bpf_register_is_null(reg) && type_may_be_null(arg_type)) {
 		/* A NULL register has a SCALAR_VALUE type, so skip
 		 * type checking.
 		 */
+		err = mark_chain_precision(env, regno);
+		if (err)
+			return err;
 		goto skip_type_check;
+	}
 
 	/* arg_btf_id and arg_size are in a union. */
 	if (base_type(arg_type) == ARG_PTR_TO_BTF_ID ||
@@ -10923,6 +10927,9 @@ static int check_helper_call(struct bpf_verifier_env *env, struct bpf_insn *insn
 			verbose(env, "get_local_storage() doesn't support non-zero flags\n");
 			return -EINVAL;
 		}
+		err = mark_chain_precision(env, BPF_REG_2);
+		if (err)
+			return err;
 		break;
 	case BPF_FUNC_for_each_map_elem:
 		err = push_callback_call(env, insn, insn_idx, meta.subprogno,
diff --git a/tools/testing/selftests/bpf/progs/verifier_subprog_precision.c b/tools/testing/selftests/bpf/progs/verifier_subprog_precision.c
index e174a905c562..dc0c7034c04f 100644
--- a/tools/testing/selftests/bpf/progs/verifier_subprog_precision.c
+++ b/tools/testing/selftests/bpf/progs/verifier_subprog_precision.c
@@ -287,9 +287,9 @@ __msg("17: (b7) r0 = 0")
 __msg("18: (95) exit")
 __msg("returning from callee:")
 __msg("to caller at 9:")
-__msg("frame 0: propagating r1,r4")
+__msg("frame 0: propagating r1,r3,r4")
 __msg("mark_precise: frame0: last_idx 9 first_idx 9 subseq_idx -1")
-__msg("mark_precise: frame0: regs=r1,r4 stack= before 18: (95) exit")
+__msg("mark_precise: frame0: regs=r1,r3,r4 stack= before 18: (95) exit")
 __msg("from 18 to 9: safe")
 __naked int callback_result_precise(void)
 {
@@ -419,9 +419,9 @@ __msg("to caller at 9:")
 /* r1, r4 are always precise for bpf_loop(),
  * r6 was marked before backtracking to callback body.
  */
-__msg("frame 0: propagating r1,r4,r6")
+__msg("frame 0: propagating r1,r3,r4,r6")
 __msg("mark_precise: frame0: last_idx 9 first_idx 9 subseq_idx -1")
-__msg("mark_precise: frame0: regs=r1,r4,r6 stack= before 16: (95) exit")
+__msg("mark_precise: frame0: regs=r1,r3,r4,r6 stack= before 16: (95) exit")
 __msg("mark_precise: frame1: regs= stack= before 15: (b7) r0 = 0")
 __msg("mark_precise: frame1: regs= stack= before 9: (85) call bpf_loop")
 __msg("mark_precise: frame0: parent state regs= stack=:")
@@ -575,9 +575,9 @@ __msg("to caller at 10:")
 /* r1, r4 are always precise for bpf_loop(),
  * fp-8 was marked before backtracking to callback body.
  */
-__msg("frame 0: propagating r1,r4,fp-8")
+__msg("frame 0: propagating r1,r3,r4,fp-8")
 __msg("mark_precise: frame0: last_idx 10 first_idx 10 subseq_idx -1")
-__msg("mark_precise: frame0: regs=r1,r4 stack=-8 before 18: (95) exit")
+__msg("mark_precise: frame0: regs=r1,r3,r4 stack=-8 before 18: (95) exit")
 __msg("mark_precise: frame1: regs= stack= before 17: (b7) r0 = 0")
 __msg("mark_precise: frame1: regs= stack= before 10: (85) call bpf_loop#181")
 __msg("mark_precise: frame0: parent state regs= stack=:")