bpf: Fix compare error in function retval_range_within

After checking lsm hook return range in verifier, the test case
"test_progs -t test_lsm" failed, and the failure log says:

libbpf: prog 'test_int_hook': BPF program load failed: Invalid argument
libbpf: prog 'test_int_hook': -- BEGIN PROG LOAD LOG --
0: R1=ctx() R10=fp0
; int BPF_PROG(test_int_hook, struct vm_area_struct *vma, @ lsm.c:89
0: (79) r0 = *(u64 *)(r1 +24) ; R0_w=scalar(smin=smin32=-4095,smax=smax32=0) R1=ctx()

[...]

24: (b4) w0 = -1 ; R0_w=0xffffffff
; int BPF_PROG(test_int_hook, struct vm_area_struct *vma, @ lsm.c:89
25: (95) exit
At program exit the register R0 has smin=4294967295 smax=4294967295 should have been in [-4095, 0]

It can be seen that instruction "w0 = -1" zero extended -1 to 64-bit
register r0, setting both smin and smax values of r0 to 4294967295.
This resulted in a false reject when r0 was checked with range [-4095, 0].

Given bpf lsm does not return 64-bit values, this patch fixes it by changing
the compare between r0 and return range from 64-bit operation to 32-bit
operation for bpf lsm.

Fixes: 8fa4ecd49b81 ("bpf: enforce exact retval range on subprog/callback exit")
Signed-off-by: Xu Kuohai <[e-mail]>
Acked-by: Shung-Hsi Yu <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Alexei Starovoitov <[e-mail]>
Signed-off-by: Andrii Nakryiko <[e-mail]>

kernel/bpf/verifier.c

···
 	return is_rbtree_lock_required_kfunc(kfunc_btf_id);
 }
 
-static bool retval_range_within(struct bpf_retval_range range, const struct bpf_reg_state *reg)
+static bool retval_range_within(struct bpf_retval_range range, const struct bpf_reg_state *reg,
+				bool return_32bit)
 {
-	return range.minval <= reg->smin_value && reg->smax_value <= range.maxval;
+	if (return_32bit)
+		return range.minval <= reg->s32_min_value && reg->s32_max_value <= range.maxval;
+	else
+		return range.minval <= reg->smin_value && reg->smax_value <= range.maxval;
 }
 
 static int prepare_func_exit(struct bpf_verifier_env *env, int *insn_idx)
···
 		if (err)
 			return err;
 
-		/* enforce R0 return value range */
-		if (!retval_range_within(callee->callback_ret_range, r0)) {
+		/* enforce R0 return value range, and bpf_callback_t returns 64bit */
+		if (!retval_range_within(callee->callback_ret_range, r0, false)) {
 			verbose_invalid_scalar(env, r0, callee->callback_ret_range,
 					       "At callback return", "R0");
 			return -EINVAL;
···
 	int err;
 	struct bpf_func_state *frame = env->cur_state->frame[0];
 	const bool is_subprog = frame->subprogno;
+	bool return_32bit = false;
 
 	/* LSM and struct_ops func-ptr's return type could be "void" */
 	if (!is_subprog || frame->in_exception_callback_fn) {
···
 			/* no restricted range, any return value is allowed */
 			if (range.minval == S32_MIN && range.maxval == S32_MAX)
 				return 0;
+			return_32bit = true;
 		} else if (!env->prog->aux->attach_func_proto->type) {
 			/* Make sure programs that attach to void
 			 * hooks don't try to modify return value.
···
 	if (err)
 		return err;
 
-	if (!retval_range_within(range, reg)) {
+	if (!retval_range_within(range, reg, return_32bit)) {
 		verbose_invalid_scalar(env, reg, range, exit_ctx, reg_name);
 		if (!is_subprog &&
 		    prog->expected_attach_type == BPF_LSM_CGROUP &&