bpf: extend is_branch_taken to registers

stable inclusion
from linux-4.19.193
commit e0b86677fb3e4622b444dcdd8546caa0dba8a689

--------------------------------

commit fb8d251e upstream

This patch extends is_branch_taken() logic from JMP+K instructions
to JMP+X instructions.
Conditional branches are often done when src and dst registers
contain known scalars. In such case the verifier can follow
the branch that is going to be taken when program executes.
That speeds up the verification and is essential feature to support
bounded loops.
Signed-off-by: NAlexei Starovoitov <ast@kernel.org>
Acked-by: NAndrii Nakryiko <andriin@fb.com>
Signed-off-by: NDaniel Borkmann <daniel@iogearbox.net>
[OP: drop is_jmp32 parameter from is_branch_taken() calls and
     adjust context]
Signed-off-by: NOvidiu Panait <ovidiu.panait@windriver.com>
Signed-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: NYang Yingliang <yangyingliang@huawei.com>

kernel/bpf/verifier.c

@@ -4415,9 +4415,10 @@ static int check_cond_jmp_op(struct bpf_verifier_env *env,
 	struct bpf_verifier_state *this_branch = env->cur_state;
 	struct bpf_verifier_state *other_branch;
 	struct bpf_reg_state *regs = this_branch->frame[this_branch->curframe]->regs;
-	struct bpf_reg_state *dst_reg, *other_branch_regs;
+	struct bpf_reg_state *dst_reg, *other_branch_regs, *src_reg = NULL;
 	u8 opcode = BPF_OP(insn->code);
 	bool is_jmp32;
+	int pred = -1;
 	int err;
 
 	/* Only conditional jumps are expected to reach here. */
@@ -4442,6 +4443,7 @@ static int check_cond_jmp_op(struct bpf_verifier_env *env,
 				insn->src_reg);
 			return -EACCES;
 		}
+		src_reg = &regs[insn->src_reg];
 	} else {
 		if (insn->src_reg != BPF_REG_0) {
 			verbose(env, "BPF_JMP/JMP32 uses reserved fields\n");
@@ -4457,20 +4459,21 @@ static int check_cond_jmp_op(struct bpf_verifier_env *env,
 	dst_reg = &regs[insn->dst_reg];
 	is_jmp32 = BPF_CLASS(insn->code) == BPF_JMP32;
 
-	if (BPF_SRC(insn->code) == BPF_K) {
-		int pred = is_branch_taken(dst_reg, insn->imm, opcode,
-					   is_jmp32);
-
-		if (pred == 1) {
-			 /* only follow the goto, ignore fall-through */
-			*insn_idx += insn->off;
-			return 0;
-		} else if (pred == 0) {
-			/* only follow fall-through branch, since
-			 * that's where the program will go
-			 */
-			return 0;
-		}
+	if (BPF_SRC(insn->code) == BPF_K)
+		pred = is_branch_taken(dst_reg, insn->imm, opcode, is_jmp32);
+	else if (src_reg->type == SCALAR_VALUE &&
+		 tnum_is_const(src_reg->var_off))
+		pred = is_branch_taken(dst_reg, src_reg->var_off.value,
+				       opcode, is_jmp32);
+	if (pred == 1) {
+		/* only follow the goto, ignore fall-through */
+		*insn_idx += insn->off;
+		return 0;
+	} else if (pred == 0) {
+		/* only follow fall-through branch, since
+		 * that's where the program will go
+		 */
+		return 0;
 	}
 
 	other_branch = push_stack(env, *insn_idx + insn->off + 1, *insn_idx,