[bpf-next PATCH v2 1/4] bpf: verifier track null pointer branch_taken with JNE and JEQ

[John Fastabend <john.fastabend@gmail.com>]

Currently, when considering the branches that may be taken for a jump
instruction if the register being compared is a pointer the verifier
assumes both branches may be taken. But, if the jump instruction
is comparing if a pointer is NULL we have this information in the
verifier encoded in the reg->type so we can do better in these cases.
Specifically, these two common cases can be handled.

 * If the instruction is BPF_JEQ and we are comparing against a
   zero value. This test is 'if ptr == 0 goto +X' then using the
   type information in reg->type we can decide if the ptr is not
   null. This allows us to avoid pushing both branches onto the
   stack and instead only use the != 0 case. For example
   PTR_TO_SOCK and PTR_TO_SOCK_OR_NULL encode the null pointer.
   Note if the type is PTR_TO_SOCK_OR_NULL we can not learn anything.
   And also if the value is non-zero we learn nothing because it
   could be any arbitrary value a different pointer for example

 * If the instruction is BPF_JNE and ware comparing against a zero
   value then a similar analysis as above can be done. The test in
   asm looks like 'if ptr != 0 goto +X'. Again using the type
   information if the non null type is set (from above PTR_TO_SOCK)
   we know the jump is taken.

In this patch we extend is_branch_taken() to consider this extra
information and to return only the branch that will be taken. This
resolves a verifier issue reported with C code like the following.
See progs/test_sk_lookup_kern.c in selftests.

 sk = bpf_sk_lookup_tcp(skb, tuple, tuple_len, BPF_F_CURRENT_NETNS, 0);
 bpf_printk("sk=%d\n", sk ? 1 : 0);
 if (sk)
   bpf_sk_release(sk);
 return sk ? TC_ACT_OK : TC_ACT_UNSPEC;

In the above the bpf_printk() will resolve the pointer from
PTR_TO_SOCK_OR_NULL to PTR_TO_SOCK. Then the second test guarding
the release will cause the verifier to walk both paths resulting
in the an unreleased sock reference. See verifier/ref_tracking.c
in selftests for an assembly version of the above.

After the above additional logic is added the C code above passes
as expected.

Suggested-by: Alexei Starovoitov <ast@kernel.org>
Reported-by: Andrey Ignatov <rdna@fb.com>
Signed-off-by: John Fastabend <john.fastabend@gmail.com>
---
 kernel/bpf/verifier.c |   36 +++++++++++++++++++++++++++++++++---
 1 file changed, 33 insertions(+), 3 deletions(-)

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 9c7d67d..4e0dc44 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -393,6 +393,15 @@ static bool type_is_sk_pointer(enum bpf_reg_type type)
 		type == PTR_TO_XDP_SOCK;
 }
 
+static bool reg_type_not_null(enum bpf_reg_type type)
+{
+	return type == PTR_TO_SOCKET ||
+		type == PTR_TO_TCP_SOCK ||
+		type == PTR_TO_MAP_VALUE ||
+		type == PTR_TO_SOCK_COMMON ||
+	        type == PTR_TO_BTF_ID;
+}
+
 static bool reg_type_may_be_null(enum bpf_reg_type type)
 {
 	return type == PTR_TO_MAP_VALUE_OR_NULL ||
@@ -6308,8 +6317,25 @@ static int is_branch64_taken(struct bpf_reg_state *reg, u64 val, u8 opcode)
 static int is_branch_taken(struct bpf_reg_state *reg, u64 val, u8 opcode,
 			   bool is_jmp32)
 {
-	if (__is_pointer_value(false, reg))
-		return -1;
+	if (__is_pointer_value(false, reg)) {
+		if (!reg_type_not_null(reg->type))
+			return -1;
+
+		/* If pointer is valid tests against zero will fail so we can
+		 * use this to direct branch taken.
+		 */
+		if (val != 0)
+			return -1;
+
+		switch (opcode) {
+		case BPF_JEQ:
+			return 0;
+		case BPF_JNE:
+			return 1;
+		default:
+			return -1;
+		}
+	}
 
 	if (is_jmp32)
 		return is_branch32_taken(reg, val, opcode);
@@ -6808,7 +6834,11 @@ static int check_cond_jmp_op(struct bpf_verifier_env *env,
 	}
 
 	if (pred >= 0) {
-		err = mark_chain_precision(env, insn->dst_reg);
+		/* If we get here with a dst_reg pointer type it is because
+		 * above is_branch_taken() special cased the 0 comparison.
+		 */
+		if (!__is_pointer_value(false, dst_reg))
+			err = mark_chain_precision(env, insn->dst_reg);
 		if (BPF_SRC(insn->code) == BPF_X && !err)
 			err = mark_chain_precision(env, insn->src_reg);
 		if (err)