[PATCH v2] bpf: fix a bug in verification logic when SUB operation taken on FRAME_PTR

[Wang Nan <wangnan0@huawei.com>]

Original code has a problem, cause following code failed to pass verifier:

 r1 <- r10
 r1 -= 8
 r2 = 8
 r3 = unsafe pointer
 call BPF_FUNC_probe_read  <-- R1 type=inv expected=fp

However, by replacing 'r1 -= 8' to 'r1 += -8' the above program can be
loaded successfully.

This is because the verifier allows only BPF_ADD instruction on a
FRAME_PTR reigster to forge PTR_TO_STACK register, but makes BPF_SUB
on FRAME_PTR reigster to get a UNKNOWN_VALUE register.

This patch fix it by adding BPF_SUB in stack_relative checking.

Signed-off-by: Wang Nan <wangnan0@huawei.com>
---

V1 is incorrect. Please ignore it and consider this one.

---
 kernel/bpf/verifier.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index a251cf6..681ac72 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -1020,7 +1020,8 @@ static int check_alu_op(struct reg_state *regs, struct bpf_insn *insn)
 		}
 
 		/* pattern match 'bpf_add Rx, imm' instruction */
-		if (opcode == BPF_ADD && BPF_CLASS(insn->code) == BPF_ALU64 &&
+		if ((opcode == BPF_ADD || opcode == BPF_SUB) &&
+		    BPF_CLASS(insn->code) == BPF_ALU64 &&
 		    regs[insn->dst_reg].type == FRAME_PTR &&
 		    BPF_SRC(insn->code) == BPF_K)
 			stack_relative = true;
-- 
1.8.3.4