bpf: don't mark FP reg as uninit

when verifier hits an internal bug don't mark register R10==FP as uninit,
since it's read only register and it's not technically correct to let
verifier run further, since it may assume that R10 has valid auxiliary state.

While developing subsequent patches this issue was discovered,
though the code eventually changed that aux reg state doesn't have
pointers any more it is still safer to avoid clearing readonly register.

Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: John Fastabend <john.fastabend@gmail.com>
Acked-by: Daniel Borkmann <daniel@iogearbox.net>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>

kernel/bpf/verifier.c

@@ -584,8 +584,8 @@ static void mark_reg_unknown(struct bpf_verifier_env *env,
 {
 	if (WARN_ON(regno >= MAX_BPF_REG)) {
 		verbose(env, "mark_reg_unknown(regs, %u)\n", regno);
-		/* Something bad happened, let's kill all regs */
-		for (regno = 0; regno < MAX_BPF_REG; regno++)
+		/* Something bad happened, let's kill all regs except FP */
+		for (regno = 0; regno < BPF_REG_FP; regno++)
 			__mark_reg_not_init(regs + regno);
 		return;
 	}
@@ -603,8 +603,8 @@ static void mark_reg_not_init(struct bpf_verifier_env *env,
 {
 	if (WARN_ON(regno >= MAX_BPF_REG)) {
 		verbose(env, "mark_reg_not_init(regs, %u)\n", regno);
-		/* Something bad happened, let's kill all regs */
-		for (regno = 0; regno < MAX_BPF_REG; regno++)
+		/* Something bad happened, let's kill all regs except FP */
+		for (regno = 0; regno < BPF_REG_FP; regno++)
 			__mark_reg_not_init(regs + regno);
 		return;
 	}