bpf: Fix a btf decl_tag bug when tagging a function

In the Linux kernel, the following vulnerability has been resolved:

bpf: Fix a btf decl_tag bug when tagging a function

syzbot reported a btf decl_tag bug with stack trace below:

general protection fault, probably for non-canonical address 0xdffffc0000000000: 0000 [#1] PREEMPT SMP KASAN

KASAN: null-ptr-deref in range [0x0000000000000000-0x0000000000000007]

CPU: 0 PID: 3592 Comm: syz-executor914 Not tainted 5.16.0-syzkaller-11424-gb7892f7d5cb2 #0

Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011

RIP: 0010:btf_type_vlen include/linux/btf.h:231 [inline]

RIP: 0010:btf_decl_tag_resolve+0x83e/0xaa0 kernel/bpf/btf.c:3910

...

Call Trace:

<TASK>

btf_resolve+0x251/0x1020 kernel/bpf/btf.c:4198

btf_check_all_types kernel/bpf/btf.c:4239 [inline]

btf_parse_type_sec kernel/bpf/btf.c:4280 [inline]

btf_parse kernel/bpf/btf.c:4513 [inline]

btf_new_fd+0x19fe/0x2370 kernel/bpf/btf.c:6047

bpf_btf_load kernel/bpf/syscall.c:4039 [inline]

__sys_bpf+0x1cbb/0x5970 kernel/bpf/syscall.c:4679

__do_sys_bpf kernel/bpf/syscall.c:4738 [inline]

__se_sys_bpf kernel/bpf/syscall.c:4736 [inline]

__x64_sys_bpf+0x75/0xb0 kernel/bpf/syscall.c:4736

do_syscall_x64 arch/x86/entry/common.c:50 [inline]

do_syscall_64+0x35/0xb0 arch/x86/entry/common.c:80

entry_SYSCALL_64_after_hwframe+0x44/0xae

The kasan error is triggered with an illegal BTF like below:

type 0: void

type 1: int

type 2: decl_tag to func type 3

type 3: func to func_proto type 8

The total number of types is 4 and the type 3 is illegal

since its func_proto type is out of range.

Currently, the target type of decl_tag can be struct/union, var or func.

Both struct/union and var implemented their own 'resolve' callback functions

and hence handled properly in kernel.

But func type doesn't have 'resolve' callback function. When

btf_decl_tag_resolve() tries to check func type, it tries to get

vlen of its func_proto type, which triggered the above kasan error.

To fix the issue, btf_decl_tag_resolve() needs to do btf_func_check()

before trying to accessing func_proto type.

In the current implementation, func type is checked with

btf_func_check() in the main checking function btf_check_all_types().

To fix the above kasan issue, let us implement 'resolve' callback

func type properly. The 'resolve' callback will be also called

in btf_check_all_types() for func types.