bpf: Remove unnecessary BTF lookups in bpf_sk_storage_tracing_allowed

When loading BPF programs, bpf_sk_storage_tracing_allowed() does a
series of lookups to get a type name from the program's attach_btf_id,
making the assumption that the type is present in the vmlinux BTF along
the way. However, this results in btf_type_by_id() returning a null
pointer if a non-vmlinux kernel BTF is attached to. Proof-of-concept on
a kernel with CONFIG_IPV6=m:

$ cat bpfcrash.c
#include <unistd.h>
#include <linux/bpf.h>
#include <sys/syscall.h>

static int bpf(enum bpf_cmd cmd, union bpf_attr *attr)
{
return syscall(__NR_bpf, cmd, attr, sizeof(*attr));
}

int main(void)
{
const int btf_fd = bpf(BPF_BTF_GET_FD_BY_ID, &(union bpf_attr) {
.btf_id = BTF_ID,
});
if (btf_fd < 0)
return 1;

const int bpf_sk_storage_get = 107;
const struct bpf_insn insns[] = {
{ .code = BPF_JMP | BPF_CALL, .imm = bpf_sk_storage_get},
{ .code = BPF_JMP | BPF_EXIT },
};
return bpf(BPF_PROG_LOAD, &(union bpf_attr) {
.prog_type = BPF_PROG_TYPE_TRACING,
.expected_attach_type = BPF_TRACE_FENTRY,
.license = (unsigned long)"GPL",
.insns = (unsigned long)&insns,
.insn_cnt = sizeof(insns) / sizeof(insns[0]),
.attach_btf_obj_fd = btf_fd,
.attach_btf_id = TYPE_ID,
});
}
$ sudo bpftool btf list | grep ipv6
2: name [ipv6] size 928200B
$ sudo bpftool btf dump id 2 | awk '$3 ~ /inet6_sock_destruct/'
[130689] FUNC 'inet6_sock_destruct' type_id=130677 linkage=static
$ gcc -D_DEFAULT_SOURCE -DBTF_ID=2 -DTYPE_ID=130689 \
bpfcrash.c -o bpfcrash
$ sudo ./bpfcrash

This causes a null pointer dereference:

Unable to handle kernel NULL pointer dereference at virtual address 0000000000000000
Call trace:
bpf_sk_storage_tracing_allowed+0x8c/0xb0 P
check_helper_call.isra.0+0xa8/0x1730
do_check+0xa18/0xb40
do_check_common+0x140/0x640
bpf_check+0xb74/0xcb8
bpf_prog_load+0x598/0x9a8
__sys_bpf+0x580/0x980
__arm64_sys_bpf+0x28/0x40
invoke_syscall.constprop.0+0x54/0xe8
do_el0_svc+0xb4/0xd0
el0_svc+0x44/0x1f8
el0t_64_sync_handler+0x13c/0x160
el0t_64_sync+0x184/0x188

Resolve this by using prog->aux->attach_func_name and removing the
lookups.

Fixes: 8e4597c627fb ("bpf: Allow using bpf_sk_storage in FENTRY/FEXIT/RAW_TP")
Suggested-by: Martin KaFai Lau <[e-mail]>
Signed-off-by: Jared Kangas <[e-mail]>
Signed-off-by: Martin KaFai Lau <[e-mail]>
Link: https://patch.msgid.link/[e-mail]
Signed-off-by: Alexei Starovoitov <[e-mail]>

net/core/bpf_sk_storage.c

···
 
 static bool bpf_sk_storage_tracing_allowed(const struct bpf_prog *prog)
 {
-	const struct btf *btf_vmlinux;
-	const struct btf_type *t;
-	const char *tname;
-	u32 btf_id;
-
 	if (prog->aux->dst_prog)
 		return false;
 
···
 		return true;
 	case BPF_TRACE_FENTRY:
 	case BPF_TRACE_FEXIT:
-		btf_vmlinux = bpf_get_btf_vmlinux();
-		if (IS_ERR_OR_NULL(btf_vmlinux))
-			return false;
-		btf_id = prog->aux->attach_btf_id;
-		t = btf_type_by_id(btf_vmlinux, btf_id);
-		tname = btf_name_by_offset(btf_vmlinux, t->name_off);
-		return !!strncmp(tname, "bpf_sk_storage",
+		return !!strncmp(prog->aux->attach_func_name, "bpf_sk_storage",
 				 strlen("bpf_sk_storage"));
 	default:
 		return false;