From 9e15db66136a14cde3f35691f1d839d950118826 Mon Sep 17 00:00:00 2001 From: Alexei Starovoitov Date: Tue, 15 Oct 2019 20:25:00 -0700 Subject: bpf: Implement accurate raw_tp context access via BTF libbpf analyzes bpf C program, searches in-kernel BTF for given type name and stores it into expected_attach_type. The kernel verifier expects this btf_id to point to something like: typedef void (*btf_trace_kfree_skb)(void *, struct sk_buff *skb, void *loc); which represents signature of raw_tracepoint "kfree_skb". Then btf_ctx_access() matches ctx+0 access in bpf program with 'skb' and 'ctx+8' access with 'loc' arguments of "kfree_skb" tracepoint. In first case it passes btf_id of 'struct sk_buff *' back to the verifier core and 'void *' in second case. Then the verifier tracks PTR_TO_BTF_ID as any other pointer type. Like PTR_TO_SOCKET points to 'struct bpf_sock', PTR_TO_TCP_SOCK points to 'struct bpf_tcp_sock', and so on. PTR_TO_BTF_ID points to in-kernel structs. If 1234 is btf_id of 'struct sk_buff' in vmlinux's BTF then PTR_TO_BTF_ID#1234 points to one of in kernel skbs. When PTR_TO_BTF_ID#1234 is dereferenced (like r2 = *(u64 *)r1 + 32) the btf_struct_access() checks which field of 'struct sk_buff' is at offset 32. Checks that size of access matches type definition of the field and continues to track the dereferenced type. If that field was a pointer to 'struct net_device' the r2's type will be PTR_TO_BTF_ID#456. Where 456 is btf_id of 'struct net_device' in vmlinux's BTF. Such verifier analysis prevents "cheating" in BPF C program. The program cannot cast arbitrary pointer to 'struct sk_buff *' and access it. C compiler would allow type cast, of course, but the verifier will notice type mismatch based on BPF assembly and in-kernel BTF. Signed-off-by: Alexei Starovoitov Signed-off-by: Daniel Borkmann Acked-by: Andrii Nakryiko Acked-by: Martin KaFai Lau Link: https://lore.kernel.org/bpf/20191016032505.2089704-7-ast@kernel.org --- kernel/bpf/btf.c | 190 +++++++++++++++++++++++++++++++++++++++++++++++++++++++ 1 file changed, 190 insertions(+) (limited to 'kernel/bpf/btf.c') diff --git a/kernel/bpf/btf.c b/kernel/bpf/btf.c index ddeab1e8d21e..271d27cd427f 100644 --- a/kernel/bpf/btf.c +++ b/kernel/bpf/btf.c @@ -3436,6 +3436,196 @@ errout: return ERR_PTR(err); } +extern struct btf *btf_vmlinux; + +bool btf_ctx_access(int off, int size, enum bpf_access_type type, + const struct bpf_prog *prog, + struct bpf_insn_access_aux *info) +{ + struct bpf_verifier_log *log = info->log; + u32 btf_id = prog->aux->attach_btf_id; + const struct btf_param *args; + const struct btf_type *t; + const char prefix[] = "btf_trace_"; + const char *tname; + u32 nr_args, arg; + + if (!btf_id) + return true; + + if (IS_ERR(btf_vmlinux)) { + bpf_log(log, "btf_vmlinux is malformed\n"); + return false; + } + + t = btf_type_by_id(btf_vmlinux, btf_id); + if (!t || BTF_INFO_KIND(t->info) != BTF_KIND_TYPEDEF) { + bpf_log(log, "btf_id is invalid\n"); + return false; + } + + tname = __btf_name_by_offset(btf_vmlinux, t->name_off); + if (strncmp(prefix, tname, sizeof(prefix) - 1)) { + bpf_log(log, "btf_id points to wrong type name %s\n", tname); + return false; + } + tname += sizeof(prefix) - 1; + + t = btf_type_by_id(btf_vmlinux, t->type); + if (!btf_type_is_ptr(t)) + return false; + t = btf_type_by_id(btf_vmlinux, t->type); + if (!btf_type_is_func_proto(t)) + return false; + + if (off % 8) { + bpf_log(log, "raw_tp '%s' offset %d is not multiple of 8\n", + tname, off); + return false; + } + arg = off / 8; + args = (const struct btf_param *)(t + 1); + /* skip first 'void *__data' argument in btf_trace_##name typedef */ + args++; + nr_args = btf_type_vlen(t) - 1; + if (arg >= nr_args) { + bpf_log(log, "raw_tp '%s' doesn't have %d-th argument\n", + tname, arg); + return false; + } + + t = btf_type_by_id(btf_vmlinux, args[arg].type); + /* skip modifiers */ + while (btf_type_is_modifier(t)) + t = btf_type_by_id(btf_vmlinux, t->type); + if (btf_type_is_int(t)) + /* accessing a scalar */ + return true; + if (!btf_type_is_ptr(t)) { + bpf_log(log, + "raw_tp '%s' arg%d '%s' has type %s. Only pointer access is allowed\n", + tname, arg, + __btf_name_by_offset(btf_vmlinux, t->name_off), + btf_kind_str[BTF_INFO_KIND(t->info)]); + return false; + } + if (t->type == 0) + /* This is a pointer to void. + * It is the same as scalar from the verifier safety pov. + * No further pointer walking is allowed. + */ + return true; + + /* this is a pointer to another type */ + info->reg_type = PTR_TO_BTF_ID; + info->btf_id = t->type; + + t = btf_type_by_id(btf_vmlinux, t->type); + /* skip modifiers */ + while (btf_type_is_modifier(t)) + t = btf_type_by_id(btf_vmlinux, t->type); + if (!btf_type_is_struct(t)) { + bpf_log(log, + "raw_tp '%s' arg%d type %s is not a struct\n", + tname, arg, btf_kind_str[BTF_INFO_KIND(t->info)]); + return false; + } + bpf_log(log, "raw_tp '%s' arg%d has btf_id %d type %s '%s'\n", + tname, arg, info->btf_id, btf_kind_str[BTF_INFO_KIND(t->info)], + __btf_name_by_offset(btf_vmlinux, t->name_off)); + return true; +} + +int btf_struct_access(struct bpf_verifier_log *log, + const struct btf_type *t, int off, int size, + enum bpf_access_type atype, + u32 *next_btf_id) +{ + const struct btf_member *member; + const struct btf_type *mtype; + const char *tname, *mname; + int i, moff = 0, msize; + +again: + tname = __btf_name_by_offset(btf_vmlinux, t->name_off); + if (!btf_type_is_struct(t)) { + bpf_log(log, "Type '%s' is not a struct", tname); + return -EINVAL; + } + + for_each_member(i, t, member) { + /* offset of the field in bits */ + moff = btf_member_bit_offset(t, member); + + if (btf_member_bitfield_size(t, member)) + /* bitfields are not supported yet */ + continue; + + if (off + size <= moff / 8) + /* won't find anything, field is already too far */ + break; + + /* type of the field */ + mtype = btf_type_by_id(btf_vmlinux, member->type); + mname = __btf_name_by_offset(btf_vmlinux, member->name_off); + + /* skip modifiers */ + while (btf_type_is_modifier(mtype)) + mtype = btf_type_by_id(btf_vmlinux, mtype->type); + + if (btf_type_is_array(mtype)) + /* array deref is not supported yet */ + continue; + + if (!btf_type_has_size(mtype) && !btf_type_is_ptr(mtype)) { + bpf_log(log, "field %s doesn't have size\n", mname); + return -EFAULT; + } + if (btf_type_is_ptr(mtype)) + msize = 8; + else + msize = mtype->size; + if (off >= moff / 8 + msize) + /* no overlap with member, keep iterating */ + continue; + /* the 'off' we're looking for is either equal to start + * of this field or inside of this struct + */ + if (btf_type_is_struct(mtype)) { + /* our field must be inside that union or struct */ + t = mtype; + + /* adjust offset we're looking for */ + off -= moff / 8; + goto again; + } + if (msize != size) { + /* field access size doesn't match */ + bpf_log(log, + "cannot access %d bytes in struct %s field %s that has size %d\n", + size, tname, mname, msize); + return -EACCES; + } + + if (btf_type_is_ptr(mtype)) { + const struct btf_type *stype; + + stype = btf_type_by_id(btf_vmlinux, mtype->type); + /* skip modifiers */ + while (btf_type_is_modifier(stype)) + stype = btf_type_by_id(btf_vmlinux, stype->type); + if (btf_type_is_struct(stype)) { + *next_btf_id = mtype->type; + return PTR_TO_BTF_ID; + } + } + /* all other fields are treated as scalars */ + return SCALAR_VALUE; + } + bpf_log(log, "struct %s doesn't have field at offset %d\n", tname, off); + return -EINVAL; +} + void btf_type_seq_show(const struct btf *btf, u32 type_id, void *obj, struct seq_file *m) { -- cgit