[bpf-next,1/2] libbpf: Don't take direct pointers into BTF data from st_ops

Commit Message

David Vernet July 24, 2024, 5:14 p.m. UTC

In struct bpf_struct_ops, we have take a pointer to a BTF type name, and
a struct btf_type. This was presumably done for convenience, but can
actually result in subtle and confusing bugs given that BTF data can be
invalidated before a program is loaded. For example, in sched_ext, we
may sometimes resize a data section after a skeleton has been opened,
but before the struct_ops scheduler map has been loaded. This may cause
the BTF data to be realloc'd, which can then cause a UAF when loading
the program because the struct_ops map has pointers directly into the
BTF data.

We're already storing the BTF type_id in struct bpf_struct_ops. Because
type_id is stable, we can therefore just update the places where we were
looking at those pointers to instead do the lookups we need from the
type_id.

Fixes: 590a00888250 ("bpf: libbpf: Add STRUCT_OPS support")
Signed-off-by: David Vernet <[e-mail]>
---
 tools/lib/bpf/libbpf.c | 23 +++++++++++++----------
 1 file changed, 13 insertions(+), 10 deletions(-)

Comments

[e-mail] July 25, 2024, 12:20 a.m. UTC | #1

Hello:

This series was applied to bpf/bpf-next.git (master)
by Andrii Nakryiko <[e-mail]>:

On Wed, 24 Jul 2024 12:14:58 -0500 you wrote:
> In struct bpf_struct_ops, we have take a pointer to a BTF type name, and
> a struct btf_type. This was presumably done for convenience, but can
> actually result in subtle and confusing bugs given that BTF data can be
> invalidated before a program is loaded. For example, in sched_ext, we
> may sometimes resize a data section after a skeleton has been opened,
> but before the struct_ops scheduler map has been loaded. This may cause
> the BTF data to be realloc'd, which can then cause a UAF when loading
> the program because the struct_ops map has pointers directly into the
> BTF data.
> 
> [...]

Here is the summary with links:
  - [bpf-next,1/2] libbpf: Don't take direct pointers into BTF data from st_ops
    https://git.kernel.org/bpf/bpf-next/c/7244100e0389
  - [bpf-next,2/2] selftests/bpf: Add test for resizing data map with struct_ops
    (no matching commit)

You are awesome, thank you!

Patch

diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
index a3be6f8fac09..e55353887439 100644
--- a/tools/lib/bpf/libbpf.c
+++ b/tools/lib/bpf/libbpf.c
@@ -496,8 +496,6 @@  struct bpf_program {
 };
 
 struct bpf_struct_ops {
-	const char *tname;
-	const struct btf_type *type;
 	struct bpf_program **progs;
 	__u32 *kern_func_off;
 	/* e.g. struct tcp_congestion_ops in bpf_prog's btf format */
@@ -1083,11 +1081,14 @@  static int bpf_object_adjust_struct_ops_autoload(struct bpf_object *obj)
 			continue;
 
 		for (j = 0; j < obj->nr_maps; ++j) {
+			const struct btf_type *type;
+
 			map = &obj->maps[j];
 			if (!bpf_map__is_struct_ops(map))
 				continue;
 
-			vlen = btf_vlen(map->st_ops->type);
+			type = btf__type_by_id(obj->btf, map->st_ops->type_id);
+			vlen = btf_vlen(type);
 			for (k = 0; k < vlen; ++k) {
 				slot_prog = map->st_ops->progs[k];
 				if (prog != slot_prog)
@@ -1121,8 +1122,8 @@  static int bpf_map__init_kern_struct_ops(struct bpf_map *map)
 	int err;
 
 	st_ops = map->st_ops;
-	type = st_ops->type;
-	tname = st_ops->tname;
+	type = btf__type_by_id(btf, st_ops->type_id);
+	tname = btf__name_by_offset(btf, type->name_off);
 	err = find_struct_ops_kern_types(obj, tname, &mod_btf,
 					 &kern_type, &kern_type_id,
 					 &kern_vtype, &kern_vtype_id,
@@ -1423,8 +1424,6 @@  static int init_struct_ops_maps(struct bpf_object *obj, const char *sec_name,
 		memcpy(st_ops->data,
 		       data->d_buf + vsi->offset,
 		       type->size);
-		st_ops->tname = tname;
-		st_ops->type = type;
 		st_ops->type_id = type_id;
 
 		pr_debug("struct_ops init: struct %s(type_id=%u) %s found at offset %u\n",
@@ -8445,11 +8444,13 @@  static int bpf_object__resolve_externs(struct bpf_object *obj,
 
 static void bpf_map_prepare_vdata(const struct bpf_map *map)
 {
+	const struct btf_type *type;
 	struct bpf_struct_ops *st_ops;
 	__u32 i;
 
 	st_ops = map->st_ops;
-	for (i = 0; i < btf_vlen(st_ops->type); i++) {
+	type = btf__type_by_id(map->obj->btf, st_ops->type_id);
+	for (i = 0; i < btf_vlen(type); i++) {
 		struct bpf_program *prog = st_ops->progs[i];
 		void *kern_data;
 		int prog_fd;
@@ -9712,6 +9713,7 @@  static struct bpf_map *find_struct_ops_map_by_offset(struct bpf_object *obj,
 static int bpf_object__collect_st_ops_relos(struct bpf_object *obj,
 					    Elf64_Shdr *shdr, Elf_Data *data)
 {
+	const struct btf_type *type;
 	const struct btf_member *member;
 	struct bpf_struct_ops *st_ops;
 	struct bpf_program *prog;
@@ -9771,13 +9773,14 @@  static int bpf_object__collect_st_ops_relos(struct bpf_object *obj,
 		}
 		insn_idx = sym->st_value / BPF_INSN_SZ;
 
-		member = find_member_by_offset(st_ops->type, moff * 8);
+		type = btf__type_by_id(btf, st_ops->type_id);
+		member = find_member_by_offset(type, moff * 8);
 		if (!member) {
 			pr_warn("struct_ops reloc %s: cannot find member at moff %u\n",
 				map->name, moff);
 			return -EINVAL;
 		}
-		member_idx = member - btf_members(st_ops->type);
+		member_idx = member - btf_members(type);
 		name = btf__name_by_offset(btf, member->name_off);
 
 		if (!resolve_func_ptr(btf, member->type, NULL)) {