Lonial reported that an out-of-bounds access in cgroup local storage
can be crafted via tail calls. Given two programs each utilizing a
cgroup local storage with a different value size, and one program
doing a tail call into the other. The verifier will validate each of
the indivial programs just fine. However, in the runtime context
the bpf_cg_run_ctx holds an bpf_prog_array_item which contains the
BPF program as well as any cgroup local storage flavor the program
uses. Helpers such as bpf_get_local_storage() pick this up from the
runtime context:

  ctx = container_of(current->bpf_ctx, struct bpf_cg_run_ctx, run_ctx);
  storage = ctx->prog_item->cgroup_storage[stype];

  if (stype == BPF_CGROUP_STORAGE_SHARED)
    ptr = &READ_ONCE(storage->buf)->data[0];
  else
    ptr = this_cpu_ptr(storage->percpu_buf);

For the second program which was called from the originally attached
one, this means bpf_get_local_storage() will pick up the former
program's map, not its own. With mismatching sizes, this can result
in an unintended out-of-bounds access.

To fix this issue, we need to extend bpf_map_owner with an array of
storage_cookie[] to match on i) the exact maps from the original
program if the second program was using bpf_get_local_storage(), or
ii) allow the tail call combination if the second program was not
using any of the cgroup local storage maps.

Fixes: 7d9c3427894f ("bpf: Make cgroup storages shared between programs on the same cgroup")
Reported-by: Lonial Con <kongln9170@gmail.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
---
 include/linux/bpf.h |  1 +
 kernel/bpf/core.c   | 11 +++++++++++
 2 files changed, 12 insertions(+)

diff --git a/include/linux/bpf.h b/include/linux/bpf.h
index a41dff574327..63169b1611cc 100644
--- a/include/linux/bpf.h
+++ b/include/linux/bpf.h
@@ -281,6 +281,7 @@ struct bpf_map_owner {
 	enum bpf_prog_type type;
 	bool jited;
 	bool xdp_has_frags;
+	u64 storage_cookie[MAX_BPF_CGROUP_STORAGE_TYPE];
 	const struct btf_type *attach_func_proto;
 };
 
diff --git a/kernel/bpf/core.c b/kernel/bpf/core.c
index 666d4019d87f..7a8201763936 100644
--- a/kernel/bpf/core.c
+++ b/kernel/bpf/core.c
@@ -2371,6 +2371,7 @@ static bool __bpf_prog_map_compatible(struct bpf_map *map,
 {
 	enum bpf_prog_type prog_type = resolve_prog_type(fp);
 	struct bpf_prog_aux *aux = fp->aux;
+	enum bpf_cgroup_storage_type i;
 	bool ret = false;
 
 	if (fp->kprobe_override)
@@ -2386,11 +2387,21 @@ static bool __bpf_prog_map_compatible(struct bpf_map *map,
 		map->owner->jited = fp->jited;
 		map->owner->xdp_has_frags = aux->xdp_has_frags;
 		map->owner->attach_func_proto = aux->attach_func_proto;
+		for_each_cgroup_storage_type(i) {
+			map->owner->storage_cookie[i] =
+				aux->cgroup_storage[i] ?
+				aux->cgroup_storage[i]->cookie : 0;
+		}
 		ret = true;
 	} else {
 		ret = map->owner->type  == prog_type &&
 		      map->owner->jited == fp->jited &&
 		      map->owner->xdp_has_frags == aux->xdp_has_frags;
+		for_each_cgroup_storage_type_cond(i, ret) {
+			u64 cookie = aux->cgroup_storage[i] ?
+				     aux->cgroup_storage[i]->cookie : 0;
+			ret = map->owner->storage_cookie[i] == cookie || !cookie;
+		}
 		if (ret &&
 		    map->owner->attach_func_proto != aux->attach_func_proto) {
 			switch (prog_type) {
-- 
2.43.0