bpf: Enforce expected_attach_type for tailcall compatibility

Yinhao et al. recently reported:

  Our fuzzer tool discovered an uninitialized pointer issue in the
  bpf_prog_test_run_xdp() function within the Linux kernel's BPF subsystem.
  This leads to a NULL pointer dereference when a BPF program attempts to
  deference the txq member of struct xdp_buff object.

The test initializes two programs of BPF_PROG_TYPE_XDP: progA acts as the
entry point for bpf_prog_test_run_xdp() and its expected_attach_type can
neither be of be BPF_XDP_DEVMAP nor BPF_XDP_CPUMAP. progA calls into a slot
of a tailcall map it owns. progB's expected_attach_type must be BPF_XDP_DEVMAP
to pass xdp_is_valid_access() validation. The program returns struct xdp_md's
egress_ifindex, and the latter is only allowed to be accessed under mentioned
expected_attach_type. progB is then inserted into the tailcall which progA
calls.

The underlying issue goes beyond XDP though. Another example are programs
of type BPF_PROG_TYPE_CGROUP_SOCK_ADDR. sock_addr_is_valid_access() as well
as sock_addr_func_proto() have different logic depending on the programs'
expected_attach_type. Similarly, a program attached to BPF_CGROUP_INET4_GETPEERNAME
should not be allowed doing a tailcall into a program which calls bpf_bind()
out of BPF which is only enabled for BPF_CGROUP_INET4_CONNECT.

In short, specifying expected_attach_type allows to open up additional
functionality or restrictions beyond what the basic bpf_prog_type enables.
The use of tailcalls must not violate these constraints. Fix it by enforcing
expected_attach_type in __bpf_prog_map_compatible().

Note that we only enforce this for tailcall maps, but not for BPF devmaps or
cpumaps: There, the programs are invoked through dev_map_bpf_prog_run*() and
cpu_map_bpf_prog_run*() which set up a new environment / context and therefore
these situations are not prone to this issue.

Fixes: 5e43f89 ("bpf: Check attach type at prog load time")
Reported-by: Yinhao Hu <[email protected]>
Reported-by: Kaiyan Mei <[email protected]>
Reviewed-by: Dongliang Mu <[email protected]>
Signed-off-by: Daniel Borkmann <[email protected]>

Author: borkmann

include/linux/bpf.h

@@ -290,6 +290,7 @@ struct bpf_map_owner {
 	bool xdp_has_frags;
 	u64 storage_cookie[MAX_BPF_CGROUP_STORAGE_TYPE];
 	const struct btf_type *attach_func_proto;
+	enum bpf_attach_type expected_attach_type;
 };
 
 struct bpf_map {

kernel/bpf/core.c

@@ -2361,6 +2361,7 @@ static bool __bpf_prog_map_compatible(struct bpf_map *map,
 		map->owner->type  = prog_type;
 		map->owner->jited = fp->jited;
 		map->owner->xdp_has_frags = aux->xdp_has_frags;
+		map->owner->expected_attach_type = fp->expected_attach_type;
 		map->owner->attach_func_proto = aux->attach_func_proto;
 		for_each_cgroup_storage_type(i) {
 			map->owner->storage_cookie[i] =
@@ -2372,6 +2373,10 @@ static bool __bpf_prog_map_compatible(struct bpf_map *map,
 		ret = map->owner->type  == prog_type &&
 		      map->owner->jited == fp->jited &&
 		      map->owner->xdp_has_frags == aux->xdp_has_frags;
+		if (ret &&
+		    map->map_type == BPF_MAP_TYPE_PROG_ARRAY &&
+		    map->owner->expected_attach_type != fp->expected_attach_type)
+			ret = false;
 		for_each_cgroup_storage_type(i) {
 			if (!ret)
 				break;