bpf: Make bpf_cgroup_acquire() KF_RCU | KF_RET_NULL

struct cgroup is already an RCU-safe type in the verifier. We can
therefore update bpf_cgroup_acquire() to be KF_RCU | KF_RET_NULL, and
subsequently remove bpf_cgroup_kptr_get(). This patch does the first of
these by updating bpf_cgroup_acquire() to be KF_RCU | KF_RET_NULL, and
also updates selftests accordingly.

Signed-off-by: David Vernet <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Alexei Starovoitov <[e-mail]>

kernel/bpf/helpers.c

···
  */
 __bpf_kfunc struct cgroup *bpf_cgroup_acquire(struct cgroup *cgrp)
 {
-	cgroup_get(cgrp);
-	return cgrp;
+	return cgroup_tryget(cgrp) ? cgrp : NULL;
 }
 
 /**
···
 BTF_ID_FLAGS(func, bpf_rbtree_first, KF_RET_NULL)
 
 #ifdef CONFIG_CGROUPS
-BTF_ID_FLAGS(func, bpf_cgroup_acquire, KF_ACQUIRE | KF_TRUSTED_ARGS)
+BTF_ID_FLAGS(func, bpf_cgroup_acquire, KF_ACQUIRE | KF_RCU | KF_RET_NULL)
 BTF_ID_FLAGS(func, bpf_cgroup_kptr_get, KF_ACQUIRE | KF_KPTR_GET | KF_RET_NULL)
 BTF_ID_FLAGS(func, bpf_cgroup_release, KF_RELEASE)
 BTF_ID_FLAGS(func, bpf_cgroup_ancestor, KF_ACQUIRE | KF_RCU | KF_RET_NULL)

tools/testing/selftests/bpf/progs/cgrp_kfunc_common.h

···
 	}
 
 	acquired = bpf_cgroup_acquire(cgrp);
+	if (!acquired) {
+		bpf_map_delete_elem(&__cgrps_kfunc_map, &id);
+		return -ENOENT;
+	}
+
 	old = bpf_kptr_xchg(&v->cgrp, acquired);
 	if (old) {
 		bpf_cgroup_release(old);

tools/testing/selftests/bpf/progs/cgrp_kfunc_failure.c

···
 
 	/* Can't invoke bpf_cgroup_acquire() on an untrusted pointer. */
 	acquired = bpf_cgroup_acquire(v->cgrp);
+	if (acquired)
+		bpf_cgroup_release(acquired);
+
+	return 0;
+}
+
+SEC("tp_btf/cgroup_mkdir")
+__failure __msg("Possibly NULL pointer passed to trusted arg0")
+int BPF_PROG(cgrp_kfunc_acquire_no_null_check, struct cgroup *cgrp, const char *path)
+{
+	struct cgroup *acquired;
+
+	acquired = bpf_cgroup_acquire(cgrp);
+	/*
+	 * Can't invoke bpf_cgroup_release() without checking the return value
+	 * of bpf_cgroup_acquire().
+	 */
 	bpf_cgroup_release(acquired);
 
 	return 0;
···
 
 	/* Can't invoke bpf_cgroup_acquire() on a random frame pointer. */
 	acquired = bpf_cgroup_acquire((struct cgroup *)&stack_cgrp);
-	bpf_cgroup_release(acquired);
+	if (acquired)
+		bpf_cgroup_release(acquired);
 
 	return 0;
 }
···
 
 	/* Can't acquire an untrusted struct cgroup * pointer. */
 	acquired = bpf_cgroup_acquire(cgrp);
-	bpf_cgroup_release(acquired);
+	if (acquired)
+		bpf_cgroup_release(acquired);
 
 	return 0;
 }
···
 
 	/* Can't invoke bpf_cgroup_acquire() on a pointer obtained from walking a trusted cgroup. */
 	acquired = bpf_cgroup_acquire(cgrp->old_dom_cgrp);
-	bpf_cgroup_release(acquired);
+	if (acquired)
+		bpf_cgroup_release(acquired);
 
 	return 0;
 }
···
 
 	/* Can't invoke bpf_cgroup_acquire() on a NULL pointer. */
 	acquired = bpf_cgroup_acquire(NULL);
-	if (!acquired)
-		return 0;
-	bpf_cgroup_release(acquired);
+	if (acquired)
+		bpf_cgroup_release(acquired);
 
 	return 0;
 }
···
 	struct cgroup *kptr, *acquired;
 
 	acquired = bpf_cgroup_acquire(cgrp);
+	if (!acquired)
+		return 0;
 
 	/* Cannot use bpf_cgroup_kptr_get() on a non-map-value, even if the kptr was acquired. */
 	kptr = bpf_cgroup_kptr_get(&acquired);
···
 		return -ENOENT;
 
 	acquired = bpf_cgroup_acquire(cgrp);
+	if (!acquired)
+		return -ENOENT;
 
 	old = bpf_kptr_xchg(&v->cgrp, acquired);
 

tools/testing/selftests/bpf/progs/cgrp_kfunc_success.c

···
 		return 0;
 
 	acquired = bpf_cgroup_acquire(cgrp);
-	bpf_cgroup_release(acquired);
+	if (!acquired)
+		err = 1;
+	else
+		bpf_cgroup_release(acquired);
 
 	return 0;
 }