bpf: Add ambient BPF runtime context stored in current

b910eaaaa4b8 ("bpf: Fix NULL pointer dereference in bpf_get_local_storage()
helper") fixed the problem with cgroup-local storage use in BPF by
pre-allocating per-CPU array of 8 cgroup storage pointers to accommodate
possible BPF program preemptions and nested executions.

While this seems to work good in practice, it introduces new and unnecessary
failure mode in which not all BPF programs might be executed if we fail to
find an unused slot for cgroup storage, however unlikely it is. It might also
not be so unlikely when/if we allow sleepable cgroup BPF programs in the
future.

Further, the way that cgroup storage is implemented as ambiently-available
property during entire BPF program execution is a convenient way to pass extra
information to BPF program and helpers without requiring user code to pass
around extra arguments explicitly. So it would be good to have a generic
solution that can allow implementing this without arbitrary restrictions.
Ideally, such solution would work for both preemptable and sleepable BPF
programs in exactly the same way.

This patch introduces such solution, bpf_run_ctx. It adds one pointer field
(bpf_ctx) to task_struct. This field is maintained by BPF_PROG_RUN family of
macros in such a way that it always stays valid throughout BPF program
execution. BPF program preemption is handled by remembering previous
current->bpf_ctx value locally while executing nested BPF program and
restoring old value after nested BPF program finishes. This is handled by two
helper functions, bpf_set_run_ctx() and bpf_reset_run_ctx(), which are
supposed to be used before and after BPF program runs, respectively.

Restoring old value of the pointer handles preemption, while bpf_run_ctx
pointer being a property of current task_struct naturally solves this problem
for sleepable BPF programs by "following" BPF program execution as it is
scheduled in and out of CPU. It would even allow CPU migration of BPF
programs, even though it's not currently allowed by BPF infra.

This patch cleans up cgroup local storage handling as a first application. The
design itself is generic, though, with bpf_run_ctx being an empty struct that
is supposed to be embedded into a specific struct for a given BPF program type
(bpf_cg_run_ctx in this case). Follow up patches are planned that will expand
this mechanism for other uses within tracing BPF programs.

To verify that this change doesn't revert the fix to the original cgroup
storage issue, I ran the same repro as in the original report ([0]) and didn't
get any problems. Replacing bpf_reset_run_ctx(old_run_ctx) with
bpf_reset_run_ctx(NULL) triggers the issue pretty quickly (so repro does work).

  [0] https://lore.kernel.org/bpf/YEEvBUiJl2pJkxTd@krava/

Fixes: b910eaaaa4b8 ("bpf: Fix NULL pointer dereference in bpf_get_local_storage() helper")
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Yonghong Song <yhs@fb.com>
Link: https://lore.kernel.org/bpf/20210712230615.3525979-1-andrii@kernel.org

1 files changed, 0 insertions, 54 deletions

diff --git a/include/linux/bpf-cgroup.h b/include/linux/bpf-cgroup.h
index 8b77d08d4b47..a74cd1c3bd87 100644
--- a/include/linux/bpf-cgroup.h
+++ b/include/linux/bpf-cgroup.h
@@ -27,19 +27,6 @@ struct task_struct;
 extern struct static_key_false cgroup_bpf_enabled_key[MAX_BPF_ATTACH_TYPE];
 #define cgroup_bpf_enabled(type) static_branch_unlikely(&cgroup_bpf_enabled_key[type])
 
-#define BPF_CGROUP_STORAGE_NEST_MAX	8
-
-struct bpf_cgroup_storage_info {
-	struct task_struct *task;
-	struct bpf_cgroup_storage *storage[MAX_BPF_CGROUP_STORAGE_TYPE];
-};
-
-/* For each cpu, permit maximum BPF_CGROUP_STORAGE_NEST_MAX number of tasks
- * to use bpf cgroup storage simultaneously.
- */
-DECLARE_PER_CPU(struct bpf_cgroup_storage_info,
-		bpf_cgroup_storage_info[BPF_CGROUP_STORAGE_NEST_MAX]);
-
 #define for_each_cgroup_storage_type(stype) \
 	for (stype = 0; stype < MAX_BPF_CGROUP_STORAGE_TYPE; stype++)
 
@@ -172,44 +159,6 @@ static inline enum bpf_cgroup_storage_type cgroup_storage_type(
 	return BPF_CGROUP_STORAGE_SHARED;
 }
 
-static inline int bpf_cgroup_storage_set(struct bpf_cgroup_storage
-					 *storage[MAX_BPF_CGROUP_STORAGE_TYPE])
-{
-	enum bpf_cgroup_storage_type stype;
-	int i, err = 0;
-
-	preempt_disable();
-	for (i = 0; i < BPF_CGROUP_STORAGE_NEST_MAX; i++) {
-		if (unlikely(this_cpu_read(bpf_cgroup_storage_info[i].task) != NULL))
-			continue;
-
-		this_cpu_write(bpf_cgroup_storage_info[i].task, current);
-		for_each_cgroup_storage_type(stype)
-			this_cpu_write(bpf_cgroup_storage_info[i].storage[stype],
-				       storage[stype]);
-		goto out;
-	}
-	err = -EBUSY;
-	WARN_ON_ONCE(1);
-
-out:
-	preempt_enable();
-	return err;
-}
-
-static inline void bpf_cgroup_storage_unset(void)
-{
-	int i;
-
-	for (i = 0; i < BPF_CGROUP_STORAGE_NEST_MAX; i++) {
-		if (unlikely(this_cpu_read(bpf_cgroup_storage_info[i].task) != current))
-			continue;
-
-		this_cpu_write(bpf_cgroup_storage_info[i].task, NULL);
-		return;
-	}
-}
-
 struct bpf_cgroup_storage *
 cgroup_storage_lookup(struct bpf_cgroup_storage_map *map,
 		      void *key, bool locked);
@@ -487,9 +436,6 @@ static inline int cgroup_bpf_prog_query(const union bpf_attr *attr,
 	return -EINVAL;
 }
 
-static inline int bpf_cgroup_storage_set(
-	struct bpf_cgroup_storage *storage[MAX_BPF_CGROUP_STORAGE_TYPE]) { return 0; }
-static inline void bpf_cgroup_storage_unset(void) {}
 static inline int bpf_cgroup_storage_assign(struct bpf_prog_aux *aux,
 					    struct bpf_map *map) { return 0; }
 static inline struct bpf_cgroup_storage *bpf_cgroup_storage_alloc(