On 8/23/23 6:38 PM, Alexei Starovoitov wrote:
On Wed, Aug 23, 2023 at 1:29 PM Yonghong Song <yonghong.song@xxxxxxxxx> wrote:
On 8/23/23 9:20 AM, Alexei Starovoitov wrote:
On Tue, Aug 22, 2023 at 11:26 PM Yonghong Song <yonghong.song@xxxxxxxxx> wrote:
On 8/21/23 12:33 PM, Dave Marchevsky wrote:
This is the final fix for the use-after-free scenario described in
commit 7793fc3babe9 ("bpf: Make bpf_refcount_acquire fallible for
non-owning refs"). That commit, by virtue of changing
bpf_refcount_acquire's refcount_inc to a refcount_inc_not_zero, fixed
the "refcount incr on 0" splat. The not_zero check in
refcount_inc_not_zero, though, still occurs on memory that could have
been free'd and reused, so the commit didn't properly fix the root
cause.
This patch actually fixes the issue by free'ing using the recently-added
bpf_mem_free_rcu, which ensures that the memory is not reused until
RCU grace period has elapsed. If that has happened then
there are no non-owning references alive that point to the
recently-free'd memory, so it can be safely reused.
Signed-off-by: Dave Marchevsky <davemarchevsky@xxxxxx>
Acked-by: Yonghong Song <yonghong.song@xxxxxxxxx>
---
kernel/bpf/helpers.c | 6 +++++-
1 file changed, 5 insertions(+), 1 deletion(-)
diff --git a/kernel/bpf/helpers.c b/kernel/bpf/helpers.c
index eb91cae0612a..945a85e25ac5 100644
--- a/kernel/bpf/helpers.c
+++ b/kernel/bpf/helpers.c
@@ -1913,7 +1913,11 @@ void __bpf_obj_drop_impl(void *p, const struct btf_record *rec)
if (rec)
bpf_obj_free_fields(rec, p);
During reviewing my percpu kptr patch with link
https://lore.kernel.org/bpf/20230814172809.1361446-1-yonghong.song@xxxxxxxxx/T/#m2f7631b8047e9f5da60a0a9cd8717fceaf1adbb7
Kumar mentioned although percpu memory itself is freed under rcu.
But its record fields are freed immediately. This will cause
the problem since there may be some active uses of these fields
within rcu cs and after bpf_obj_free_fields(), some fields may
be re-initialized with new memory but they do not have chances
to free any more.
Do we have problem here as well?
I think it's not an issue here or in your percpu patch,
since bpf_obj_free_fields() calls __bpf_obj_drop_impl() which will
call bpf_mem_free_rcu() (after this patch set lands).
The following is my understanding.
void bpf_obj_free_fields(const struct btf_record *rec, void *obj)
{
const struct btf_field *fields;
int i;
if (IS_ERR_OR_NULL(rec))
return;
fields = rec->fields;
for (i = 0; i < rec->cnt; i++) {
struct btf_struct_meta *pointee_struct_meta;
const struct btf_field *field = &fields[i];
void *field_ptr = obj + field->offset;
void *xchgd_field;
switch (fields[i].type) {
case BPF_SPIN_LOCK:
break;
case BPF_TIMER:
bpf_timer_cancel_and_free(field_ptr);
break;
case BPF_KPTR_UNREF:
WRITE_ONCE(*(u64 *)field_ptr, 0);
break;
case BPF_KPTR_REF:
......
break;
case BPF_LIST_HEAD:
if (WARN_ON_ONCE(rec->spin_lock_off < 0))
continue;
bpf_list_head_free(field, field_ptr, obj +
rec->spin_lock_off);
break;
case BPF_RB_ROOT:
if (WARN_ON_ONCE(rec->spin_lock_off < 0))
continue;
bpf_rb_root_free(field, field_ptr, obj +
rec->spin_lock_off);
break;
case BPF_LIST_NODE:
case BPF_RB_NODE:
case BPF_REFCOUNT:
break;
default:
WARN_ON_ONCE(1);
continue;
}
}
}
For percpu kptr, the remaining possible actiionable fields are
BPF_LIST_HEAD and BPF_RB_ROOT
So BPF_LIST_HEAD and BPF_RB_ROOT will try to go through all
list/rb nodes to unlink them from the list_head/rb_root.
So yes, rb_nodes and list nodes will call __bpf_obj_drop_impl().
Depending on whether the correspondingrec
with rb_node/list_node has ref count or not,
it may call bpf_mem_free() or bpf_mem_free_rcu(). If
bpf_mem_free() is called, then the field is immediately freed
but it may be used by some bpf prog (under rcu) concurrently,
could this be an issue?
I see. Yeah. Looks like percpu makes such fields refcount-like.
For non-percpu non-refcount only one bpf prog on one cpu can observe
that object. That's why we're doing plain bpf_mem_free() for them.
So this patch is a good fix for refcounted, but you and Kumar are
correct that it's not sufficient for the case when percpu struct
includes multiple rb_roots. One for each cpu.
Changing bpf_mem_free() in
__bpf_obj_drop_impl() to bpf_mem_free_rcu() should fix this problem.
I guess we can do that when obj is either refcount or can be
insider percpu, but it might not be enough. More below.
Another thing is related to list_head/rb_root.
During bpf_obj_free_fields(), is it possible that another cpu
may allocate a list_node/rb_node and add to list_head/rb_root?
It's not an issue for the single owner case and for refcounted.
Access to rb_root/list_head is always lock protected.
For refcounted the obj needs to be acquired (from the verifier pov)
meaning to have refcount =1 to be able to do spin_lock and
operate on list_head.
Martin and I came up with the following example early today like below,
assuming the map value struct contains a list_head and a spin_lock.
cpu 0 cpu 1
key = 1;
v = bpf_map_lookup(&map, &key);
key = 1;
bpf_map_delete_elem(&map, &key);
/* distruction continues and
* bpf_obj_free_fields() are
* called.
*/
/* in bpf_list_head_free():
* __bpf_spin_lock_irqsave(...)
* ...
* __bpf_spin_unlock_irqrestore();
*/
n = bpf_obj_new(...)
bpf_spin_lock(&v->lock);
bpf_list_push_front(&v->head, &v->node);
bpf_spin_lock(&v->lock);
In cpu 1 'bpf_obj_free_fields', there is a list head, so
bpf_list_head_free() is called. In bpf_list_head_free(), we do
__bpf_spin_lock_irqsave(spin_lock);
if (!head->next || list_empty(head))
goto unlock;
head = head->next;
unlock:
INIT_LIST_HEAD(orig_head);
__bpf_spin_unlock_irqrestore(spin_lock);
while (head != orig_head) {
void *obj = head;
obj -= field->graph_root.node_offset;
head = head->next;
/* The contained type can also have resources, including a
* bpf_list_head which needs to be freed.
*/
migrate_disable();
__bpf_obj_drop_impl(obj, field->graph_root.value_rec);
migrate_enable();
}
So it is possible the cpu 0 may add one element to the list_head
which will never been freed.
This happens to say list_head or rb_root too. I am aware that
this may be an existing issue for some maps, e.g. hash map.
So it may not be a big problem. Just want to mention this though.
But bpf_rb_root_free is indeed an issue for percpu, since each
percpu has its own rb root field with its own bpf_spinlock, but
for_each_cpu() {bpf_obj_free_fields();} violates access contract.
Could you explain what 'access contract' mean here? For non-percpu
case, 'x' special fields may be checked. For percpu case, it is
just 'x * nr_cpus' special fields to be checked.
percpu and rb_root creates such a maze of dependencies that
I think it's better to disallow rb_root-s and kptr-s inside percpu
for now.
I can certainly disallow rb_root and list_head. Just want to
understand what kind of issues we face specially for percpu kptr.
Current kptrs cannot be nested since the first argument of
bpf_kptr_xchg() must be a map_value. So only impactful fields
(w.r.t. bpf_obj_free_fields()) are rb_root and list_head.
If this is true, then we might have a memory leak.
But I don't whether this is possible or not.
I think local kptr has the issue as percpu kptr.
Let's tackle one at a time.
I still think Dave's patch set is a good fix for recounted,
while we need to think more about percpu case.
I agree that Dave's patch indeed fixed an existing issue.
We can resolve other issues (like above) gradually.
