[PATCH bpf-next] bpf, cgroup: Don't lose errors in effective BPF_LSM_CGROUP queries

[Shakeel Butt <shakeel.butt@linux.dev>]

An effective BPF_LSM_CGROUP query walks one attach type per LSM hook.
__cgroup_bpf_query() sets ret to -ENOSPC before the loop when prog_cnt
is too small, but the effective branch then overwrites ret with each
type's copy result:

- If the buffer runs out exactly at the end of a type, the last copy
  returns 0 and the query returns 0 instead of -ENOSPC.
- If one type's copy faults and a later one succeeds, the query returns
  0 instead of -EFAULT.

Return -EFAULT right away when a copy faults, as the non-effective
branch does, and leave ret alone otherwise.  The copy helper then no
longer needs to return -ENOSPC.  That also removes a false -ENOSPC it
gave when a dummy entry followed the last copied program.

Reproduced in a VM with two BPF_LSM_CGROUP programs on different hooks.
An effective query with prog_cnt=1 returned 0, and so did one whose
first id slot was not writable.  With this patch they fail with -ENOSPC
and -EFAULT.  Queries of other attach types already did.

Fixes: b79c9fc9551b ("bpf: implement BPF_PROG_QUERY for BPF_LSM_CGROUP")
Reported-by: Sashiko <sashiko-bot@kernel.org>
Closes: https://lore.kernel.org/bpf/sashiko-outbox-211618@kernel.org/
Signed-off-by: Shakeel Butt <shakeel.butt@linux.dev>
---
This is independent of the "bpf, cgroup: Don't wait for cgroup_mutex to
read or free cgroup bpf state" series [1], which changes nearby lines.
The two apply cleanly in either order.

[1] https://lore.kernel.org/bpf/20261112172721.440275-1-shakeel.butt@linux.dev/

 kernel/bpf/cgroup.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/kernel/bpf/cgroup.c b/kernel/bpf/cgroup.c
index 57a785241d42..2bffa9f659cd 100644
--- a/kernel/bpf/cgroup.c
+++ b/kernel/bpf/cgroup.c
@@ -424,7 +424,7 @@ static int bpf_cgroup_array_copy_to_user(struct bpf_prog_array *array,
 			return -EFAULT;
 		i++;
 	}
-	return item->prog ? -ENOSPC : 0;
+	return 0;
 }
 
 static int bpf_cgroup_array_delete_safe_at(struct bpf_prog_array *array,
@@ -1505,7 +1505,8 @@ static int __cgroup_bpf_query(struct cgroup *cgrp, const union bpf_attr *attr,
 			effective = rcu_dereference_protected(cgrp->bpf.effective[atype],
 							      lockdep_is_held(&cgroup_mutex));
 			cnt = min_t(int, bpf_cgroup_array_length(effective, atype), total_cnt);
-			ret = bpf_cgroup_array_copy_to_user(effective, prog_ids, cnt, atype);
+			if (bpf_cgroup_array_copy_to_user(effective, prog_ids, cnt, atype))
+				return -EFAULT;
 		} else {
 			struct hlist_head *progs;
 			struct bpf_prog_list *pl;

base-commit: 2e14a351662dc5a3b7421d210049c59edd0425d2
-- 
2.53.0-Meta