[PATCH 1/2] bpf: cgroup: fix sysctl new value replacement

[Dawei Feng]

Commit 4e63acdff864 ("bpf: Introduce bpf_sysctl_{get,set}_new_value
helpers") changed the success return value to 0, but failed to update the
corresponding check in __cgroup_bpf_run_filter_sysctl(). Since
bpf_prog_run_array_cg() now returns 0 on success, the legacy ret == 1
condition is never satisfied. As a result, the modified value is ignored,
and bpf_sysctl_set_new_value() fails to replace the write buffer.

Fix this by checking for a return value of 0 instead, so cgroup/sysctl
programs can correctly replace the pending sysctl buffer.

This bug was discovered during a manual code review. Tested via a
cgroup/sysctl BPF reproducer overriding writes to a target sysctl.
Pre-fix, bpf_sysctl_set_new_value("foo") was silently ignored: the write
returned 8192 and the value remained "600". Post-fix, the BPF replacement
buffer properly propagates: the write returns 3 and the value updates to
"foo".

Fixes: 4e63acdff864 ("bpf: Introduce bpf_sysctl_{get,set}_new_value helpers")
Cc: stable@xxxxxxxxxxxxxxx

Signed-off-by: Zilin Guan <zilin@xxxxxxxxxx>
Signed-off-by: Dawei Feng <dawei.feng@xxxxxxxxxx>
---
 kernel/bpf/cgroup.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/bpf/cgroup.c b/kernel/bpf/cgroup.c
index 876f6a81a9b6..8715a014c21d 100644
--- a/kernel/bpf/cgroup.c
+++ b/kernel/bpf/cgroup.c
@@ -1935,7 +1935,7 @@ int __cgroup_bpf_run_filter_sysctl(struct ctl_table_header *head,
 
 	kfree(ctx.cur_val);
 
-	if (ret == 1 && ctx.new_updated) {
+	if (!ret && ctx.new_updated) {
 		kfree(*buf);
 		*buf = ctx.new_val;
 		*pcount = ctx.new_len;
-- 
2.34.1