[PATCH 6.6.y] bpf, cgroup: Fix storage null-ptr-deref after replacing prog

[Artem Dinaburg]

From: Pu Lehui <pulehui@xxxxxxxxxx>

[ Upstream commit 3f562c537e9ecf4bc5e206cfffc2cc047f1b7e94 ]

Syzkaller reported a storage null-ptr-deref issue after replacing prog.
This occurs in the following scenario:
1. prog A, an empty prog, is attached to a cgrp.
2. prog B uses BPF_MAP_TYPE_PERCPU_CGROUP_STORAGE and calls the
   bpf_get_local_storage helper.
3. link_update is called to replace prog A with prog B.

The reason is that __cgroup_bpf_replace fails to alloc and assign the
required cgrp storage for the incoming replacement prog. Consequently,
the new prog inherits an uninit storage, leading to null-ptr-deref panic
when kick the new prog.

Fix this by rejecting a link update if new_prog's cgroup storage is
incompatible with link->prog.

Fixes: 0c991ebc8c69 ("bpf: Implement bpf_prog replacement for an active bpf_cgroup_link")
Signed-off-by: Pu Lehui <pulehui@xxxxxxxxxx>
Signed-off-by: Andrii Nakryiko <andrii@xxxxxxxxxx>
Reviewed-by: Amery Hung <ameryhung@xxxxxxxxx>
Acked-by: Leon Hwang <leon.hwang@xxxxxxxxx>
Link: https://lore.kernel.org/bpf/20260728132336.2857800-1-pulehui@xxxxxxxxxxxxxxx [0]
Link: https://lore.kernel.org/bpf/f87b53c0-8f00-45a6-82db-8242fa9b143f@xxxxxxxxxxxxxxx [1]
Link: https://lore.kernel.org/bpf/20260807104403.1013064-1-pulehui@xxxxxxxxxxxxxxx
Assisted-by: LLM
Signed-off-by: Artem Dinaburg <artem@xxxxxxxxxxxxxxx>
---
Hi,

Please consider this backport of the fix for CVE-2026-90250 to 6.6.y.

The code changes are identical to upstream.

The complete fix is already present in 6.12.y, 6.18.y and 7.2.y.

Build-tested only as part of a cumulative 6.6.y series on x86_64
(allmodconfig, CONFIG_WERROR=y); no runtime testing.

Thanks,
Artem Dinaburg

 kernel/bpf/cgroup.c |   17 +++++++++++++++++
 1 file changed, 17 insertions(+)

diff --git a/kernel/bpf/cgroup.c b/kernel/bpf/cgroup.c
index fa6f818c8eabb7e3db0de27cc6d58821e91189ae..a810bc0192de8e74aedfb2351f54c118eb8be584 100644
--- a/kernel/bpf/cgroup.c
+++ b/kernel/bpf/cgroup.c
@@ -856,6 +856,20 @@ static void replace_effective_prog(struct cgroup *cgrp,
 	}
 }
 
+static bool cgroup_bpf_storages_compatible(struct bpf_prog *old_prog,
+					   struct bpf_prog *new_prog)
+{
+	enum bpf_cgroup_storage_type stype;
+
+	for_each_cgroup_storage_type(stype) {
+		if (old_prog->aux->cgroup_storage[stype] !=
+		    new_prog->aux->cgroup_storage[stype])
+			return false;
+	}
+
+	return true;
+}
+
 /**
  * __cgroup_bpf_replace() - Replace link's program and propagate the change
  *                          to descendants
@@ -894,6 +908,9 @@ static int __cgroup_bpf_replace(struct cgroup *cgrp,
 	if (!found)
 		return -ENOENT;
 
+	if (!cgroup_bpf_storages_compatible(link->link.prog, new_prog))
+		return -EINVAL;
+
 	old_prog = xchg(&link->link.prog, new_prog);
 	replace_effective_prog(cgrp, atype, pl);
 	bpf_prog_put(old_prog);