From: Pu Lehui 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 link update with -EPERM if new_prog's cgroup storage is incompatible with link->prog. Fixes: 0c991ebc8c69 ("bpf: Implement bpf_prog replacement for an active bpf_cgroup_link") Reviewed-by: Amery Hung Signed-off-by: Pu Lehui --- v5: - Reject link update if new_prog's storage is incompatible with link->prog. (Andrii) v4: https://lore.kernel.org/bpf/20260728133058.2858367-1-pulehui@huaweicloud.com - Extract the fix for __cgroup_bpf_attach() into a standalone patch [0]. - Add Reviewed-by tag by Amery. - Separate from patchset [1]. (Andrii) Link: https://lore.kernel.org/bpf/20260728132336.2857800-1-pulehui@huaweicloud.com [0] Link: https://lore.kernel.org/bpf/f87b53c0-8f00-45a6-82db-8242fa9b143f@huaweicloud.com [1] v3: https://lore.kernel.org/bpf/20260720134547.1289964-5-pulehui@huaweicloud.com - Include the storage and flags rollbacks to patch4 for sake of code rigor, as it's hard to make update_effective_progs fail in __cgroup_bpf_attach. v2: https://lore.kernel.org/bpf/20260717073343.958862-1-pulehui@huaweicloud.com - Fix invalid access for in-place update when storage changed. (Sashiko) v1: https://lore.kernel.org/bpf/20260714014659.401063-1-pulehui@huaweicloud.com kernel/bpf/cgroup.c | 17 +++++++++++++++++ 1 file changed, 17 insertions(+) diff --git a/kernel/bpf/cgroup.c b/kernel/bpf/cgroup.c index 57fd1d784e44..31c63ff276fa 100644 --- a/kernel/bpf/cgroup.c +++ b/kernel/bpf/cgroup.c @@ -1026,6 +1026,20 @@ 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 @@ -1064,6 +1078,9 @@ static int __cgroup_bpf_replace(struct cgroup *cgrp, if (!found) return -ENOENT; + if (!cgroup_bpf_storages_compatible(link->link.prog, new_prog)) + return -EINVAL; + cgrp->bpf.revisions[atype] += 1; old_prog = xchg(&link->link.prog, new_prog); replace_effective_prog(cgrp, atype, pl); -- 2.34.1