Re: [PATCH bpf v2] bpf: Fix invalid mem access when update_effective_progs fails in __cgroup_bpf_detach

[Pu Lehui <pulehui@huawei.com>]

CI report the following warning, and I've post new version [0]. Sorry 
for the noise.

warning: symbol '__pcpu_scope___dummy_stats' was not declared. Should it 
be static?

Link: 
https://lore.kernel.org/bpf/20251110092536.4082324-1-pulehui@huaweicloud.com/ 
[0]

On 2025/11/10 15:17, Pu Lehui wrote:
> From: Pu Lehui <pulehui@huawei.com>
> 
> Syzkaller triggers an invalid memory access issue following fault
> injection in update_effective_progs. The issue can be described as
> follows:
> 
> __cgroup_bpf_detach
>    update_effective_progs
>      compute_effective_progs
>        bpf_prog_array_alloc <-- fault inject
>    purge_effective_progs
>      /* change to dummy_bpf_prog */
>      array->items[index] = &dummy_bpf_prog.prog
> 
> ---softirq start---
> __do_softirq
>    ...
>      __cgroup_bpf_run_filter_skb
>        __bpf_prog_run_save_cb
>          bpf_prog_run
>            stats = this_cpu_ptr(prog->stats)
>            /* invalid memory access */
>            flags = u64_stats_update_begin_irqsave(&stats->syncp)
> ---softirq end---
> 
>    static_branch_dec(&cgroup_bpf_enabled_key[atype])
> 
> The reason is that fault injection caused update_effective_progs to fail
> and then changed the original prog into dummy_bpf_prog.prog in
> purge_effective_progs. Then a softirq came, and accessing the stats of
> dummy_bpf_prog.prog in the softirq triggers invalid mem access.
> 
> To fix it, we can use static per-cpu variables to initialize the stats
> of dummy_bpf_prog.prog.
> 
> Fixes: 4c46091ee985 ("bpf: Fix KASAN use-after-free Read in compute_effective_progs")
> Signed-off-by: Pu Lehui <pulehui@huawei.com>
> ---
> v2:
> - Use static per-cpu variables to initialize the stats of
>    dummy_bpf_prog.prog suggested by Eduard.
> 
> v1: https://lore.kernel.org/all/20251105100302.2968475-1-pulehui@huaweicloud.com
> 
>   kernel/bpf/core.c | 3 +++
>   1 file changed, 3 insertions(+)
> 
> diff --git a/kernel/bpf/core.c b/kernel/bpf/core.c
> index d595fe512498..c7c9c78f171a 100644
> --- a/kernel/bpf/core.c
> +++ b/kernel/bpf/core.c
> @@ -2536,11 +2536,14 @@ static unsigned int __bpf_prog_ret1(const void *ctx,
>   	return 1;
>   }
>   
> +DEFINE_PER_CPU(struct bpf_prog_stats, __dummy_stats);
> +
>   static struct bpf_prog_dummy {
>   	struct bpf_prog prog;
>   } dummy_bpf_prog = {
>   	.prog = {
>   		.bpf_func = __bpf_prog_ret1,
> +		.stats = &__dummy_stats,
>   	},
>   };
>