Re: [PATCH v3] net/socket: Check cgroup_bpf_enabled() only once in do_sock_getsockopt()

[Martin KaFai Lau <martin.lau@linux.dev>]

On 8/20/24 2:29 AM, Tze-nan Wu wrote:
> The return value from `cgroup_bpf_enabled(CGROUP_GETSOCKOPT)` can change
> between the invocations of `BPF_CGROUP_GETSOCKOPT_MAX_OPTLEN` and
> `BPF_CGROUP_RUN_PROG_GETSOCKOPT`.
> 
> If `cgroup_bpf_enabled(CGROUP_GETSOCKOPT)` changes from "false" to
> "true" between the invocations of `BPF_CGROUP_GETSOCKOPT_MAX_OPTLEN` and
> `BPF_CGROUP_RUN_PROG_GETSOCKOPT`, `BPF_CGROUP_RUN_PROG_GETSOCKOPT` will
> receive an -EFAULT from `__cgroup_bpf_run_filter_getsockopt(max_optlen=0)`
> due to `get_user()` was not reached in `BPF_CGROUP_GETSOCKOPT_MAX_OPTLEN`.
> 
> Scenario shown as below:
> 
>             `process A`                      `process B`
>             -----------                      ------------
>    BPF_CGROUP_GETSOCKOPT_MAX_OPTLEN
>                                              enable CGROUP_GETSOCKOPT
>    BPF_CGROUP_RUN_PROG_GETSOCKOPT (-EFAULT)
> 
> To prevent this, invoke `cgroup_bpf_enabled()` only once and cache the
> result in a newly added local variable `enabled`.
> Both `BPF_CGROUP_*` macros in `do_sock_getsockopt` will then check their
> condition using the same `enabled` variable as the condition variable,
> instead of using the return values from `cgroup_bpf_enabled` called by
> themselves as the condition variable(which could yield different results).
> This ensures that either both `BPF_CGROUP_*` macros pass the condition
> or neither does.
> 
> Co-developed-by: Yanghui Li <yanghui.li@mediatek.com>
> Signed-off-by: Yanghui Li <yanghui.li@mediatek.com>
> Co-developed-by: Cheng-Jui Wang <cheng-jui.wang@mediatek.com>
> Signed-off-by: Cheng-Jui Wang <cheng-jui.wang@mediatek.com>
> Signed-off-by: Tze-nan Wu <Tze-nan.Wu@mediatek.com>

Please tag bpf in the subject and add a Fixes tag.

[cc: Stanislav]

pw-bot: cr