Re: [PATCH net v4] bpf, net: Check cgroup_bpf_enabled() only once in do_sock_getsockopt()

[Yonghong Song <yonghong.song@linux.dev>]

On 8/21/24 2:30 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.
>
> Fixes: 0d01da6afc54 ("bpf: implement getsockopt and setsockopt hooks")
> 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>
> ---
>
> Chagnes from v1 to v2: https://lore.kernel.org/all/20240819082513.27176-1-Tze-nan.Wu@mediatek.com/
>    Instead of using cgroup_lock in the fastpath, invoke cgroup_bpf_enabled
>    only once and cache the value in the newly added variable `enabled`.
>    `BPF_CGROUP_*` macros in do_sock_getsockopt can then both check their
>    condition with the new variable `enable`, ensuring that either they both
>    passing the condition or both do not.
>
> Chagnes from v2 to v3: https://lore.kernel.org/all/20240819155627.1367-1-Tze-nan.Wu@mediatek.com/
>    Hide cgroup_bpf_enabled in the macro, and some modifications to adapt
>    the coding style.
>
> Chagnes from v3 to v4: https://lore.kernel.org/all/20240820092942.16654-1-Tze-nan.Wu@mediatek.com/
>    Add bpf tag to subject, and Fixes tag in body.
>
> ---
>   include/linux/bpf-cgroup.h | 15 ++++++++-------
>   net/socket.c               |  5 +++--
>   2 files changed, 11 insertions(+), 9 deletions(-)
>
> diff --git a/include/linux/bpf-cgroup.h b/include/linux/bpf-cgroup.h
> index fb3c3e7181e6..5afa2ac76aae 100644
> --- a/include/linux/bpf-cgroup.h
> +++ b/include/linux/bpf-cgroup.h
> @@ -390,20 +390,20 @@ static inline bool cgroup_bpf_sock_enabled(struct sock *sk,
>   	__ret;								       \
>   })
>   
> -#define BPF_CGROUP_GETSOCKOPT_MAX_OPTLEN(optlen)			       \
> +#define BPF_CGROUP_GETSOCKOPT_MAX_OPTLEN(optlen, enabled)		       \
>   ({									       \
>   	int __ret = 0;							       \
> -	if (cgroup_bpf_enabled(CGROUP_GETSOCKOPT))			       \
> +	enabled = cgroup_bpf_enabled(CGROUP_GETSOCKOPT);		       \
> +	if (enabled)							       \
>   		copy_from_sockptr(&__ret, optlen, sizeof(int));		       \
>   	__ret;								       \
>   })
>   
>   #define BPF_CGROUP_RUN_PROG_GETSOCKOPT(sock, level, optname, optval, optlen,   \
> -				       max_optlen, retval)		       \
> +				       max_optlen, retval, enabled)	       \
>   ({									       \
>   	int __ret = retval;						       \
> -	if (cgroup_bpf_enabled(CGROUP_GETSOCKOPT) &&			       \
> -	    cgroup_bpf_sock_enabled(sock, CGROUP_GETSOCKOPT))		       \
> +	if (enabled && cgroup_bpf_sock_enabled(sock, CGROUP_GETSOCKOPT))       \
>   		if (!(sock)->sk_prot->bpf_bypass_getsockopt ||		       \
>   		    !INDIRECT_CALL_INET_1((sock)->sk_prot->bpf_bypass_getsockopt, \
>   					tcp_bpf_bypass_getsockopt,	       \
> @@ -518,9 +518,10 @@ static inline int bpf_percpu_cgroup_storage_update(struct bpf_map *map,
>   #define BPF_CGROUP_RUN_PROG_SOCK_OPS(sock_ops) ({ 0; })
>   #define BPF_CGROUP_RUN_PROG_DEVICE_CGROUP(atype, major, minor, access) ({ 0; })
>   #define BPF_CGROUP_RUN_PROG_SYSCTL(head,table,write,buf,count,pos) ({ 0; })
> -#define BPF_CGROUP_GETSOCKOPT_MAX_OPTLEN(optlen) ({ 0; })
> +#define BPF_CGROUP_GETSOCKOPT_MAX_OPTLEN(optlen, enabled) ({ 0; })
>   #define BPF_CGROUP_RUN_PROG_GETSOCKOPT(sock, level, optname, optval, \
> -				       optlen, max_optlen, retval) ({ retval; })
> +				       optlen, max_optlen, retval, \
> +				       enabled) ({ retval; })
>   #define BPF_CGROUP_RUN_PROG_GETSOCKOPT_KERN(sock, level, optname, optval, \
>   					    optlen, retval) ({ retval; })
>   #define BPF_CGROUP_RUN_PROG_SETSOCKOPT(sock, level, optname, optval, optlen, \
> diff --git a/net/socket.c b/net/socket.c
> index fcbdd5bc47ac..0b465dc8a789 100644
> --- a/net/socket.c
> +++ b/net/socket.c
> @@ -2363,6 +2363,7 @@ int do_sock_getsockopt(struct socket *sock, bool compat, int level,
>   		       int optname, sockptr_t optval, sockptr_t optlen)
>   {
>   	int max_optlen __maybe_unused;
> +	bool enabled __maybe_unused;
>   	const struct proto_ops *ops;
>   	int err;
>   
> @@ -2371,7 +2372,7 @@ int do_sock_getsockopt(struct socket *sock, bool compat, int level,
>   		return err;
>   
>   	if (!compat)
> -		max_optlen = BPF_CGROUP_GETSOCKOPT_MAX_OPTLEN(optlen);
> +		max_optlen = BPF_CGROUP_GETSOCKOPT_MAX_OPTLEN(optlen, enabled);

Here, 'enabled' is actually assigned with a value in the macro. I am not sure
whether this is a common practice or not. At least from macro, it is not clear
about this.

Maybe we can do
	max_optlen = BPF_CGROUP_GETSOCKOPT_MAX_OPTLEN(optlen, &enabled);

The &enabled signals that its value could change. And indeed
the macro will store the proper value to &enabled properly.

Just my 2 cents.

>   
>   	ops = READ_ONCE(sock->ops);
>   	if (level == SOL_SOCKET) {
> @@ -2390,7 +2391,7 @@ int do_sock_getsockopt(struct socket *sock, bool compat, int level,
>   	if (!compat)
>   		err = BPF_CGROUP_RUN_PROG_GETSOCKOPT(sock->sk, level, optname,
>   						     optval, optlen, max_optlen,
> -						     err);
> +						     err, enabled);
>   
>   	return err;
>   }