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

["Tze-nan Wu (吳澤南)" <Tze-nan.Wu@mediatek.com>]

On Wed, 2024-08-21 at 14:01 -0700, Alexei Starovoitov wrote:
>  	 
> External email : Please do not click links or open attachments until
> you have verified the sender or the content.
>  On Wed, Aug 21, 2024 at 2:30 AM Tze-nan Wu <Tze-nan.Wu@mediatek.com>
> 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)
> 
> 
> I suspect the compiler generates slow code after such a patch.
> pw-bot: cr
> 
> What is the problem with double cgroup_bpf_enabled() check?
> yes it might return two different values, so?

Depending on where the -EFAULT occurs, the problem could be different.
In our case, the -EFAULT is returned from getsockopt() during a
"bootup-critical property setting" flow in Android. As a result, the
property setting fails due it get -EFAULT from getsockopt(), causing
the device to fail the boot process.

Should the userspace caller always anticipate an -EFAULT from
getsockopt() if there's another process enables CGROUP_GETSOCKOPT
(possibly through the bpf() syscall) at the same time?
If that's the case, then I will handle this in userspace.

Thanks,
--tze-nan