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Message-ID: <49d74e2c74e0e1786b976c0b12cb1cdd680c5f58.camel@mediatek.com>
Date: Thu, 22 Aug 2024 07:01:57 +0000
From: Tze-nan Wu (吳澤南) <Tze-nan.Wu@...iatek.com>
To: "alexei.starovoitov@...il.com" <alexei.starovoitov@...il.com>
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Subject: Re: [PATCH net v4] bpf, net: Check cgroup_bpf_enabled() only once in
do_sock_getsockopt()
On Thu, 2024-08-22 at 11:16 +0800, Tze-nan Wu wrote:
> 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@...iatek.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@...iatek.com>
> > > Signed-off-by: Yanghui Li <yanghui.li@...iatek.com>
> > > Co-developed-by: Cheng-Jui Wang <cheng-jui.wang@...iatek.com>
> > > Signed-off-by: Cheng-Jui Wang <cheng-jui.wang@...iatek.com>
> > > Signed-off-by: Tze-nan Wu <Tze-nan.Wu@...iatek.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.
>
BTW, If this should be handled in kernel, modification shown below
could fix the issue without breaking the "static_branch" usage in both
macros:
+++ /include/linux/bpf-cgroup.h:
-#define BPF_CGROUP_GETSOCKOPT_MAX_OPTLEN(optlen)
+#define BPF_CGROUP_GETSOCKOPT_MAX_OPTLEN(optlen, compat)
({
int __ret = 0;
if (cgroup_bpf_enabled(CGROUP_GETSOCKOPT))
copy_from_sockptr(&__ret, optlen, sizeof(int));
+ else
+ *compat = true;
__ret;
})
#define BPF_CGROUP_RUN_PROG_GETSOCKOPT(sock, level, optname,
optval, optlen, max_optlen, retval)
({
int __ret = retval;
- if (cgroup_bpf_enabled(CGROUP_GETSOCKOPT) &&
- cgroup_bpf_sock_enabled(sock, CGROUP_GETSOCKOPT))
+ if (cgroup_bpf_sock_enabled(sock, CGROUP_GETSOCKOPT))
if (!(sock)->sk_prot->bpf_bypass_getsockopt ||
...
+++ /net/socket.c:
int do_sock_getsockopt(struct socket *sock, bool compat, int level,
{
...
...
+ /* The meaning of `compat` variable could be changed here
+ * to indicate if cgroup_bpf_enabled(CGROUP_SOCK_OPS) is
false.
+ */
if (!compat)
- max_optlen = BPF_CGROUP_GETSOCKOPT_MAX_OPTLEN(optlen);
+ max_optlen = BPF_CGROUP_GETSOCKOPT_MAX_OPTLEN(optlen,
&compat);
> Thanks,
> --tze-nan
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