Re: [PATCH] ipc: Limit sysctl value to IPCMNI

From: Andrew Morton
Date: Mon Jun 11 2018 - 19:18:52 EST


On Sat, 09 Jun 2018 08:48:48 +0200 Takashi Iwai <tiwai@xxxxxxx> wrote:

> On Fri, 08 Jun 2018 23:16:59 +0200,
> Andrew Morton wrote:
> >
> > On Fri, 8 Jun 2018 15:49:49 +0200 Takashi Iwai <tiwai@xxxxxxx> wrote:
> >
> > > Currently shmmni proc entry accepts all entered integer values, but
> > > the practical limit is IPCMNI (32768). This confuses user as if a
> > > bigger value were accepted but not applied correctly.
> > >
> > > This patch changes the proc entry to use *_minmax variant to limit the
> > > accepted values accordingly.
> >
> > Waiman Long was working on a (vastly more complicated) patchset to
> > address this.
>
> That's great. Any patch available for testing?

I think
http://lkml.kernel.org/r/1520885744-1546-1-git-send-email-longman@xxxxxxxxxx
is the most recent version.

>
> > > --- a/ipc/ipc_sysctl.c
> > > +++ b/ipc/ipc_sysctl.c
> > > @@ -99,6 +99,7 @@ static int proc_ipc_auto_msgmni(struct ctl_table *table, int write,
> > > static int zero;
> > > static int one = 1;
> > > static int int_max = INT_MAX;
> > > +static int ipcmni = IPCMNI;
> > >
> > > static struct ctl_table ipc_kern_table[] = {
> > > {
> > > @@ -120,7 +121,9 @@ static struct ctl_table ipc_kern_table[] = {
> > > .data = &init_ipc_ns.shm_ctlmni,
> > > .maxlen = sizeof(init_ipc_ns.shm_ctlmni),
> > > .mode = 0644,
> > > - .proc_handler = proc_ipc_dointvec,
> > > + .proc_handler = proc_ipc_dointvec_minmax,
> > > + .extra1 = &zero,
> > > + .extra2 = &ipcmni,
> > > },
> > > {
> > > .procname = "shm_rmid_forced",
> >
> > What is the back-compatibility situation here?
>
> It's obviously an error to set such a high value and suppose that it
> were accepted. So relying on that behavior must be broken in
> anyway...

Well the present behaviour is to convert higher values downwards, yes?

int ipc_addid(struct ipc_ids *ids, struct kern_ipc_perm *new, int limit)
{
kuid_t euid;
kgid_t egid;
int id, err;

if (limit > IPCMNI)
limit = IPCMNI;

So if someone out there is presently setting this to 999999 then their
kernel will work just fine. After your proposed change, it will no
longer do so - the tuning attempt will fail with -EINVAL.

It really does us no good to say "you shouldn't have been doing that".
The fact that they *are* doing it and that it works OK is the kernel
developers' fault for not applying suitable checking on day one. I
think we're stuck with continuing to accept such input.