Re: [PATCH 1/3] kernel/sysctl: support setting sysctl parameters from kernel command line

Luis Chamberlain
2020-04-02T20:59:37Z

On Thu, Apr 02, 2020 at 10:23:13AM -0700, Kees Cook wrote:
> On Thu, Apr 02, 2020 at 04:04:42PM +0000, Luis Chamberlain wrote:
> > On Wed, Apr 01, 2020 at 01:01:47PM +0200, Vlastimil Babka wrote:
> > > On 3/31/20 12:44 AM, Luis Chamberlain wrote:
> > > >> + } else if (wret != len) {
> > > >> + pr_err("Wrote only %ld bytes of %d writing to proc file %s to set sysctl parameter '%s=%s'",
> > > >> + wret, len, path, param, val);
> > > >> + }
> > > >> +
> > > >> + err = filp_close(file, NULL);
> > > >> + if (err)
> > > >> + pr_err("Error %pe closing proc file to set sysctl parameter '%s=%s'",
> > > >> + ERR_PTR(err), param, val);
> > > >> +out:
> > > >> + kfree(path);
> > > >> + return 0;
> > > >> +}
> > > >> +
> > > >> +void do_sysctl_args(void)
> > > >> +{
> > > >> + char *command_line;
> > > >> + struct vfsmount *proc_mnt = NULL;
> > > >> +
> > > >> + command_line = kstrdup(saved_command_line, GFP_KERNEL);
> > > >
> > > > can you use kstrndup() ? And then use kfree_const()? Yes, feel free to
> > >
> > > I don't follow, what am I missing? Do you mean this?
> > >
> > > size_t len = strlen(saved_command_line);
> > > command_line = kstrndup(saved_command_line, len, GFP_KERNEL);
> > >
> > > What would be the advantage over plain kstrdup()?
> > > As for kfree_const(), when would command_line be .rodata? I don't see using
> > > kstrndup() resulting in that.
> >
> > The const nature of using kstrdup() comes with using const for your
> > purpose. ie:
> >
> > const char *const_command_line = saved_command_line;
> >
> > The point of a kstrncpy() then is to ensure force a const throughout
> > your use if you know you don't need modifications.
>
> I'm not following this suggestion. It _is_ modifying it. That's why it's
> making a copy. What am I missing?

We modify the copied bootparams to allow new sysctls to map to old boot params?

If so, then yes, this cannot be used.

> > > >> + parse_args("Setting sysctl args", command_line,
> > > >> + NULL, 0, -1, -1, &proc_mnt, process_sysctl_arg);
> > > >> +
> > > >> + if (proc_mnt)
> > > >> + kern_unmount(proc_mnt);
> > > >> +
> > > >> + kfree(command_line);
> > > >> +}
> > > >
> > > > Then, can we get this tested as part of lib/test_sysctl.c with its
> > > > respective tools/testing/selftests/sysctl/sysctl.sh ?
> > >
> > > Hmm so I add some sysctl to the test "module" (in fact the 'config' file says it
> > > should be build with 'y', which would be needed anyway) and expand the test
> > > instructions so that the test kernel boot has to include it on the command line,
> > > and then I verify it has been set? Or do you see a better way?
> >
> > We don't necessarily have a way to test the use boot params today.
> > That reveals an are which we should eventually put some focus on
> > in the future. In the meantime we have to deal with what we have.
> >
> > So let's think about this:
> >
> > You are adding a new cmdline sysctl boot param, and also a wrapper
> > for those old boot bootparams to also work using both new sysctl
> > path and old path. Testing just these both should suffice.
> >
> > How about this:
> >
> > For testing the new feature you are adding, can you extend the default
> > boot params *always* if a new CONFIG_TEST_SYSCTL_CMDLINE is set? Then
> > upon boot we can verify the proc handlers for these new boot params got
> > kicked, and likewise some other proc handlers which also can be used
> > from the cmdline are *not* set. For this later set, we already have
> > a series of test syctls you can use. In fact, you can use the existing
> > syctls for both cases already I believe, its just a matter of adding
> > this new CONFIG_TEST_SYSCTL_CMDLINE which would extend the cmdline,
> > and these tests would take place *first* on the script.
>
> This seems... messy.

It is all we have.

> I'm all for testing this,

OK so we do want to test it.

> but I'd rather this not be internally driven.

This is the least cumbersome solution I could think of. Other things
would require things like using qemu, etc. That seems much more messsy.

> This is an external interface (boot params), so
> I'd rather an external driver handle that testing. We don't have a
> common method to do that with the kernel, though.

Right... which begs the question now -- how do we test this sort of
stuff? The above would at least get us coverage while we iron something
more generic out for boot params.

> > That would test both cases with one kernel.
> >
> > You could then also add a bogus new sysctl which also expands to a silly
> > raw boot param to test the wrapper you are providing. That would be the
> > only new test syctl you would need to add.
>
> Sure, that seems reasonable. Supporting externally driven testing makes
> sense for this.

But again, what exactly?

Luis