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

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Date:   Thu, 2 Apr 2020 10:23:13 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Luis Chamberlain <mcgrof@...nel.org>
Cc:     Vlastimil Babka <vbabka@...e.cz>,
        Iurii Zaikin <yzaikin@...gle.com>,
        linux-kernel@...r.kernel.org, linux-api@...r.kernel.org,
        linux-mm@...ck.org, Ivan Teterevkov <ivan.teterevkov@...anix.com>,
        Michal Hocko <mhocko@...nel.org>,
        David Rientjes <rientjes@...gle.com>,
        Matthew Wilcox <willy@...radead.org>,
        "Eric W . Biederman" <ebiederm@...ssion.com>,
        "Guilherme G . Piccoli" <gpiccoli@...onical.com>,
        Alexey Dobriyan <adobriyan@...il.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Christian Brauner <christian.brauner@...ntu.com>
Subject: Re: [PATCH 1/3] kernel/sysctl: support setting sysctl parameters
 from kernel command line

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?

> > >> +	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. I'm all for testing this, but I'd rather this not
be internally driven. 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.

> 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.

-- 
Kees Cook