linux-kernel - Re: [PATCH v4 1/1] exec: seal system mappings

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Message-ID: <CABi2SkX3ES24y=18_kjmi+gzJbmaDsqepMT5jrogB7K5GTnUSA@mail.gmail.com>
Date: Mon, 2 Dec 2024 11:57:58 -0800
From: Jeff Xu <jeffxu@...omium.org>
To: Matthew Wilcox <willy@...radead.org>
Cc: akpm@...ux-foundation.org, keescook@...omium.org, jannh@...gle.com, 
	torvalds@...ux-foundation.org, adhemerval.zanella@...aro.org, oleg@...hat.com, 
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	ojeda@...nel.org, adobriyan@...il.com, anna-maria@...utronix.de, 
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	groeck@...omium.org, mpe@...erman.id.au, Vlastimil Babka <vbabka@...e.cz>, 
	Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
Subject: Re: [PATCH v4 1/1] exec: seal system mappings

On Mon, Dec 2, 2024 at 9:22 AM Jeff Xu <jeffxu@...omium.org> wrote:
>
> On Mon, Nov 25, 2024 at 12:40 PM Matthew Wilcox <willy@...radead.org> wrote:
> >
> > On Mon, Nov 25, 2024 at 08:20:21PM +0000, jeffxu@...omium.org wrote:
> > > +/*
> > > + * Kernel cmdline override for CONFIG_SEAL_SYSTEM_MAPPINGS
> > > + */
> > > +enum seal_system_mappings_type {
> > > +     SEAL_SYSTEM_MAPPINGS_DISABLED,
> > > +     SEAL_SYSTEM_MAPPINGS_ENABLED
> > > +};
> > > +
> > > +static enum seal_system_mappings_type seal_system_mappings_v __ro_after_init =
> > > +     IS_ENABLED(CONFIG_SEAL_SYSTEM_MAPPINGS) ? SEAL_SYSTEM_MAPPINGS_ENABLED :
> > > +     SEAL_SYSTEM_MAPPINGS_DISABLED;
> > > +
> > > +static const struct constant_table value_table_sys_mapping[] __initconst = {
> > > +     { "no", SEAL_SYSTEM_MAPPINGS_DISABLED},
> > > +     { "yes", SEAL_SYSTEM_MAPPINGS_ENABLED},
> > > +     { }
> > > +};
> > > +
> > > +static int __init early_seal_system_mappings_override(char *buf)
> > > +{
> > > +     if (!buf)
> > > +             return -EINVAL;
> > > +
> > > +     seal_system_mappings_v = lookup_constant(value_table_sys_mapping,
> > > +                     buf, seal_system_mappings_v);
> > > +     return 0;
> > > +}
> > > +
> > > +early_param("exec.seal_system_mappings", early_seal_system_mappings_override);
> >
> > Are you paid by the line?
> > This all seems ridiculously overcomplicated.
> > Look at (first example I found) kgdbwait:
> >
> The example you provided doesn't seem to support the kernel cmd-line ?
>
> > static int __init opt_kgdb_wait(char *str)
> > {
> >         kgdb_break_asap = 1;
> >
> >         kdb_init(KDB_INIT_EARLY);
> >         if (kgdb_io_module_registered &&
> >             IS_ENABLED(CONFIG_ARCH_HAS_EARLY_DEBUG))
> >                 kgdb_initial_breakpoint();
> >
> >         return 0;
> > }
> > early_param("kgdbwait", opt_kgdb_wait);
> >
> There is an existing pattern of supporting kernel cmd line + KCONFIG
> which I followed [1],
> IMO, this fits this user-case really well, if you have a better
> example, I'm happy to look.
>
> [1] https://lore.kernel.org/lkml/20240802080225.89408-1-adrian.ratiu@collabora.com/
>
Sorry, I miss-understood the code. This code also uses the kernel cmd
line, it is just not using keyword=yes/no pattern, but checking the
existence of "keyword" in the kernel cmd line.

Current pattern allows values beyond "yes"/"no", so if we ever need
extension (e.g. a new system mapping type, or pre-process control), we
have flexibility to do so.

On second thought, that might be over-thinking, I will switch to this
(simpler) pattern in the next version.

Thanks

> > I don't understand why you've created a new 'exec' namespace, and why
> > this feature fits in 'exec'.  That seems like an implementation detail.
> > I'd lose the "exec." prefix.
>
> I would prefer some prefix to group these types of features.
> vdso/vvar are sealed during the execve() call, so I choose "exec".
> The next work I'm planning is sealing the NX stack, it would start
> with the same prefix.
>
>  If exec is not an intuitive prefix, I'm also happy with "process." prefix.
>
> Thanks for reviewing
>
> -Jeff