Re: [PATCH] module: Taint the kernel when write-protecting ro_after_init fails

[Kees Cook <kees@kernel.org>]

On Fri, Mar 14, 2025 at 05:48:00PM +0100, Christophe Leroy wrote:
> 
> 
> Le 12/03/2025 à 17:30, Kees Cook a écrit :
> > On Wed, Mar 12, 2025 at 04:45:24PM +0100, Vlastimil Babka wrote:
> > > On 3/6/25 17:57, Luis Chamberlain wrote:
> > > > + linux-mm since we're adding TAINT_BAD_PAGE
> > > > 
> > > > On Thu, Mar 06, 2025 at 11:36:55AM +0100, Petr Pavlu wrote:
> > > > > In the unlikely case that setting ro_after_init data to read-only fails, it
> > > > > is too late to cancel loading of the module. The loader then issues only
> > > > > a warning about the situation. Given that this reduces the kernel's
> > > > > protection, it was suggested to make the failure more visible by tainting
> > > > > the kernel.
> > > > > 
> > > > > Allow TAINT_BAD_PAGE to be set per-module and use it in this case. The flag
> > > > > is set in similar situations and has the following description in
> > > > > Documentation/admin-guide/tainted-kernels.rst: "bad page referenced or some
> > > > > unexpected page flags".
> > > > > 
> > > > > Adjust the warning that reports the failure to avoid references to internal
> > > > > functions and to add information about the kernel being tainted, both to
> > > > > match the style of other messages in the file. Additionally, merge the
> > > > > message on a single line because checkpatch.pl recommends that for the
> > > > > ability to grep for the string.
> > > > > 
> > > > > Suggested-by: Kees Cook <kees@kernel.org>
> > > > > Signed-off-by: Petr Pavlu <petr.pavlu@suse.com>
> > > > > ---
> > > > > I opted to use TAINT_BAD_PAGE for now because it seemed unnecessary to me
> > > > > to introduce a new flag only for this specific case. However, if we end up
> > > > > similarly checking set_memory_*() in the boot context, a separate flag
> > > > > would be probably better.
> > > > > ---
> > > > >   kernel/module/main.c | 7 ++++---
> > > > >   kernel/panic.c       | 2 +-
> > > > >   2 files changed, 5 insertions(+), 4 deletions(-)
> > > > > 
> > > > > diff --git a/kernel/module/main.c b/kernel/module/main.c
> > > > > index 1fb9ad289a6f..8f424a107b92 100644
> > > > > --- a/kernel/module/main.c
> > > > > +++ b/kernel/module/main.c
> > > > > @@ -3030,10 +3030,11 @@ static noinline int do_init_module(struct module *mod)
> > > > >   	rcu_assign_pointer(mod->kallsyms, &mod->core_kallsyms);
> > > > >   #endif
> > > > >   	ret = module_enable_rodata_ro_after_init(mod);
> > > > > -	if (ret)
> > > > > -		pr_warn("%s: module_enable_rodata_ro_after_init() returned %d, "
> > > > > -			"ro_after_init data might still be writable\n",
> > > > > +	if (ret) {
> > > > > +		pr_warn("%s: write-protecting ro_after_init data failed with %d, the data might still be writable - tainting kernel\n",
> > > > >   			mod->name, ret);
> > > > > +		add_taint_module(mod, TAINT_BAD_PAGE, LOCKDEP_STILL_OK);
> > > > > +	}
> > > > >   	mod_tree_remove_init(mod);
> > > > >   	module_arch_freeing_init(mod);
> > > > > diff --git a/kernel/panic.c b/kernel/panic.c
> > > > > index d8635d5cecb2..794c443bfb5c 100644
> > > > > --- a/kernel/panic.c
> > > > > +++ b/kernel/panic.c
> > > > > @@ -497,7 +497,7 @@ const struct taint_flag taint_flags[TAINT_FLAGS_COUNT] = {
> > > > >   	TAINT_FLAG(CPU_OUT_OF_SPEC,		'S', ' ', false),
> > > > >   	TAINT_FLAG(FORCED_RMMOD,		'R', ' ', false),
> > > > >   	TAINT_FLAG(MACHINE_CHECK,		'M', ' ', false),
> > > > > -	TAINT_FLAG(BAD_PAGE,			'B', ' ', false),
> > > > > +	TAINT_FLAG(BAD_PAGE,			'B', ' ', true),
> > > > >   	TAINT_FLAG(USER,			'U', ' ', false),
> > > > >   	TAINT_FLAG(DIE,				'D', ' ', false),
> > > > >   	TAINT_FLAG(OVERRIDDEN_ACPI_TABLE,	'A', ' ', false),
> > > > 
> > > > Reviewed-by: Luis Chamberlain <mcgrof@kernel.org>
> > > > 
> > > > For our needs this makes sense, however I am curious if TAINT_BAD_PAGE
> > > > is too broadly generic, and also if we're going to add it, if there are
> > > > other mm uses for such a thing.
> > > 
> > > I'm not sure BAD_PAGE is a good fit. If there was a new flag that meant "a
> > > hardening measure failed", would that have other possible uses? The
> > > semantics would be that the kernel self-protection was weakened wrt
> > > expectations, even if not yet a corruption due to attack would be detected.
> > > Some admins could opt-in to panic in such case anyway, etc. Any other
> > > hardening features where such "failure to harden" is possible and could use
> > > this too? Kees?
> > 
> > Yeah, it could certainly be used. The direction the hardening stuff has
> > taken is to use WARN() (as Linus requires no direct BUG() usage), and to
> > recommend that end users tune their warn_limit sysctl as needed.
> > 
> > Being able to TAINT might be useful, but I don't have any places that
> > immediately come to mind that seem appropriate for it (besides this
> > case). Hm, well, maybe in the case of a W^X test failure? (I note that
> > this is also a "safe memory permission" failure...)
> 
> Can be anything that fails in function mark_readonly() ? :
> 
> 		jump_label_init_ro();
> 		mark_rodata_ro();
> 		debug_checkwx();
> 		rodata_test();

Yeah, works for me!

-- 
Kees Cook