linux-kernel - Re: [PATCH] module: Taint the kernel when write-protecting ro_after

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Message-ID: <6e3ce71a-da5a-4d69-a5ea-4caca761d00f@csgroup.eu>
Date: Fri, 14 Mar 2025 17:48:00 +0100
From: Christophe Leroy <christophe.leroy@...roup.eu>
To: Kees Cook <kees@...nel.org>, Vlastimil Babka <vbabka@...e.cz>
Cc: Luis Chamberlain <mcgrof@...nel.org>, Petr Pavlu <petr.pavlu@...e.com>,
 Sami Tolvanen <samitolvanen@...gle.com>, Daniel Gomez
 <da.gomez@...sung.com>, Petr Mladek <pmladek@...e.com>,
 Jani Nikula <jani.nikula@...el.com>,
 Andrew Morton <akpm@...ux-foundation.org>,
 John Ogness <john.ogness@...utronix.de>,
 Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
 linux-mm <linux-mm@...ck.org>, linux-modules@...r.kernel.org,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH] module: Taint the kernel when write-protecting
 ro_after_init fails



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@...nel.org>
>>>> Signed-off-by: Petr Pavlu <petr.pavlu@...e.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@...nel.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();


Christophe