Re: [RFC PATCH 2/3] module: Don't fail module loading when setting ro_after_init section RO failed

[Petr Pavlu <petr.pavlu@suse.com>]

On 11/12/24 10:43, Daniel Gomez wrote:
> On Mon Nov 11, 2024 at 7:53 PM CET, Christophe Leroy wrote:
>>
>>
>> Le 09/11/2024 à 23:17, Daniel Gomez a écrit :
>>> On Sat Nov 9, 2024 at 11:35 AM CET, Christophe Leroy wrote:
>>>> Once module init has succeded it is too late to cancel loading.
>>>> If setting ro_after_init data section to read-only fails, all we
>>>> can do is to inform the user through a warning.
>>>>
>>>> Reported-by: Thomas Gleixner <tglx@linutronix.de>
>>>> Closes: https://protect2.fireeye.com/v1/url?k=d3deb284-b2a35ac3-d3df39cb-74fe485fff30-288375d7d91e4ad9&q=1&e=701066ca-634d-4525-a77d-1a44451f897a&u=https%3A%2F%2Feur01.safelinks.protection.outlook.com%2F%3Furl%3Dhttps%253A%252F%252Flore.kernel.org%252Fall%252F20230915082126.4187913-1-ruanjinjie%2540huawei.com%252F%26data%3D05%257C02%257Cchristophe.leroy%2540csgroup.eu%257C26b5ca7363e54210439b08dd010c4865%257C8b87af7d86474dc78df45f69a2011bb5%257C0%257C0%257C638667874457200373%257CUnknown%257CTWFpbGZsb3d8eyJFbXB0eU1hcGkiOnRydWUsIlYiOiIwLjAuMDAwMCIsIlAiOiJXaW4zMiIsIkFOIjoiTWFpbCIsIldUIjoyfQ%253D%253D%257C0%257C%257C%257C%26sdata%3DZeJ%252F3%252B2Nx%252FBf%252FWLFEkhxKlDhZk8LNkz0fs%252Fg2xMcOjY%253D%26reserved%3D0
>>>> Fixes: d1909c022173 ("module: Don't ignore errors from set_memory_XX()")
>>>> Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
>>>> ---
>>>>   kernel/module/main.c | 6 +++---
>>>>   1 file changed, 3 insertions(+), 3 deletions(-)
>>>>
>>>> diff --git a/kernel/module/main.c b/kernel/module/main.c
>>>> index 2de4ad7af335..1bf4b0db291b 100644
>>>> --- a/kernel/module/main.c
>>>> +++ b/kernel/module/main.c
>>>> @@ -2583,7 +2583,9 @@ static noinline int do_init_module(struct module *mod)
>>>>   #endif
>>>>   	ret = module_enable_rodata_ro_after_init(mod);
>>>>   	if (ret)
>>>> -		goto fail_mutex_unlock;
>>>> +		pr_warn("%s: %s() returned %d, ro_after_init data might still be writable\n",
>>>> +			mod->name, __func__, ret);
>>>> +
>>>>   	mod_tree_remove_init(mod);
>>>>   	module_arch_freeing_init(mod);
>>>>   	for_class_mod_mem_type(type, init) {
>>>> @@ -2622,8 +2624,6 @@ static noinline int do_init_module(struct module *mod)
>>>>   
>>>>   	return 0;
>>>
>>> I think it would make sense to propagate the error. But that would
>>> require changing modprobe.c. What kind of error can we expect when this
>>> happens?
>>
>> AFAIK, on powerpc it fails with EINVAL when
>> - The area is a vmalloc or module area and is a hugepage area
>> - The area is not vmalloc or io register and MMU is not powerpc radix MMU
>>
>> Otherwise it propagates the error from apply_to_existing_page_range(). 
>> IIUC it will return EINVAL when it hits a leaf PTE in upper directories.
> 
> Looking at that path I see we can also fail at __apply_to_page_range()
> -> apply_to_p4d_range() and return with -ENOMEM.
> 
> My proposal was to do something like the change below in modprobe:
> 
> diff --git a/tools/modprobe.c b/tools/modprobe.c
> index ec66e6f..8876e27 100644
> --- a/tools/modprobe.c
> +++ b/tools/modprobe.c
> @@ -572,6 +572,11 @@ static int insmod_insert(struct kmod_module *mod, int flags, const char *extra_o
>                 err = 0;
>         else {
>                 switch (err) {
> +               case -EINVAL:
> +                       ERR("module '%s'inserted: ro_after_init data might"
> +                           "still be writable (see dmesg)\n",
> +                           kmod_module_get_name(mod));
> +                       break;
>                 case -EEXIST:
>                         ERR("could not insert '%s': Module already in kernel\n",
>                             kmod_module_get_name(mod));
> 
> But I think these error codes may be also be reported in other parts
> such as simplify_symbols() so may not be a good idea after all.

It isn't really possible to make a sensible use of the return code from
init_module(), besides some basic check for -EEXIST. The problem is that
any error code from a module's init function is also propagated as
a result from the syscall.

> 
> Maybe we just need to change the default/catch all error message in
> modprobe.c and to indicate/include this case:
> 
> diff --git a/tools/modprobe.c b/tools/modprobe.c
> index ec66e6f..3647d37 100644
> --- a/tools/modprobe.c
> +++ b/tools/modprobe.c
> @@ -582,7 +582,8 @@ static int insmod_insert(struct kmod_module *mod, int flags, const char *extra_o
>                             kmod_module_get_name(mod));
>                         break;
>                 default:
> -                       ERR("could not insert '%s': %s\n", kmod_module_get_name(mod),
> +                       ERR("could not insert '%s' or inserted with error %s, "
> +                           "(see dmesg)\n", kmod_module_get_name(mod),
>                             strerror(-err));
>                         break;
>                 }
> 
> 
>>
>> On other architectures it can be different, I know some architecture try 
>> to split the pages when they hit hugepages and that can fail.
> 
> Is it worth it adding an error code for this case in case we want to
> report it back?

I feel that the proposed kernel warning about this situation is
sufficient and the loader should then return 0 to indicate that the
module got loaded. It would be more confusing to return an error but
with the module actually remaining inserted.

A module loaded without having its RO-after-init section changed
properly to RO is still fully functional. In practice, if this final
set_memory_ro() call fails, the system is already in such a state where
the additional warning is the least of the issues?

-- 
Thanks,
Petr