[PATCH v15 05/11] LSM: Create lsm_list_modules system call

[Casey Schaufler]

On 3/12/2024 2:50 PM, Kees Cook wrote:
> On Tue, Mar 12, 2024 at 08:28:20PM +0200, Dmitry V. Levin wrote:
>> On Tue, Mar 12, 2024 at 10:44:38AM -0700, Casey Schaufler wrote:
>>> On 3/12/2024 10:06 AM, Paul Moore wrote:
>>>> On Tue, Mar 12, 2024 at 11:27 AM Casey Schaufler <casey at schaufler-ca.com> wrote:
>>>>> On 3/12/2024 6:25 AM, Paul Moore wrote:
>>>>>> On Tue, Mar 12, 2024 at 6:16 AM Dmitry V. Levin <ldv at strace.io> wrote:
>>>>>>> On Tue, Sep 12, 2023 at 01:56:50PM -0700, Casey Schaufler wrote:
>>>>>>> [...]
>>>>>>>> --- a/security/lsm_syscalls.c
>>>>>>>> +++ b/security/lsm_syscalls.c
>>>>>>>> @@ -55,3 +55,42 @@ SYSCALL_DEFINE4(lsm_get_self_attr, unsigned int, attr, struct lsm_ctx __user *,
>>>>>>>>  {
>>>>>>>>       return security_getselfattr(attr, ctx, size, flags);
>>>>>>>>  }
>>>>>>>> +
>>>>>>>> +/**
>>>>>>>> + * sys_lsm_list_modules - Return a list of the active security modules
>>>>>>>> + * @ids: the LSM module ids
>>>>>>>> + * @size: pointer to size of @ids, updated on return
>>>>>>>> + * @flags: reserved for future use, must be zero
>>>>>>>> + *
>>>>>>>> + * Returns a list of the active LSM ids. On success this function
>>>>>>>> + * returns the number of @ids array elements. This value may be zero
>>>>>>>> + * if there are no LSMs active. If @size is insufficient to contain
>>>>>>>> + * the return data -E2BIG is returned and @size is set to the minimum
>>>>>>>> + * required size. In all other cases a negative value indicating the
>>>>>>>> + * error is returned.
>>>>>>>> + */
>>>>>>>> +SYSCALL_DEFINE3(lsm_list_modules, u64 __user *, ids, size_t __user *, size,
>>>>>>>> +             u32, flags)
>>>>>>> I'm sorry but the size of userspace size_t is different from the kernel one
>>>>>>> on 32-bit compat architectures.
>>>>>> D'oh, yes, thanks for pointing that out.  It would have been nice to
>>>>>> have caught that before v6.8 was released, but I guess it's better
>>>>>> than later.
>>>>>>
>>>>>>> Looks like there has to be a COMPAT_SYSCALL_DEFINE3(lsm_list_modules, ..)
>>>>>>> now.  Other two added lsm syscalls also have this issue.
>>>>>> Considering that Linux v6.8, and by extension these syscalls, are only
>>>>>> a few days old, I think I'd rather see us just modify the syscalls and
>>>>>> avoid the compat baggage.  I'm going to be shocked if anyone has
>>>>>> shifted to using the new syscalls yet, and even if they have (!!),
>>>>>> moving from a "size_t" type to a "u64" should be mostly transparent
>>>>>> for the majority of native 64-bit systems.  Those running the absolute
>>>>>> latest kernels on 32-bit systems with custom or bleeding edge
>>>>>> userspace *may* see a slight hiccup, but I think that user count is in
>>>>>> the single digits, if not zero.
>>>>>>
>>>>>> Let's fix this quickly with /size_t/u64/ in v6.8.1 and avoid the
>>>>>> compat shim if we can.
>>>>>>
>>>>>> Casey, do you have time to put together a patch for this (you should
>>>>>> fix the call chains below the syscalls too)?  If not, please let me
>>>>>> know and I'll get a patch out ASAP.
>>>>> Grumble. Yes, I'll get right on it.
>>>> Great, thanks Casey.
>>> Look like lsm_get_self_attr() needs the same change. lsm_set_self_attr()
>>> doesn't, need it, but I'm tempted to change it as well for consistency.
>>> Thoughts?
>> As lsm_get_self_attr() has the same issue, it needs the same treatment.
>>
>> lsm_set_self_attr() could be left unchanged.  In fact, changing the type
>> of syscall arguments from size_t to an explicit 64-bit type would be
>> problematic because 32-bit syscalls cannot have 64-bit arguments.
> Using u32 should be totally fine for both. Nearly ever kernel internal
> limits sizes to INT_MAX anyway. :)

My initial patch changes the size_t pointers in lsm_list_modules()
and lsm_get_self_attr() to __u64 pointers. I could change them to
__u32 pointers and include lsm_set_self_attr() as well. Let's see
who screams how loudly.