Re: [PATCH v12 05/11] LSM: Create lsm_list_modules system call

[Casey Schaufler <casey@schaufler-ca.com>]

On 7/11/2023 8:36 AM, Mickaël Salaün wrote:
>
> On 29/06/2023 21:55, Casey Schaufler wrote:
>> Create a system call to report the list of Linux Security Modules
>> that are active on the system. The list is provided as an array
>> of LSM ID numbers.
>>
>> The calling application can use this list determine what LSM
>> specific actions it might take. That might include choosing an
>> output format, determining required privilege or bypassing
>> security module specific behavior.
>>
>> Signed-off-by: Casey Schaufler <casey@schaufler-ca.com>
>> Reviewed-by: Kees Cook <keescook@chromium.org>
>> Reviewed-by: Serge Hallyn <serge@hallyn.com>
>> ---
>>   Documentation/userspace-api/lsm.rst |  3 +++
>>   include/linux/syscalls.h            |  1 +
>>   kernel/sys_ni.c                     |  1 +
>>   security/lsm_syscalls.c             | 39 +++++++++++++++++++++++++++++
>>   4 files changed, 44 insertions(+)
>>
>> diff --git a/Documentation/userspace-api/lsm.rst
>> b/Documentation/userspace-api/lsm.rst
>> index e6c3f262addc..9edae18a2688 100644
>> --- a/Documentation/userspace-api/lsm.rst
>> +++ b/Documentation/userspace-api/lsm.rst
>> @@ -63,6 +63,9 @@ Get the specified security attributes of the
>> current process
>>   .. kernel-doc:: security/lsm_syscalls.c
>>       :identifiers: sys_lsm_get_self_attr
>>   +.. kernel-doc:: security/lsm_syscalls.c
>> +    :identifiers: sys_lsm_list_modules
>> +
>>   Additional documentation
>>   ========================
>>   diff --git a/include/linux/syscalls.h b/include/linux/syscalls.h
>> index 9a94c31bf6b6..ddbcc333f3c3 100644
>> --- a/include/linux/syscalls.h
>> +++ b/include/linux/syscalls.h
>> @@ -1063,6 +1063,7 @@ asmlinkage long sys_lsm_get_self_attr(unsigned
>> int attr, struct lsm_ctx *ctx,
>>                         size_t *size, __u32 flags);
>>   asmlinkage long sys_lsm_set_self_attr(unsigned int attr, struct
>> lsm_ctx *ctx,
>>                         size_t size, __u32 flags);
>> +asmlinkage long sys_lsm_list_modules(u64 *ids, size_t *size, u32
>> flags);
>>     /*
>>    * Architecture-specific system calls
>> diff --git a/kernel/sys_ni.c b/kernel/sys_ni.c
>> index d03c78ef1562..ceb3d21a62d0 100644
>> --- a/kernel/sys_ni.c
>> +++ b/kernel/sys_ni.c
>> @@ -265,6 +265,7 @@ COND_SYSCALL(mremap);
>>   /* security/lsm_syscalls.c */
>>   COND_SYSCALL(lsm_get_self_attr);
>>   COND_SYSCALL(lsm_set_self_attr);
>> +COND_SYSCALL(lsm_list_modules);
>>     /* security/keys/keyctl.c */
>>   COND_SYSCALL(add_key);
>> diff --git a/security/lsm_syscalls.c b/security/lsm_syscalls.c
>> index ee3881159241..f03f2d17ab49 100644
>> --- a/security/lsm_syscalls.c
>> +++ b/security/lsm_syscalls.c
>> @@ -53,3 +53,42 @@ 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,
>
> As explained in patch 4/12, it would be more flexible to return a list
> of:
>
> struct lsm_entry {
>     __u64 id;
>     __u64 flags;
> };

Any additional information should be obtained using lsm_get_self_attr().
If there's a "flag" value if should be provided as a LSM_FLAG there.

>
>
>> +        u32, flags)
>> +{
>> +    size_t total_size = lsm_active_cnt * sizeof(*ids);
>> +    size_t usize;
>> +    int i;
>> +
>> +    if (flags)
>> +        return -EINVAL;
>> +
>> +    if (get_user(usize, size))
>> +        return -EFAULT;
>> +
>> +    if (put_user(total_size, size) != 0)
>> +        return -EFAULT;
>> +
>> +    if (usize < total_size)
>> +        return -E2BIG;
>> +
>> +    for (i = 0; i < lsm_active_cnt; i++)
>> +        if (put_user(lsm_idlist[i]->id, ids++))
>
> What about putting the returned fixed-size list on the stack and only
> call copy_to_user() once?

There are objections to using the stack, doing an allocation, and doing
multiple put_user() calls. I just picked one. There's usually only going
to be one call, so this is the best in most cases.

>
>
>> +            return -EFAULT;
>> +
>> +    return lsm_active_cnt;
>> +}