Re: [PATCH v6 04/11] LSM: syscalls for current process attributes

[Casey Schaufler]

On 3/7/2023 3:51 AM, Mickaël Salaün wrote:
>
> On 22/02/2023 21:08, Casey Schaufler wrote:
>> Create a system call lsm_get_self_attr() to provide the security
>> module maintained attributes of the current process.
>> Create a system call lsm_set_self_attr() to set a security
>> module maintained attribute of the current process.
>> Historically these attributes have been exposed to user space via
>> entries in procfs under /proc/self/attr.
>>
>> The attribute value is provided in a lsm_ctx structure. The structure
>> identifys the size of the attribute, and the attribute value. The format
>> of the attribute value is defined by the security module. A flags field
>> is included for LSM specific information. It is currently unused and
>> must
>> be 0. The total size of the data, including the lsm_ctx structure and
>> any
>> padding, is maintained as well.
>>
>> struct lsm_ctx {
>>          __u64   id;
>>          __u64   flags;
>>          __u64   len;
>>          __u64   ctx_len;
>>          __u8    ctx[];
>> };
>>
>> Two new LSM hooks are used to interface with the LSMs.
>> security_getselfattr() collects the lsm_ctx values from the
>> LSMs that support the hook, accounting for space requirements.
>> security_setselfattr() identifies which LSM the attribute is
>> intended for and passes it along.
>>
>> Signed-off-by: Casey Schaufler <casey@xxxxxxxxxxxxxxxx>
>> ---
>>   Documentation/userspace-api/lsm.rst |  15 ++++
>>   include/linux/lsm_hook_defs.h       |   4 ++
>>   include/linux/lsm_hooks.h           |   9 +++
>>   include/linux/security.h            |  19 +++++
>>   include/linux/syscalls.h            |   4 ++
>>   include/uapi/linux/lsm.h            |  33 +++++++++
>>   kernel/sys_ni.c                     |   4 ++
>>   security/Makefile                   |   1 +
>>   security/lsm_syscalls.c             | 104 ++++++++++++++++++++++++++++
>>   security/security.c                 |  82 ++++++++++++++++++++++
>>   10 files changed, 275 insertions(+)
>>   create mode 100644 security/lsm_syscalls.c
>>
>
> [...]
>
>> +/**
>> + * security_setselfattr - Set an LSM attribute on the current process.
>> + * @attr: which attribute to return
>> + * @ctx: the user-space source for the information
>> + * @size: the size of the data
>> + *
>> + * Set an LSM attribute for the current process. The LSM, attribute
>> + * and new value are included in @ctx.
>> + *
>> + * Returns 0 on seccess, an LSM specific value on failure.
>> + */
>> +int security_setselfattr(u64 __user attr, struct lsm_ctx __user *ctx,
>> +             size_t __user size)
>> +{
>> +    struct security_hook_list *hp;
>> +    struct lsm_ctx lctx;
>> +
>> +    if (size < sizeof(*ctx))
>
> If the lsm_ctx struct could grow in the future, we should check the
> size of the struct to the last field for compatibility reasons, see
> Landlock's copy_min_struct_from_user().

Because the lsm_ctx structure ends with the variable length context there's
no way to append new fields to it. The structure can't grow.

>
>
>> +        return -EINVAL;
>> +    if (copy_from_user(&lctx, ctx, sizeof(*ctx)))
>> +        return -EFAULT;
>> +
>> +    hlist_for_each_entry(hp, &security_hook_heads.setselfattr, list)
>> +        if ((hp->lsmid->id) == lctx.id)
>> +            return hp->hook.setselfattr(attr, ctx, size);
>> +
>> +    return LSM_RET_DEFAULT(setselfattr);
>> +}
>> +
>>   int security_getprocattr(struct task_struct *p, int lsmid, const
>> char *name,
>>                char **value)
>>   {