Re: [PATCH v14 4/11] LSM: syscalls for current process attributes

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

On 9/6/2023 4:22 PM, Paul Moore wrote:
> On Aug 28, 2023 Casey Schaufler <casey@schaufler-ca.com> 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
>> identifies 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@schaufler-ca.com>
>> Reviewed-by: Kees Cook <keescook@chromium.org>
>> Reviewed-by: Serge Hallyn <serge@hallyn.com>
>> Reviewed-by: John Johansen <john.johansen@canonical.com>
>> ---
>>  Documentation/userspace-api/lsm.rst |  70 +++++++++++++
>>  include/linux/lsm_hook_defs.h       |   4 +
>>  include/linux/lsm_hooks.h           |   1 +
>>  include/linux/security.h            |  19 ++++
>>  include/linux/syscalls.h            |   5 +
>>  include/uapi/linux/lsm.h            |  36 +++++++
>>  kernel/sys_ni.c                     |   2 +
>>  security/Makefile                   |   1 +
>>  security/lsm_syscalls.c             |  57 +++++++++++
>>  security/security.c                 | 146 ++++++++++++++++++++++++++++
>>  10 files changed, 341 insertions(+)
>>  create mode 100644 Documentation/userspace-api/lsm.rst
>>  create mode 100644 security/lsm_syscalls.c
> ..
>
>> diff --git a/security/security.c b/security/security.c
>> index 82253294069c..aa4ade1f71b9 100644
>> --- a/security/security.c
>> +++ b/security/security.c
>> @@ -3798,6 +3798,152 @@ void security_d_instantiate(struct dentry *dentry, struct inode *inode)
>>  }
>>  EXPORT_SYMBOL(security_d_instantiate);
>>  
>> +/**
>> + * security_getselfattr - Read an LSM attribute of the current process.
>> + * @attr: which attribute to return
>> + * @uctx: the user-space destination for the information, or NULL
>> + * @size: pointer to the size of space available to receive the data
>> + * @flags: special handling options. LSM_FLAG_SINGLE indicates that only
>> + * attributes associated with the LSM identified in the passed @ctx be
>> + * reported.
>> + *
>> + * A NULL value for @uctx can be used to get both the number of attributes
>> + * and the size of the data.
>> + *
>> + * Returns the number of attributes found on success, negative value
>> + * on error. @size is reset to the total size of the data.
>> + * If @size is insufficient to contain the data -E2BIG is returned.
>> + */
>> +int security_getselfattr(unsigned int attr, struct lsm_ctx __user *uctx,
>> +			 size_t __user *size, u32 flags)
>> +{
>> +	struct security_hook_list *hp;
>> +	struct lsm_ctx lctx = { .id = LSM_ID_UNDEF, };
>> +	u8 __user *base = (u8 __user *)uctx;
>> +	size_t total = 0;
>> +	size_t entrysize;
>> +	size_t left;
>> +	bool toobig = false;
>> +	bool single = false;
>> +	int count = 0;
>> +	int rc;
>> +
>> +	if (attr == LSM_ATTR_UNDEF)
>> +		return -EINVAL;
>> +	if (size == NULL)
>> +		return -EINVAL;
>> +	if (get_user(left, size))
>> +		return -EFAULT;
>> +
>> +	if (flags) {
>> +		/*
>> +		 * Only flag supported is LSM_FLAG_SINGLE
>> +		 */
>> +		if (flags & LSM_FLAG_SINGLE)
>> +			return -EINVAL;
> Should this be something like the following?
>
>   if (flags & ~LSM_FLAG_SINGLE)
>     return -EINVAL;

Yes. I have a fix ready. There are a couple other touch-ups, too.

>
>> +		if (uctx &&
>> +		    copy_struct_from_user(&lctx, sizeof(lctx), uctx, left))
>> +			return -EFAULT;
>> +		/*
>> +		 * If the LSM ID isn't specified it is an error.
>> +		 */
>> +		if (lctx.id == LSM_ID_UNDEF)
>> +			return -EINVAL;
>> +		single = true;
>> +	}
>> +
>> +	/*
>> +	 * In the usual case gather all the data from the LSMs.
>> +	 * In the single case only get the data from the LSM specified.
>> +	 */
>> +	hlist_for_each_entry(hp, &security_hook_heads.getselfattr, list) {
>> +		if (single) {
>> +			if (count > 0)
>> +				break;
>> +			if (lctx.id != hp->lsmid->id)
>> +				continue;
>> +		}
>> +		entrysize = left;
>> +		if (base)
>> +			uctx = (struct lsm_ctx __user *)(base + total);
>> +		rc = hp->hook.getselfattr(attr, uctx, &entrysize, flags);
>> +		if (rc == -EOPNOTSUPP) {
>> +			rc = 0;
>> +			continue;
>> +		}
>> +		if (rc == -E2BIG) {
>> +			toobig = true;
>> +			left = 0;
>> +			total += entrysize;
>> +			continue;
>> +		}
>> +		if (rc < 0)
>> +			return rc;
>> +
>> +		left -= entrysize;
>> +		total += entrysize;
>> +		count += rc;
>> +	}
>> +	if (put_user(total, size))
>> +		return -EFAULT;
>> +	if (toobig)
>> +		return -E2BIG;
>> +	if (count == 0)
>> +		return LSM_RET_DEFAULT(getselfattr);
>> +	return count;
>> +}
>
> --
> paul-moore.com