On Mon, Aug 28, 2023 at 12:57:54PM -0700, 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
> 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@xxxxxxxxxxxxxxxx>
> Reviewed-by: Kees Cook <keescook@xxxxxxxxxxxx>
> Reviewed-by: Serge Hallyn <serge@xxxxxxxxxx>
> Reviewed-by: John Johansen <john.johansen@xxxxxxxxxxxxx>
> ---
> 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
>
> +/**
> + * 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;
Much easier to read with these booleans.
> + 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;
> + if (uctx &&
> + copy_struct_from_user(&lctx, sizeof(lctx), uctx, left))
Again, I'm not sure is copy_struct_from_user() should be used here
because it checks that the user space structures ends with zeros, which
is inconsistent with the case without LSM_FLAG_SINGLE. Anyway, this code
should at least properly handle the copy_struct_from_user() error codes
which includes EFAULT *and* E2BIG.
> + 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;
Not a big deal but with LSM_FLAG_SINGLE, if the selected LSM doesn't
implement this hook, this will uselessly loop over all LSMs.
I'd add:
if (single)
/* Still try to write 0 as the total size for consistency. */
break;
> + continue;
> + }
> + if (rc == -E2BIG) {
> + toobig = true;
> + left = 0;
> + total += entrysize;
I'm not sure what is the desired behavior with LSM_FLAG_SINGLE in this
case but I'd add:
if (single)
break;
> + continue;
> + }
> + if (rc < 0)
> + return rc;
I think this should be a break instead of the return rc for consistency
reasons.
> +
> + left -= entrysize;
> + total += entrysize;
> + count += rc;
You could simplify a bit by replacing the first single check with this
one:
if (single)
break;
> + }
> + if (put_user(total, size))
> + return -EFAULT;
> + if (toobig)
> + return -E2BIG;
> + if (count == 0)
> + return LSM_RET_DEFAULT(getselfattr);
> + return count;
> +}
> +
It would be nice to add a comment before all these three syscall
documentations to keep up-to-date the related user space documentation
in lsm_syscalls.c
> +/**
> + * security_setselfattr - Set an LSM attribute on the current process.
> + * @attr: which attribute to set
> + * @uctx: the user-space source for the information
> + * @size: the size of the data
> + * @flags: reserved for future use, must be 0
> + *
> + * Set an LSM attribute for the current process. The LSM, attribute
> + * and new value are included in @uctx.
> + *
> + * Returns 0 on success, -EINVAL if the input is inconsistent, -EFAULT
> + * if the user buffer is inaccessible or an LSM specific failure.
> + */
> +int security_setselfattr(unsigned int attr, struct lsm_ctx __user *uctx,
> + size_t size, u32 flags)
> +{
> + struct security_hook_list *hp;
> + struct lsm_ctx *lctx;
> + int rc = LSM_RET_DEFAULT(setselfattr);
> +
> + if (flags)
> + return -EINVAL;
> + if (size < sizeof(*lctx) || size > PAGE_SIZE)
I would be better to return -E2BIG if size > PAGE_SIZE, and update the
(two) documentations accordingly.
> + return -EINVAL;
> +
> + lctx = kmalloc(size, GFP_KERNEL);
> + if (lctx == NULL)
> + return -ENOMEM;
> +
> + if (copy_from_user(lctx, uctx, size)) {
> + rc = -EFAULT;
> + goto free_out;
> + }
> +
> + if (size < lctx->len || size < lctx->ctx_len + sizeof(*lctx) ||
> + lctx->len < lctx->ctx_len + sizeof(*lctx)) {
> + rc = -EINVAL;
> + goto free_out;
> + }
> +
> + hlist_for_each_entry(hp, &security_hook_heads.setselfattr, list)
> + if ((hp->lsmid->id) == lctx->id) {
> + rc = hp->hook.setselfattr(attr, lctx, size, flags);
There is no need (at least for now) to directly expose lctx->id nor
lctx->flags. It would be simpler to only pass lctx->ctx and
lctx->ctx_len to the hooks.
> + break;
> + }
> +
> +free_out:
> + kfree(lctx);
> + return rc;
> +}
> +
> /**
> * security_getprocattr() - Read an attribute for a task
> * @p: the task
> --
> 2.41.0
>
