Re: [PATCH v14 04/11] LSM: syscalls for current process attributes

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@schaufler-ca.com>
Reviewed-by: Kees Cook <redacted>
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

+/**
+ * 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