On 9/6/2023 4:22 PM, Paul Moore wrote:

On Aug 28, 2023 Casey Schaufler [off-list ref] wrote:

quoted

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

..

quoted

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.

quoted

+		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;
+}

--
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