keys: Cache result of request_key*() temporarily in task_struct

If a filesystem uses keys to hold authentication tokens, then it needs a
token for each VFS operation that might perform an authentication check -
either by passing it to the server, or using to perform a check based on
authentication data cached locally.

For open files this isn't a problem, since the key should be cached in the
file struct since it represents the subject performing operations on that
file descriptor.

During pathwalk, however, there isn't anywhere to cache the key, except
perhaps in the nameidata struct - but that isn't exposed to the
filesystems.  Further, a pathwalk can incur a lot of operations, calling
one or more of the following, for instance:

	->lookup()
	->permission()
	->d_revalidate()
	->d_automount()
	->get_acl()
	->getxattr()

on each dentry/inode it encounters - and each one may need to call
request_key().  And then, at the end of pathwalk, it will call the actual
operation:

	->mkdir()
	->mknod()
	->getattr()
	->open()
	...

which may need to go and get the token again.

However, it is very likely that all of the operations on a single
dentry/inode - and quite possibly a sequence of them - will all want to use
the same authentication token, which suggests that caching it would be a
good idea.

To this end:

 (1) Make it so that a positive result of request_key() and co. that didn't
     require upcalling to userspace is cached temporarily in task_struct.

 (2) The cache is 1 deep, so a new result displaces the old one.

 (3) The key is released by exit and by notify-resume.

 (4) The cache is cleared in a newly forked process.

Signed-off-by: David Howells <dhowells@redhat.com>

1 files changed, 18 insertions, 0 deletions

diff --git a/security/keys/Kconfig b/security/keys/Kconfig
index 6462e6654ccf..12f70b556d09 100644
--- a/security/keys/Kconfig
+++ b/security/keys/Kconfig
@@ -24,6 +24,24 @@ config KEYS_COMPAT
 	def_bool y
 	depends on COMPAT && KEYS
 
+config KEYS_REQUEST_CACHE
+	bool "Enable temporary caching of the last request_key() result"
+	depends on KEYS
+	help
+	  This option causes the result of the last successful request_key()
+	  call that didn't upcall to the kernel to be cached temporarily in the
+	  task_struct.  The cache is cleared by exit and just prior to the
+	  resumption of userspace.
+
+	  This allows the key used for multiple step processes where each step
+	  wants to request a key that is likely the same as the one requested
+	  by the last step to save on the searching.
+
+	  An example of such a process is a pathwalk through a network
+	  filesystem in which each method needs to request an authentication
+	  key.  Pathwalk will call multiple methods for each dentry traversed
+	  (permission, d_revalidate, lookup, getxattr, getacl, ...).
+
 config PERSISTENT_KEYRINGS
 	bool "Enable register of persistent per-UID keyrings"
 	depends on KEYS