lsm: ensure the correct LSM context releaser

Add a new lsm_context data structure to hold all the information about a
"security context", including the string, its size and which LSM allocated
the string. The allocation information is necessary because LSMs have
different policies regarding the lifecycle of these strings. SELinux
allocates and destroys them on each use, whereas Smack provides a pointer
to an entry in a list that never goes away.

Update security_release_secctx() to use the lsm_context instead of a
(char *, len) pair. Change its callers to do likewise.  The LSMs
supporting this hook have had comments added to remind the developer
that there is more work to be done.

The BPF security module provides all LSM hooks. While there has yet to
be a known instance of a BPF configuration that uses security contexts,
the possibility is real. In the existing implementation there is
potential for multiple frees in that case.

Cc: [email protected]
Cc: [email protected]
Cc: [email protected]
Cc: [email protected]
To: Pablo Neira Ayuso <[email protected]>
Cc: [email protected]
Cc: Todd Kjos <[email protected]>
Signed-off-by: Casey Schaufler <[email protected]>
[PM: subject tweak]
Signed-off-by: Paul Moore <[email protected]>

1 files changed, 9 insertions, 2 deletions

diff --git a/security/selinux/hooks.c b/security/selinux/hooks.c
index f5a08f94e094..aabc724f4d44 100644
--- a/security/selinux/hooks.c
+++ b/security/selinux/hooks.c
@@ -6657,9 +6657,16 @@ static int selinux_secctx_to_secid(const char *secdata, u32 seclen, u32 *secid)
 				       secid, GFP_KERNEL);
 }
 
-static void selinux_release_secctx(char *secdata, u32 seclen)
+static void selinux_release_secctx(struct lsm_context *cp)
 {
-	kfree(secdata);
+	/*
+	 * stacking scaffolding:
+	 * When it is possible for more than one LSM to provide a
+	 * release hook, do this check:
+	 * if (cp->id == LSM_ID_SELINUX || cp->id == LSM_ID_UNDEF)
+	 */
+
+	kfree(cp->context);
 }
 
 static void selinux_inode_invalidate_secctx(struct inode *inode)