[RFC PATCH 2/2] SELinux: cache inode checks inside struct inode

[Eric Paris]

This patch adds a cache of selinux security checks into struct inode.
It is protected by the seq counter against updates by other nodes.  This
has a measurable impact on one benchmark Linus mentioned.  The cpu
time using make to check a huge project for changes.  It is going to
have a negative impact on cases where tasks with different labels are
accessing the same object.  In these cases each one will grab the i_lock
to reset the in inode cache.  The only place I imagine this would be
common would be with shared libraries.  But as those are typically
openned and mmapped, they don't have continuous checks...

Stock Kernel:
8.23%      make  [k] __d_lookup_rcu
6.27%      make  [k] link_path_walk
3.91%      make  [k] selinux_inode_permission <----
3.37%      make  [k] avc_has_perm_noaudit <----
2.26%      make  [k] lookup_fast
2.12%      make  [k] system_call
1.86%      make  [k] path_lookupat
1.82%      make  [k] inode_has_perm.isra.32.constprop.61 <----
1.57%      make  [k] copy_user_enhanced_fast_string
1.48%      make  [k] generic_permission
1.34%      make  [k] __audit_syscall_exit
1.31%      make  [k] kmem_cache_free
1.24%      make  [k] kmem_cache_alloc
1.20%      make  [k] generic_fillattr
1.12%      make  [k] __inode_permission
1.06%      make  [k] dput
1.04%      make  [k] strncpy_from_user
1.04%      make  [k] _raw_spin_lock
Total: 3.91 + 3.37 + 1.82 = 9.1%

My Changes:
8.54%      make  [k] __d_lookup_rcu
6.52%      make  [k] link_path_walk
3.93%      make  [k] inode_has_perm <----
2.31%      make  [k] lookup_fast
2.05%      make  [k] system_call
1.79%      make  [k] path_lookupat
1.72%      make  [k] generic_permission
1.50%      make  [k] __audit_syscall_exit
1.49%      make  [k] selinux_inode_permission <----
1.47%      make  [k] copy_user_enhanced_fast_string
1.28%      make  [k] __inode_permission
1.23%      make  [k] kmem_cache_alloc
1.19%      make  [k] _raw_spin_lock
1.15%      make  [k] lg_local_lock
1.10%      make  [k] strncpy_from_user
1.10%      make  [k] dput
1.08%      make  [k] kmem_cache_free
1.08%      make  [k] generic_fillattr
Total: 3.93 + 1.49 = 5.42

In inode_has_perm the big time takers are loading cred->sid and the
raw_seqcount_begin(inode->i_security_seccount).  I'm not certain how to
make either of those much faster.

In selinux_inode_permission() we spend time in getting current->cred and
in calling inode_has_perm().

Signed-off-by: Eric Paris <eparis@xxxxxxxxxx>
---
 include/linux/fs.h                  |  5 +++
 security/selinux/hooks.c            | 62 ++++++++++++++++++++++++++++++++++---
 security/selinux/include/security.h |  1 +
 security/selinux/ss/services.c      |  5 +++
 4 files changed, 69 insertions(+), 4 deletions(-)

diff --git a/include/linux/fs.h b/include/linux/fs.h
index 43db02e..5268cf3 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -535,6 +535,11 @@ struct inode {
 	struct address_space	*i_mapping;
 
 #ifdef CONFIG_SECURITY
+	seqcount_t		i_security_seqcount;
+	u32			i_last_task_sid;
+	u32			i_last_granting;
+	u32			i_last_perms;
+	u32			i_audit_allow;
 	void			*i_security;
 #endif
 
diff --git a/security/selinux/hooks.c b/security/selinux/hooks.c
index cfecb52..00dd6d9 100644
--- a/security/selinux/hooks.c
+++ b/security/selinux/hooks.c
@@ -82,6 +82,7 @@
 #include <linux/user_namespace.h>
 #include <linux/export.h>
 #include <linux/msg.h>
+#include <linux/seqlock.h>
 #include <linux/shm.h>
 
 #include "avc.h"
@@ -207,6 +208,7 @@ static int inode_alloc_security(struct inode *inode)
 	if (!isec)
 		return -ENOMEM;
 
+	seqcount_init(&inode->i_security_seqcount);
 	mutex_init(&isec->lock);
 	INIT_LIST_HEAD(&isec->list);
 	isec->inode = inode;
@@ -1516,6 +1518,44 @@ static noinline int audit_inode_permission(struct inode *inode,
 	return 0;
 }
 
+static bool inode_has_perm_cached(u32 sid, struct inode *inode, u32 perms)
+{
+	unsigned seq;
+	u32 last_task_sid;
+	u32 last_perms;
+	u32 last_granting;
+
+	seq = raw_seqcount_begin(&inode->i_security_seqcount);
+	last_task_sid = inode->i_last_task_sid;
+	last_perms = inode->i_last_perms;
+	last_granting = inode->i_last_granting;
+
+	/* something changed, bail! */
+	if (read_seqcount_retry(&inode->i_security_seqcount, seq))
+		return false;
+
+	return sid == last_task_sid && (perms & last_perms) == perms &&
+	       security_get_latest_granting() == last_granting;
+}
+
+static void inode_set_perm_cache(struct inode *inode, u32 task_sid, u32 perms,
+				 u32 granting, u32 audit_allow)
+{
+	spin_lock(&inode->i_lock);
+	write_seqcount_begin(&inode->i_security_seqcount);
+	if (inode->i_last_task_sid == task_sid &&
+	    inode->i_last_granting == granting) {
+		inode->i_last_perms |= perms;
+	} else {
+		inode->i_last_task_sid = task_sid;
+		inode->i_last_perms = perms;
+		inode->i_last_granting = granting;
+		inode->i_audit_allow = audit_allow;
+	}
+	write_seqcount_end(&inode->i_security_seqcount);
+	spin_unlock(&inode->i_lock);
+}
+
 /* Check whether a task has a particular permission to an inode.
    The 'adp' parameter is optional and allows other audit
    data to be passed (e.g. the dentry). */
@@ -1525,7 +1565,6 @@ static int inode_has_perm(const struct cred *cred,
 			  struct common_audit_data *adp,
 			  unsigned flags)
 {
-	struct inode_security_struct *isec;
 	struct av_decision avd;
 	u32 sid, denied, audited;
 	int rc, rc2;
@@ -1536,9 +1575,23 @@ static int inode_has_perm(const struct cred *cred,
 		return 0;
 
 	sid = cred_sid(cred);
-	isec = inode->i_security;
 
-	rc = avc_has_perm_noaudit(sid, isec->sid, isec->sclass, perms, 0, &avd);
+	if (inode_has_perm_cached(sid, inode, perms)) {
+		rc = 0;
+	        avd.allowed = -1;
+        	avd.auditallow = inode->i_audit_allow;
+        	avd.auditdeny = -1;
+        	avd.seqno = 0;
+        	avd.flags = 0;
+	} else {
+		struct inode_security_struct *isec;
+
+		isec = inode->i_security;
+
+		rc = avc_has_perm_noaudit(sid, isec->sid, isec->sclass, perms, 0, &avd);
+		if (!rc)
+			inode_set_perm_cache(inode, sid, perms, avd.seqno, avd.auditallow);
+	}
 	audited = avc_audit_required(perms, &avd, rc, dontaudit, &denied);
 	if (likely(!audited))
 		return rc;
@@ -1546,7 +1599,7 @@ static int inode_has_perm(const struct cred *cred,
 	rc2 = audit_inode_permission(inode, adp, perms, audited, denied, flags);
 	if (rc2)
 		return rc2;
-	return avc_has_perm_flags(sid, isec->sid, isec->sclass, perms, adp, flags);
+	return rc;
 }
 
 /* Same as inode_has_perm, but pass explicit audit data containing
@@ -2841,6 +2894,7 @@ static void selinux_inode_post_setxattr(struct dentry *dentry, const char *name,
 		return;
 	}
 
+	inode_set_perm_cache(inode, 0, 0, 0, 0);
 	isec->sid = newsid;
 	return;
 }
diff --git a/security/selinux/include/security.h b/security/selinux/include/security.h
index 6d38851..ec7d984 100644
--- a/security/selinux/include/security.h
+++ b/security/selinux/include/security.h
@@ -85,6 +85,7 @@ extern int selinux_policycap_openperm;
 /* limitation of boundary depth  */
 #define POLICYDB_BOUNDS_MAXDEPTH	4
 
+u32 security_get_latest_granting(void);
 int security_mls_enabled(void);
 
 int security_load_policy(void *data, size_t len);
diff --git a/security/selinux/ss/services.c b/security/selinux/ss/services.c
index b4feecc3..c6687ab 100644
--- a/security/selinux/ss/services.c
+++ b/security/selinux/ss/services.c
@@ -87,6 +87,11 @@ int ss_initialized;
  */
 static u32 latest_granting;
 
+u32 security_get_latest_granting(void)
+{
+	return latest_granting;
+}
+
 /* Forward declaration. */
 static int context_struct_to_string(struct context *context, char **scontext,
 				    u32 *scontext_len);
-- 
1.8.2.1

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