IMA: use i_writecount rather than a private counter

IMA tracks the number of struct files which are holding a given inode
readonly and the number which are holding the inode write or r/w.  It
needs this information so when a new reader or writer comes in it can
tell if this new file will be able to invalidate results it already made
about existing files.

aka if a task is holding a struct file open RO, IMA measured the file
and recorded those measurements and then a task opens the file RW IMA
needs to note in the logs that the old measurement may not be correct.
It's called a "Time of Measure Time of Use" (ToMToU) issue.  The same is
true is a RO file is opened to an inode which has an open writer.  We
cannot, with any validity, measure the file in question since it could
be changing.

This patch attempts to use the i_writecount field to track writers.  The
i_writecount field actually embeds more information in it's value than
IMA needs but it should work for our purposes and allow us to shrink the
struct inode even more.

Signed-off-by: Eric Paris <eparis@redhat.com>
Acked-by: Mimi Zohar <zohar@linux.vnet.ibm.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

security/integrity/ima/ima.h

@@ -108,7 +108,6 @@ struct ima_iint_cache {
 	struct mutex mutex;	/* protects: version, flags, digest */
 	/* protected by inode->i_lock */
 	unsigned int readcount;	/* measured files readcount */
-	unsigned int writecount;/* measured files writecount */
 	struct kref refcount;	/* ima_iint_cache reference count */
 };
 

security/integrity/ima/ima_iint.c

@@ -129,11 +129,6 @@ void iint_free(struct kref *kref)
 		       iint->readcount);
 		iint->readcount = 0;
 	}
-	if (iint->writecount != 0) {
-		printk(KERN_INFO "%s: writecount: %u\n", __func__,
-		       iint->writecount);
-		iint->writecount = 0;
-	}
 	kref_init(&iint->refcount);
 	kmem_cache_free(iint_cache, iint);
 }
@@ -166,7 +161,6 @@ static void init_once(void *foo)
 	iint->flags = 0UL;
 	mutex_init(&iint->mutex);
 	iint->readcount = 0;
-	iint->writecount = 0;
 	kref_init(&iint->refcount);
 }
 

security/integrity/ima/ima_main.c

@@ -94,8 +94,6 @@ static void ima_inc_counts(struct ima_iint_cache *iint, fmode_t mode)
 
 	if ((mode & (FMODE_READ | FMODE_WRITE)) == FMODE_READ)
 		iint->readcount++;
-	if (mode & FMODE_WRITE)
-		iint->writecount++;
 }
 
 /*
@@ -173,18 +171,16 @@ static void ima_dec_counts(struct ima_iint_cache *iint, struct inode *inode,
 		iint->readcount--;
 	}
 	if (mode & FMODE_WRITE) {
-		if (unlikely(iint->writecount == 0))
+		if (atomic_read(&inode->i_writecount) <= 0)
 			dump = true;
-		iint->writecount--;
-		if (iint->writecount == 0) {
-			if (iint->version != inode->i_version)
-				iint->flags &= ~IMA_MEASURED;
-		}
+		if (atomic_read(&inode->i_writecount) == 1 &&
+		    iint->version != inode->i_version)
+			iint->flags &= ~IMA_MEASURED;
 	}
 
 	if (dump && !ima_limit_imbalance(file)) {
-		printk(KERN_INFO "%s: open/free imbalance (r:%u w:%u)\n",
-		       __func__, iint->readcount, iint->writecount);
+		printk(KERN_INFO "%s: open/free imbalance (r:%u)\n",
+		       __func__, iint->readcount);
 		dump_stack();
 	}
 }