Merge tag 'fsverity-for-linus' of git://git.kernel.org/pub/scm/fs/fscrypt/fscrypt

Pull fsverity update from Eric Biggers:
 "One fix for fs/verity/ to strengthen a memory barrier which might be
  too weak. This mirrors a similar fix in fs/crypto/"

* tag 'fsverity-for-linus' of git://git.kernel.org/pub/scm/fs/fscrypt/fscrypt:
  fs-verity: use smp_load_acquire() for ->i_verity_info

2 files changed, 19 insertions, 5 deletions

diff --git a/fs/verity/open.c b/fs/verity/open.c
index d007db0c9304..bfe0280c14e4 100644
--- a/fs/verity/open.c
+++ b/fs/verity/open.c
@@ -221,11 +221,20 @@ out:
 void fsverity_set_info(struct inode *inode, struct fsverity_info *vi)
 {
 	/*
-	 * Multiple processes may race to set ->i_verity_info, so use cmpxchg.
-	 * This pairs with the READ_ONCE() in fsverity_get_info().
+	 * Multiple tasks may race to set ->i_verity_info, so use
+	 * cmpxchg_release().  This pairs with the smp_load_acquire() in
+	 * fsverity_get_info().  I.e., here we publish ->i_verity_info with a
+	 * RELEASE barrier so that other tasks can ACQUIRE it.
 	 */
-	if (cmpxchg(&inode->i_verity_info, NULL, vi) != NULL)
+	if (cmpxchg_release(&inode->i_verity_info, NULL, vi) != NULL) {
+		/* Lost the race, so free the fsverity_info we allocated. */
 		fsverity_free_info(vi);
+		/*
+		 * Afterwards, the caller may access ->i_verity_info directly,
+		 * so make sure to ACQUIRE the winning fsverity_info.
+		 */
+		(void)fsverity_get_info(inode);
+	}
 }
 
 void fsverity_free_info(struct fsverity_info *vi)
diff --git a/include/linux/fsverity.h b/include/linux/fsverity.h
index 78201a6d35f6..c1144a450392 100644
--- a/include/linux/fsverity.h
+++ b/include/linux/fsverity.h
@@ -115,8 +115,13 @@ struct fsverity_operations {
 
 static inline struct fsverity_info *fsverity_get_info(const struct inode *inode)
 {
-	/* pairs with the cmpxchg() in fsverity_set_info() */
-	return READ_ONCE(inode->i_verity_info);
+	/*
+	 * Pairs with the cmpxchg_release() in fsverity_set_info().
+	 * I.e., another task may publish ->i_verity_info concurrently,
+	 * executing a RELEASE barrier.  We need to use smp_load_acquire() here
+	 * to safely ACQUIRE the memory the other task published.
+	 */
+	return smp_load_acquire(&inode->i_verity_info);
 }
 
 /* enable.c */