From: Eric Biggers <ebiggers@xxxxxxxxxx>
Normally smp_store_release() or cmpxchg_release() is paired with
smp_load_acquire(). Sometimes smp_load_acquire() can be replaced with
the more lightweight READ_ONCE(). However, for this to be safe, all the
published memory must only be accessed in a way that involves the
pointer itself. This may not be the case if allocating the object also
involves initializing a static or global variable, for example.
fsverity_info::tree_params.hash_alg->tfm is a crypto_ahash object that's
internal to and is allocated by the crypto subsystem. So by using
READ_ONCE() for ->i_verity_info, we're relying on internal
implementation details of the crypto subsystem.
Remove this fragile assumption by using smp_load_acquire() instead.
Also fix the cmpxchg logic to correctly execute an ACQUIRE barrier when
losing the cmpxchg race, since cmpxchg doesn't guarantee a memory
barrier on failure.
(Note: I haven't seen any real-world problems here. This change is just
fixing the code to be guaranteed correct and less fragile.)
Fixes: fd2d1acfcadf ("fs-verity: add the hook for file ->open()")
Signed-off-by: Eric Biggers <ebiggers@xxxxxxxxxx>
---
fs/verity/open.c | 15 ++++++++++++---
include/linux/fsverity.h | 9 +++++++--
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 @@ struct fsverity_info *fsverity_create_info(const struct inode *inode,
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 */
--
2.27.0
