From f3db0bed458314a835ccef5ccb130270c5b2cf04 Mon Sep 17 00:00:00 2001 From: Eric Biggers Date: Tue, 21 Jul 2020 15:59:20 -0700 Subject: fs-verity: use smp_load_acquire() for ->i_verity_info 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()") Link: https://lore.kernel.org/r/20200721225920.114347-6-ebiggers@kernel.org Signed-off-by: Eric Biggers --- include/linux/fsverity.h | 9 +++++++-- 1 file changed, 7 insertions(+), 2 deletions(-) (limited to 'include') 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 */ -- cgit v1.2.3