From: Eric Biggers <ebiggers@kernel.org>
To: linux-fscrypt@vger.kernel.org
Cc: linux-fsdevel@vger.kernel.org, linux-ext4@vger.kernel.org,
linux-f2fs-devel@lists.sourceforge.net
Subject: [PATCH 5/5] fs-verity: use smp_load_acquire() for ->i_verity_info
Date: Tue, 21 Jul 2020 15:59:20 -0700 [thread overview]
Message-ID: <20200721225920.114347-6-ebiggers@kernel.org> (raw)
In-Reply-To: <20200721225920.114347-1-ebiggers@kernel.org>
From: Eric Biggers <ebiggers@google.com>
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@google.com>
---
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
next prev parent reply other threads:[~2020-07-21 23:01 UTC|newest]
Thread overview: 7+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-07-21 22:59 [PATCH 0/5] fscrypt, fs-verity: one-time init fixes Eric Biggers
2020-07-21 22:59 ` [PATCH 1/5] fscrypt: switch fscrypt_do_sha256() to use the SHA-256 library Eric Biggers
2020-07-21 22:59 ` [PATCH 2/5] fscrypt: use smp_load_acquire() for fscrypt_prepared_key Eric Biggers
2020-07-21 22:59 ` [PATCH 3/5] fscrypt: use smp_load_acquire() for ->s_master_keys Eric Biggers
2020-07-21 22:59 ` [PATCH 4/5] fscrypt: use smp_load_acquire() for ->i_crypt_info Eric Biggers
2020-07-21 22:59 ` Eric Biggers [this message]
2020-07-27 16:38 ` [PATCH 0/5] fscrypt, fs-verity: one-time init fixes Eric Biggers
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=20200721225920.114347-6-ebiggers@kernel.org \
--to=ebiggers@kernel.org \
--cc=linux-ext4@vger.kernel.org \
--cc=linux-f2fs-devel@lists.sourceforge.net \
--cc=linux-fscrypt@vger.kernel.org \
--cc=linux-fsdevel@vger.kernel.org \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).