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authorEric Biggers <ebiggers@google.com>2020-07-21 15:59:19 -0700
committerEric Biggers <ebiggers@google.com>2020-07-21 16:02:13 -0700
commitab673b987488c4fab7a0bc4824a48211f9d910e3 (patch)
tree16ec1d6324aa8c0fe60f3d5f45f35b65de43f2ae
parent777afe4e68d7ac37711d36098bea65650ec305a0 (diff)
fscrypt: use smp_load_acquire() for ->i_crypt_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. fscrypt_info includes various sub-objects which are internal to and are allocated by other kernel subsystems such as keyrings and crypto. So by using READ_ONCE() for ->i_crypt_info, we're relying on internal implementation details of these other kernel subsystems. Remove this fragile assumption by using smp_load_acquire() instead. (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: e37a784d8b6a ("fscrypt: use READ_ONCE() to access ->i_crypt_info") Link: https://lore.kernel.org/r/20200721225920.114347-5-ebiggers@kernel.org Signed-off-by: Eric Biggers <ebiggers@google.com>
-rw-r--r--fs/crypto/keysetup.c12
-rw-r--r--fs/crypto/policy.c4
-rw-r--r--include/linux/fscrypt.h29
3 files changed, 37 insertions, 8 deletions
diff --git a/fs/crypto/keysetup.c b/fs/crypto/keysetup.c
index 7f85fc645602..fea6226afc2b 100644
--- a/fs/crypto/keysetup.c
+++ b/fs/crypto/keysetup.c
@@ -518,7 +518,17 @@ int fscrypt_get_encryption_info(struct inode *inode)
if (res)
goto out;
+ /*
+ * Multiple tasks may race to set ->i_crypt_info, so use
+ * cmpxchg_release(). This pairs with the smp_load_acquire() in
+ * fscrypt_get_info(). I.e., here we publish ->i_crypt_info with a
+ * RELEASE barrier so that other tasks can ACQUIRE it.
+ */
if (cmpxchg_release(&inode->i_crypt_info, NULL, crypt_info) == NULL) {
+ /*
+ * We won the race and set ->i_crypt_info to our crypt_info.
+ * Now link it into the master key's inode list.
+ */
if (master_key) {
struct fscrypt_master_key *mk =
master_key->payload.data[0];
@@ -589,7 +599,7 @@ EXPORT_SYMBOL(fscrypt_free_inode);
*/
int fscrypt_drop_inode(struct inode *inode)
{
- const struct fscrypt_info *ci = READ_ONCE(inode->i_crypt_info);
+ const struct fscrypt_info *ci = fscrypt_get_info(inode);
const struct fscrypt_master_key *mk;
/*
diff --git a/fs/crypto/policy.c b/fs/crypto/policy.c
index 8e667aadf271..2d73fd39ad96 100644
--- a/fs/crypto/policy.c
+++ b/fs/crypto/policy.c
@@ -352,7 +352,7 @@ static int fscrypt_get_policy(struct inode *inode, union fscrypt_policy *policy)
union fscrypt_context ctx;
int ret;
- ci = READ_ONCE(inode->i_crypt_info);
+ ci = fscrypt_get_info(inode);
if (ci) {
/* key available, use the cached policy */
*policy = ci->ci_policy;
@@ -641,7 +641,7 @@ int fscrypt_inherit_context(struct inode *parent, struct inode *child,
if (res < 0)
return res;
- ci = READ_ONCE(parent->i_crypt_info);
+ ci = fscrypt_get_info(parent);
if (ci == NULL)
return -ENOKEY;
diff --git a/include/linux/fscrypt.h b/include/linux/fscrypt.h
index bb257411365f..991ff8575d0e 100644
--- a/include/linux/fscrypt.h
+++ b/include/linux/fscrypt.h
@@ -74,10 +74,15 @@ struct fscrypt_operations {
struct request_queue **devs);
};
-static inline bool fscrypt_has_encryption_key(const struct inode *inode)
+static inline struct fscrypt_info *fscrypt_get_info(const struct inode *inode)
{
- /* pairs with cmpxchg_release() in fscrypt_get_encryption_info() */
- return READ_ONCE(inode->i_crypt_info) != NULL;
+ /*
+ * Pairs with the cmpxchg_release() in fscrypt_get_encryption_info().
+ * I.e., another task may publish ->i_crypt_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_crypt_info);
}
/**
@@ -234,9 +239,9 @@ static inline void fscrypt_set_ops(struct super_block *sb,
}
#else /* !CONFIG_FS_ENCRYPTION */
-static inline bool fscrypt_has_encryption_key(const struct inode *inode)
+static inline struct fscrypt_info *fscrypt_get_info(const struct inode *inode)
{
- return false;
+ return NULL;
}
static inline bool fscrypt_needs_contents_encryption(const struct inode *inode)
@@ -620,6 +625,20 @@ static inline bool fscrypt_inode_uses_fs_layer_crypto(const struct inode *inode)
}
/**
+ * fscrypt_has_encryption_key() - check whether an inode has had its key set up
+ * @inode: the inode to check
+ *
+ * Return: %true if the inode has had its encryption key set up, else %false.
+ *
+ * Usually this should be preceded by fscrypt_get_encryption_info() to try to
+ * set up the key first.
+ */
+static inline bool fscrypt_has_encryption_key(const struct inode *inode)
+{
+ return fscrypt_get_info(inode) != NULL;
+}
+
+/**
* fscrypt_require_key() - require an inode's encryption key
* @inode: the inode we need the key for
*