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Message-Id: <20201228191211.138300-2-ebiggers@kernel.org>
Date: Mon, 28 Dec 2020 11:12:08 -0800
From: Eric Biggers <ebiggers@...nel.org>
To: stable@...r.kernel.org
Cc: linux-fscrypt@...r.kernel.org, linux-ext4@...r.kernel.org,
linux-f2fs-devel@...ts.sourceforge.net,
linux-mtd@...ts.infradead.org
Subject: [PATCH 4.19 1/4] fscrypt: add fscrypt_is_nokey_name()
From: Eric Biggers <ebiggers@...gle.com>
commit 159e1de201b6fca10bfec50405a3b53a561096a8 upstream.
It's possible to create a duplicate filename in an encrypted directory
by creating a file concurrently with adding the encryption key.
Specifically, sys_open(O_CREAT) (or sys_mkdir(), sys_mknod(), or
sys_symlink()) can lookup the target filename while the directory's
encryption key hasn't been added yet, resulting in a negative no-key
dentry. The VFS then calls ->create() (or ->mkdir(), ->mknod(), or
->symlink()) because the dentry is negative. Normally, ->create() would
return -ENOKEY due to the directory's key being unavailable. However,
if the key was added between the dentry lookup and ->create(), then the
filesystem will go ahead and try to create the file.
If the target filename happens to already exist as a normal name (not a
no-key name), a duplicate filename may be added to the directory.
In order to fix this, we need to fix the filesystems to prevent
->create(), ->mkdir(), ->mknod(), and ->symlink() on no-key names.
(->rename() and ->link() need it too, but those are already handled
correctly by fscrypt_prepare_rename() and fscrypt_prepare_link().)
In preparation for this, add a helper function fscrypt_is_nokey_name()
that filesystems can use to do this check. Use this helper function for
the existing checks that fs/crypto/ does for rename and link.
Cc: stable@...r.kernel.org
Link: https://lore.kernel.org/r/20201118075609.120337-2-ebiggers@kernel.org
Signed-off-by: Eric Biggers <ebiggers@...gle.com>
---
fs/crypto/hooks.c | 10 +++++-----
include/linux/fscrypt_notsupp.h | 5 +++++
include/linux/fscrypt_supp.h | 29 +++++++++++++++++++++++++++++
3 files changed, 39 insertions(+), 5 deletions(-)
diff --git a/fs/crypto/hooks.c b/fs/crypto/hooks.c
index 042d5b44f4ed9..aa86cb2db8236 100644
--- a/fs/crypto/hooks.c
+++ b/fs/crypto/hooks.c
@@ -58,8 +58,8 @@ int __fscrypt_prepare_link(struct inode *inode, struct inode *dir,
if (err)
return err;
- /* ... in case we looked up ciphertext name before key was added */
- if (dentry->d_flags & DCACHE_ENCRYPTED_NAME)
+ /* ... in case we looked up no-key name before key was added */
+ if (fscrypt_is_nokey_name(dentry))
return -ENOKEY;
if (!fscrypt_has_permitted_context(dir, inode))
@@ -83,9 +83,9 @@ int __fscrypt_prepare_rename(struct inode *old_dir, struct dentry *old_dentry,
if (err)
return err;
- /* ... in case we looked up ciphertext name(s) before key was added */
- if ((old_dentry->d_flags | new_dentry->d_flags) &
- DCACHE_ENCRYPTED_NAME)
+ /* ... in case we looked up no-key name(s) before key was added */
+ if (fscrypt_is_nokey_name(old_dentry) ||
+ fscrypt_is_nokey_name(new_dentry))
return -ENOKEY;
if (old_dir != new_dir) {
diff --git a/include/linux/fscrypt_notsupp.h b/include/linux/fscrypt_notsupp.h
index 24b261e49dc14..93304cfeb6016 100644
--- a/include/linux/fscrypt_notsupp.h
+++ b/include/linux/fscrypt_notsupp.h
@@ -24,6 +24,11 @@ static inline bool fscrypt_dummy_context_enabled(struct inode *inode)
return false;
}
+static inline bool fscrypt_is_nokey_name(const struct dentry *dentry)
+{
+ return false;
+}
+
/* crypto.c */
static inline void fscrypt_enqueue_decrypt_work(struct work_struct *work)
{
diff --git a/include/linux/fscrypt_supp.h b/include/linux/fscrypt_supp.h
index 8641e20694ce9..0409c14ae1de4 100644
--- a/include/linux/fscrypt_supp.h
+++ b/include/linux/fscrypt_supp.h
@@ -58,6 +58,35 @@ static inline bool fscrypt_dummy_context_enabled(struct inode *inode)
inode->i_sb->s_cop->dummy_context(inode);
}
+/**
+ * fscrypt_is_nokey_name() - test whether a dentry is a no-key name
+ * @dentry: the dentry to check
+ *
+ * This returns true if the dentry is a no-key dentry. A no-key dentry is a
+ * dentry that was created in an encrypted directory that hasn't had its
+ * encryption key added yet. Such dentries may be either positive or negative.
+ *
+ * When a filesystem is asked to create a new filename in an encrypted directory
+ * and the new filename's dentry is a no-key dentry, it must fail the operation
+ * with ENOKEY. This includes ->create(), ->mkdir(), ->mknod(), ->symlink(),
+ * ->rename(), and ->link(). (However, ->rename() and ->link() are already
+ * handled by fscrypt_prepare_rename() and fscrypt_prepare_link().)
+ *
+ * This is necessary because creating a filename requires the directory's
+ * encryption key, but just checking for the key on the directory inode during
+ * the final filesystem operation doesn't guarantee that the key was available
+ * during the preceding dentry lookup. And the key must have already been
+ * available during the dentry lookup in order for it to have been checked
+ * whether the filename already exists in the directory and for the new file's
+ * dentry not to be invalidated due to it incorrectly having the no-key flag.
+ *
+ * Return: %true if the dentry is a no-key name
+ */
+static inline bool fscrypt_is_nokey_name(const struct dentry *dentry)
+{
+ return dentry->d_flags & DCACHE_ENCRYPTED_NAME;
+}
+
/* crypto.c */
extern void fscrypt_enqueue_decrypt_work(struct work_struct *);
extern struct fscrypt_ctx *fscrypt_get_ctx(const struct inode *, gfp_t);
--
2.29.2
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