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![Eric Biggers](/assets/img/avatar_default.png)
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: fd2d1acfca
("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 <ebiggers@google.com>
224 lines
6.5 KiB
C
224 lines
6.5 KiB
C
/* SPDX-License-Identifier: GPL-2.0 */
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/*
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* fs-verity: read-only file-based authenticity protection
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*
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* This header declares the interface between the fs/verity/ support layer and
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* filesystems that support fs-verity.
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*
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* Copyright 2019 Google LLC
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*/
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#ifndef _LINUX_FSVERITY_H
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#define _LINUX_FSVERITY_H
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#include <linux/fs.h>
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#include <uapi/linux/fsverity.h>
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/* Verity operations for filesystems */
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struct fsverity_operations {
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/**
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* Begin enabling verity on the given file.
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*
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* @filp: a readonly file descriptor for the file
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*
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* The filesystem must do any needed filesystem-specific preparations
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* for enabling verity, e.g. evicting inline data. It also must return
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* -EBUSY if verity is already being enabled on the given file.
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*
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* i_rwsem is held for write.
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*
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* Return: 0 on success, -errno on failure
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*/
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int (*begin_enable_verity)(struct file *filp);
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/**
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* End enabling verity on the given file.
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*
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* @filp: a readonly file descriptor for the file
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* @desc: the verity descriptor to write, or NULL on failure
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* @desc_size: size of verity descriptor, or 0 on failure
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* @merkle_tree_size: total bytes the Merkle tree took up
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*
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* If desc == NULL, then enabling verity failed and the filesystem only
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* must do any necessary cleanups. Else, it must also store the given
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* verity descriptor to a fs-specific location associated with the inode
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* and do any fs-specific actions needed to mark the inode as a verity
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* inode, e.g. setting a bit in the on-disk inode. The filesystem is
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* also responsible for setting the S_VERITY flag in the VFS inode.
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*
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* i_rwsem is held for write, but it may have been dropped between
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* ->begin_enable_verity() and ->end_enable_verity().
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*
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* Return: 0 on success, -errno on failure
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*/
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int (*end_enable_verity)(struct file *filp, const void *desc,
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size_t desc_size, u64 merkle_tree_size);
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/**
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* Get the verity descriptor of the given inode.
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*
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* @inode: an inode with the S_VERITY flag set
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* @buf: buffer in which to place the verity descriptor
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* @bufsize: size of @buf, or 0 to retrieve the size only
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*
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* If bufsize == 0, then the size of the verity descriptor is returned.
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* Otherwise the verity descriptor is written to 'buf' and its actual
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* size is returned; -ERANGE is returned if it's too large. This may be
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* called by multiple processes concurrently on the same inode.
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*
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* Return: the size on success, -errno on failure
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*/
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int (*get_verity_descriptor)(struct inode *inode, void *buf,
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size_t bufsize);
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/**
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* Read a Merkle tree page of the given inode.
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*
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* @inode: the inode
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* @index: 0-based index of the page within the Merkle tree
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* @num_ra_pages: The number of Merkle tree pages that should be
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* prefetched starting at @index if the page at @index
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* isn't already cached. Implementations may ignore this
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* argument; it's only a performance optimization.
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*
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* This can be called at any time on an open verity file, as well as
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* between ->begin_enable_verity() and ->end_enable_verity(). It may be
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* called by multiple processes concurrently, even with the same page.
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*
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* Note that this must retrieve a *page*, not necessarily a *block*.
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*
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* Return: the page on success, ERR_PTR() on failure
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*/
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struct page *(*read_merkle_tree_page)(struct inode *inode,
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pgoff_t index,
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unsigned long num_ra_pages);
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/**
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* Write a Merkle tree block to the given inode.
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*
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* @inode: the inode for which the Merkle tree is being built
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* @buf: block to write
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* @index: 0-based index of the block within the Merkle tree
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* @log_blocksize: log base 2 of the Merkle tree block size
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*
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* This is only called between ->begin_enable_verity() and
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* ->end_enable_verity().
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*
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* Return: 0 on success, -errno on failure
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*/
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int (*write_merkle_tree_block)(struct inode *inode, const void *buf,
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u64 index, int log_blocksize);
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};
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#ifdef CONFIG_FS_VERITY
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static inline struct fsverity_info *fsverity_get_info(const struct inode *inode)
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{
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/*
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* Pairs with the cmpxchg_release() in fsverity_set_info().
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* I.e., another task may publish ->i_verity_info concurrently,
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* executing a RELEASE barrier. We need to use smp_load_acquire() here
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* to safely ACQUIRE the memory the other task published.
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*/
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return smp_load_acquire(&inode->i_verity_info);
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}
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/* enable.c */
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int fsverity_ioctl_enable(struct file *filp, const void __user *arg);
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/* measure.c */
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int fsverity_ioctl_measure(struct file *filp, void __user *arg);
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/* open.c */
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int fsverity_file_open(struct inode *inode, struct file *filp);
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int fsverity_prepare_setattr(struct dentry *dentry, struct iattr *attr);
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void fsverity_cleanup_inode(struct inode *inode);
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/* verify.c */
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bool fsverity_verify_page(struct page *page);
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void fsverity_verify_bio(struct bio *bio);
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void fsverity_enqueue_verify_work(struct work_struct *work);
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#else /* !CONFIG_FS_VERITY */
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static inline struct fsverity_info *fsverity_get_info(const struct inode *inode)
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{
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return NULL;
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}
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/* enable.c */
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static inline int fsverity_ioctl_enable(struct file *filp,
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const void __user *arg)
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{
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return -EOPNOTSUPP;
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}
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/* measure.c */
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static inline int fsverity_ioctl_measure(struct file *filp, void __user *arg)
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{
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return -EOPNOTSUPP;
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}
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/* open.c */
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static inline int fsverity_file_open(struct inode *inode, struct file *filp)
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{
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return IS_VERITY(inode) ? -EOPNOTSUPP : 0;
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}
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static inline int fsverity_prepare_setattr(struct dentry *dentry,
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struct iattr *attr)
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{
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return IS_VERITY(d_inode(dentry)) ? -EOPNOTSUPP : 0;
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}
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static inline void fsverity_cleanup_inode(struct inode *inode)
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{
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}
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/* verify.c */
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static inline bool fsverity_verify_page(struct page *page)
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{
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WARN_ON(1);
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return false;
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}
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static inline void fsverity_verify_bio(struct bio *bio)
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{
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WARN_ON(1);
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}
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static inline void fsverity_enqueue_verify_work(struct work_struct *work)
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{
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WARN_ON(1);
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}
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#endif /* !CONFIG_FS_VERITY */
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/**
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* fsverity_active() - do reads from the inode need to go through fs-verity?
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* @inode: inode to check
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*
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* This checks whether ->i_verity_info has been set.
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*
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* Filesystems call this from ->readpages() to check whether the pages need to
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* be verified or not. Don't use IS_VERITY() for this purpose; it's subject to
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* a race condition where the file is being read concurrently with
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* FS_IOC_ENABLE_VERITY completing. (S_VERITY is set before ->i_verity_info.)
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*
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* Return: true if reads need to go through fs-verity, otherwise false
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*/
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static inline bool fsverity_active(const struct inode *inode)
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{
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return fsverity_get_info(inode) != NULL;
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}
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#endif /* _LINUX_FSVERITY_H */
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