From: Eric Biggers <ebiggers@xxxxxxxxxx> Add a function for filesystems to call to implement the FS_IOC_MEASURE_VERITY ioctl. This ioctl retrieves the file measurement hash that fs-verity calculated for the given file and is enforcing for reads; i.e., reads that don't match this hash will fail. This ioctl can be used to implement lightweight auditing or authentication of file hashes in userspace, as an alternative to an in-kernel policy such as an IMA policy. Note that due to fs-verity's use of a Merkle tree, opening a file and executing this ioctl takes constant time, regardless of the file's size. Signed-off-by: Eric Biggers <ebiggers@xxxxxxxxxx> --- fs/verity/ioctl.c | 49 ++++++++++++++++++++++++++++++++++++++++ fs/verity/setup.c | 4 +++- include/linux/fsverity.h | 6 +++++ 3 files changed, 58 insertions(+), 1 deletion(-) diff --git a/fs/verity/ioctl.c b/fs/verity/ioctl.c index 993f2afdcc734..3da9b0f441c13 100644 --- a/fs/verity/ioctl.c +++ b/fs/verity/ioctl.c @@ -119,3 +119,52 @@ int fsverity_ioctl_enable(struct file *filp, const void __user *arg) return err; } EXPORT_SYMBOL_GPL(fsverity_ioctl_enable); + +/** + * fsverity_ioctl_measure - get a verity file's measurement + * + * The FS_IOC_MEASURE_VERITY ioctl retrieves the file measurement that the + * kernel is enforcing for reads from a verity file. + * + * No privileges are required to use this ioctl, since it is a read-only + * operation on a single regular file. + * + * Return: 0 on success, -errno on failure + */ +int fsverity_ioctl_measure(struct file *filp, void __user *_uarg) +{ + const struct inode *inode = file_inode(filp); + struct fsverity_digest __user *uarg = _uarg; + const struct fsverity_info *vi; + const struct fsverity_hash_alg *hash_alg; + struct fsverity_digest arg; + + vi = get_fsverity_info(inode); + if (!vi) + return -ENODATA; /* not a verity file */ + hash_alg = vi->hash_alg; + + /* + * The user specifies the digest_size their buffer has space for; we can + * return the digest if it fits in the available space. We write back + * the actual size, which may be shorter than the user-specified size. + */ + + if (get_user(arg.digest_size, &uarg->digest_size)) + return -EFAULT; + if (arg.digest_size < hash_alg->digest_size) + return -EOVERFLOW; + + memset(&arg, 0, sizeof(arg)); + arg.digest_algorithm = hash_alg - fsverity_hash_algs; + arg.digest_size = hash_alg->digest_size; + + if (copy_to_user(uarg, &arg, sizeof(arg))) + return -EFAULT; + + if (copy_to_user(uarg->digest, vi->measurement, hash_alg->digest_size)) + return -EFAULT; + + return 0; +} +EXPORT_SYMBOL_GPL(fsverity_ioctl_measure); diff --git a/fs/verity/setup.c b/fs/verity/setup.c index 84cc2edeca25b..3f5cb9526dbc9 100644 --- a/fs/verity/setup.c +++ b/fs/verity/setup.c @@ -842,7 +842,9 @@ static int __init fsverity_module_init(void) goto error; err = -ENOMEM; - fsverity_info_cachep = KMEM_CACHE(fsverity_info, SLAB_RECLAIM_ACCOUNT); + fsverity_info_cachep = KMEM_CACHE_USERCOPY(fsverity_info, + SLAB_RECLAIM_ACCOUNT, + measurement); if (!fsverity_info_cachep) goto error_free_workqueue; diff --git a/include/linux/fsverity.h b/include/linux/fsverity.h index c710b6b5fb4a6..9d3371dbd262f 100644 --- a/include/linux/fsverity.h +++ b/include/linux/fsverity.h @@ -23,6 +23,7 @@ struct fsverity_operations { /* ioctl.c */ extern int fsverity_ioctl_enable(struct file *filp, const void __user *arg); +extern int fsverity_ioctl_measure(struct file *filp, void __user *arg); /* setup.c */ extern int fsverity_file_open(struct inode *inode, struct file *filp); @@ -46,6 +47,11 @@ static inline int fsverity_ioctl_enable(struct file *filp, return -EOPNOTSUPP; } +static inline int fsverity_ioctl_measure(struct file *filp, void __user *arg) +{ + return -EOPNOTSUPP; +} + /* setup.c */ static inline int fsverity_file_open(struct inode *inode, struct file *filp) -- 2.18.0
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