ovl_maybe_lookup_lowerdata
On Thu, Apr 20, 2023 at 10:44 AM Alexander Larsson <alexl@xxxxxxxxxx> wrote:
>
> When resolving lowerdata (lazily or non-lazily) we chech the
> overlay.verity xattr on the metadata inode, and if set verify that the
> source lowerdata inode matches it (according to the verity options
> enabled).
>
> Signed-off-by: Alexander Larsson <alexl@xxxxxxxxxx>
> ---
> fs/overlayfs/namei.c | 34 ++++++++++++++
> fs/overlayfs/overlayfs.h | 6 +++
> fs/overlayfs/util.c | 97 ++++++++++++++++++++++++++++++++++++++++
> 3 files changed, 137 insertions(+)
>
> diff --git a/fs/overlayfs/namei.c b/fs/overlayfs/namei.c
> index ba2b156162ca..49f3715c582d 100644
> --- a/fs/overlayfs/namei.c
> +++ b/fs/overlayfs/namei.c
> @@ -892,6 +892,7 @@ static int ovl_fix_origin(struct ovl_fs *ofs, struct dentry *dentry,
> /* Lazy lookup of lowerdata */
> int ovl_maybe_lookup_lowerdata(struct dentry *dentry)
> {
> + struct ovl_fs *ofs = dentry->d_sb->s_fs_info;
> struct inode *inode = d_inode(dentry);
> const char *redirect = ovl_lowerdata_redirect(inode);
> struct ovl_path datapath = {};
> @@ -919,6 +920,21 @@ int ovl_maybe_lookup_lowerdata(struct dentry *dentry)
> if (err)
> goto out_err;
>
> + if (ofs->config.verity_validate) {
> + struct path data = { .mnt = datapath.layer->mnt, .dentry = datapath.dentry, };
> + struct path metapath = {};
> +
> + ovl_path_real(dentry, &metapath);
> + if (!metapath.dentry) {
> + err = -EIO;
> + goto out_err;
> + }
> +
> + err = ovl_validate_verity(ofs, &metapath, &data);
> + if (err)
> + goto out_err;
> + }
> +
> err = ovl_dentry_set_lowerdata(dentry, &datapath);
> if (err)
> goto out_err;
> @@ -1186,6 +1202,24 @@ struct dentry *ovl_lookup(struct inode *dir, struct dentry *dentry,
> if (err)
> goto out_put;
>
> + /* Validate verity of lower-data */
> + if (ofs->config.verity_validate &&
> + !d.is_dir && (uppermetacopy || ctr > 1)) {
> + struct path datapath;
> +
> + ovl_entry_path_lowerdata(&oe, &datapath);
> +
> + /* Is NULL for lazy lookup, will be verified later */
> + if (datapath.dentry) {
> + struct path metapath;
> +
> + ovl_entry_path_real(ofs, &oe, upperdentry, &metapath);
> + err = ovl_validate_verity(ofs, &metapath, &datapath);
> + if (err < 0)
> + goto out_free_oe;
> + }
> + }
> +
> if (upperopaque)
> ovl_dentry_set_opaque(dentry);
>
> diff --git a/fs/overlayfs/overlayfs.h b/fs/overlayfs/overlayfs.h
> index 3d14770dc711..b1d639ccd5ac 100644
> --- a/fs/overlayfs/overlayfs.h
> +++ b/fs/overlayfs/overlayfs.h
> @@ -38,6 +38,7 @@ enum ovl_xattr {
> OVL_XATTR_UPPER,
> OVL_XATTR_METACOPY,
> OVL_XATTR_PROTATTR,
> + OVL_XATTR_VERITY,
> };
>
> enum ovl_inode_flag {
> @@ -467,6 +468,11 @@ int ovl_lock_rename_workdir(struct dentry *workdir, struct dentry *upperdir);
> int ovl_check_metacopy_xattr(struct ovl_fs *ofs, const struct path *path);
> bool ovl_is_metacopy_dentry(struct dentry *dentry);
> char *ovl_get_redirect_xattr(struct ovl_fs *ofs, const struct path *path, int padding);
> +int ovl_get_verity_xattr(struct ovl_fs *ofs, const struct path *path,
> + u8 *digest_buf, int *buf_length);
> +int ovl_validate_verity(struct ovl_fs *ofs,
> + struct path *metapath,
> + struct path *datapath);
> int ovl_sync_status(struct ovl_fs *ofs);
>
> static inline void ovl_set_flag(unsigned long flag, struct inode *inode)
> diff --git a/fs/overlayfs/util.c b/fs/overlayfs/util.c
> index 17eff3e31239..55e90aa0978a 100644
> --- a/fs/overlayfs/util.c
> +++ b/fs/overlayfs/util.c
> @@ -10,7 +10,9 @@
> #include <linux/cred.h>
> #include <linux/xattr.h>
> #include <linux/exportfs.h>
> +#include <linux/file.h>
> #include <linux/fileattr.h>
> +#include <linux/fsverity.h>
> #include <linux/uuid.h>
> #include <linux/namei.h>
> #include <linux/ratelimit.h>
> @@ -742,6 +744,7 @@ bool ovl_path_check_dir_xattr(struct ovl_fs *ofs, const struct path *path,
> #define OVL_XATTR_UPPER_POSTFIX "upper"
> #define OVL_XATTR_METACOPY_POSTFIX "metacopy"
> #define OVL_XATTR_PROTATTR_POSTFIX "protattr"
> +#define OVL_XATTR_VERITY_POSTFIX "verity"
>
> #define OVL_XATTR_TAB_ENTRY(x) \
> [x] = { [false] = OVL_XATTR_TRUSTED_PREFIX x ## _POSTFIX, \
> @@ -756,6 +759,7 @@ const char *const ovl_xattr_table[][2] = {
> OVL_XATTR_TAB_ENTRY(OVL_XATTR_UPPER),
> OVL_XATTR_TAB_ENTRY(OVL_XATTR_METACOPY),
> OVL_XATTR_TAB_ENTRY(OVL_XATTR_PROTATTR),
> + OVL_XATTR_TAB_ENTRY(OVL_XATTR_VERITY),
> };
>
> int ovl_check_setxattr(struct ovl_fs *ofs, struct dentry *upperdentry,
> @@ -1188,6 +1192,99 @@ char *ovl_get_redirect_xattr(struct ovl_fs *ofs, const struct path *path, int pa
> return ERR_PTR(res);
> }
>
> +int ovl_get_verity_xattr(struct ovl_fs *ofs, const struct path *path,
> + u8 *digest_buf, int *buf_length)
> +{
> + int res;
> +
> + res = ovl_path_getxattr(ofs, path, OVL_XATTR_VERITY, digest_buf, *buf_length);
> + if (res == -ENODATA || res == -EOPNOTSUPP)
> + return -ENODATA;
> + if (res < 0) {
> + pr_warn_ratelimited("failed to get digest (%i)\n", res);
> + return res;
> + }
> +
> + *buf_length = res;
> + return 0;
> +}
> +
> +static int ovl_ensure_verity_loaded(struct ovl_fs *ofs,
> + struct path *datapath)
> +{
> + struct inode *inode = d_inode(datapath->dentry);
> + const struct fsverity_info *vi;
> + const struct cred *old_cred;
> + struct file *filp;
> +
> + vi = fsverity_get_info(inode);
> + if (vi == NULL && IS_VERITY(inode)) {
> + /*
> + * If this inode was not yet opened, the verity info hasn't been
> + * loaded yet, so we need to do that here to force it into memory.
> + */
> + old_cred = override_creds(ofs->creator_cred);
Even though it may work, that's the wrong place for override_creds(),
because you are calling this helper from within ovl_lookup() with
mounter creds already.
Better to move revert_creds() in ovl_maybe_lookup_lowerdata()
to out_revert_creds: goto label and call ovl_validate_verity() with
mounter creds from all call sites.
> + filp = dentry_open(datapath, O_RDONLY, current_cred());
You could use open_with_fake_path() here to avoid ENFILE
not sure if this is critical.
> + revert_creds(old_cred);
> + if (IS_ERR(filp))
> + return PTR_ERR(filp);
> + fput(filp);
> + }
> +
> + return 0;
> +}
> +
> +int ovl_validate_verity(struct ovl_fs *ofs,
> + struct path *metapath,
> + struct path *datapath)
> +{
> + u8 required_digest[FS_VERITY_MAX_DIGEST_SIZE];
> + u8 actual_digest[FS_VERITY_MAX_DIGEST_SIZE];
> + enum hash_algo verity_algo;
> + int digest_len;
> + int err;
> +
> + if (!ofs->config.verity_validate ||
> + /* Verity only works on regular files */
> + !S_ISREG(d_inode(metapath->dentry)->i_mode))
> + return 0;
> +
> + digest_len = sizeof(required_digest);
> + err = ovl_get_verity_xattr(ofs, metapath, required_digest, &digest_len);
> + if (err == -ENODATA) {
> + if (ofs->config.verity_require) {
> + pr_warn_ratelimited("metacopy file '%pd' has no overlay.verity xattr\n",
> + metapath->dentry);
> + return -EIO;
> + }
> + return 0;
> + }
Thinking out loud: feels to me that verity xattr is a property that is
always "coupled" with metacopy xattr.
I wonder if we should check and store them together during lookup.
On one hand that means using a bit more memory per inode
before we need it.
OTOH, getxattr on metapath during lazy lookup may incur extra
IO to the metapath inode xattr block that will have been amortized
if done during lookup.
I have no strong feelings one way or the other.
Thanks,
Amir.
> + if (err < 0)
> + return err;
> +
> + err = ovl_ensure_verity_loaded(ofs, datapath);
> + if (err < 0) {
> + pr_warn_ratelimited("lower file '%pd' failed to load fs-verity info\n",
> + datapath->dentry);
> + return -EIO;
> + }
> +
> + err = fsverity_get_digest(d_inode(datapath->dentry), actual_digest, &verity_algo);
> + if (err < 0) {
> + pr_warn_ratelimited("lower file '%pd' has no fs-verity digest\n", datapath->dentry);
> + return -EIO;
> + }
> +
> + if (digest_len != hash_digest_size[verity_algo] ||
> + memcmp(required_digest, actual_digest, digest_len) != 0) {
> + pr_warn_ratelimited("lower file '%pd' has the wrong fs-verity digest\n",
> + datapath->dentry);
> + return -EIO;
> + }
> +
> + return 0;
> +}
> +
> /*
> * ovl_sync_status() - Check fs sync status for volatile mounts
> *
> --
> 2.39.2
>
