Re: [PATCH v8 2/2] overlayfs: override_creds=off option bypass creator_cred

From: Vivek Goyal
Date: Thu Nov 08 2018 - 15:01:12 EST

On Tue, Nov 06, 2018 at 03:01:15PM -0800, Mark Salyzyn wrote:
> By default, all access to the upper, lower and work directories is the
> recorded mounter's MAC and DAC credentials. The incoming accesses are
> checked against the caller's credentials.

Ok, I am trying to think of scenarios where override_creds=off can
provide any privilege escalation. How about following.

$ mkdir lower lower/foo upper upper/foo work merged
$ touch lower/foo/bar.txt
$ chmod 700 lower/foo/

# Mount overlay with override_creds=off

$ mount -t overlay -o
lowerdir=lower,upperdir=upper,workdir=work,override_creds=off none merged

# Try to read lower/foo as unpriviliged user. Say "test"
# su test
# ls merged/foo/
ls: cannot access 'merged/foo/': Operation not permitted

# Now first try to do same operation as root and retry as test user.
$ exit
$ ls merged/foo
bar.txt
$ su test
$ ls merged/foo
bar.txt

lower/foo/ is not readable by user "test". So it fails in first try. Later
"root" accesses it and it populates cache in overlayfs. When test retries,
it gets these entries from cache.

With override_creds=on this is not a problem because overlay provides
this as functionality as long as mounter as access to lower/foo/.

But with override_creds=off, mounter is not providing any such
functionality and we are exposing an issue where cache will make
something available which is not normally available.

I think it probably is a good idea to do something about it?

Thanks
Vivek


>
> If the principles of least privilege are applied, the mounter's
> credentials might not overlap the credentials of the caller's when
> accessing the overlayfs filesystem. For example, a file that a lower
> DAC privileged caller can execute, is MAC denied to the generally
> higher DAC privileged mounter, to prevent an attack vector.
>
> We add the option to turn off override_creds in the mount options; all
> subsequent operations after mount on the filesystem will be only the
> caller's credentials. The module boolean parameter and mount option
> override_creds is also added as a presence check for this "feature",
> existence of /sys/module/overlay/parameters/override_creds.
>
> It was not always this way. Circa 4.4 there was no recorded mounter's
> credentials, instead privileged access to upper or work directories
> were temporarily increased to perform the operations. The MAC
> (selinux) policies were caller's in all cases. override_creds=off
> partially returns us to this older access model minus the insecure
> temporary credential increases. This is to permit use in a system
> with non-overlapping security models for each executable including
> the agent that mounts the overlayfs filesystem. In Android
> this is the case since init, which performs the mount operations,
> has a minimal MAC set of privileges to reduce any attack surface,
> and services that use the content have a different set of MAC
> privileges (eg: read, for vendor labelled configuration, execute for
> vendor libraries and modules).
>
> Signed-off-by: Mark Salyzyn <salyzyn@xxxxxxxxxxx>
> Cc: Miklos Szeredi <miklos@xxxxxxxxxx>
> Cc: Jonathan Corbet <corbet@xxxxxxx>
> Cc: Vivek Goyal <vgoyal@xxxxxxxxxx>
> Cc: Eric W. Biederman <ebiederm@xxxxxxxxxxxx>
> Cc: Amir Goldstein <amir73il@xxxxxxxxx>
> Cc: Randy Dunlap <rdunlap@xxxxxxxxxxxxx>
> Cc: Stephen Smalley <sds@xxxxxxxxxxxxx>
> Cc: linux-unionfs@xxxxxxxxxxxxxxx
> Cc: linux-doc@xxxxxxxxxxxxxxx
> Cc: linux-kernel@xxxxxxxxxxxxxxx
> Cc: kernel-team@xxxxxxxxxxx
> ---
> v8:
> - drop pr_warn message after straw poll to remove it.
> - added a use case in the commit message
>
> v7:
> - change name of internal parameter to ovl_override_creds_def
> - report override_creds only if different than default
>
> v6:
> - Drop CONFIG_OVERLAY_FS_OVERRIDE_CREDS.
> - Do better with the documentation.
> - pr_warn message adjusted to report consequences.
>
> v5:
> - beefed up the caveats in the Documentation
> - Is dependent on
> "overlayfs: check CAP_DAC_READ_SEARCH before issuing exportfs_decode_fh"
> "overlayfs: check CAP_MKNOD before issuing vfs_whiteout"
> - Added prwarn when override_creds=off
>
> v4:
> - spelling and grammar errors in text
>
> v3:
> - Change name from caller_credentials / creator_credentials to the
> boolean override_creds.
> - Changed from creator to mounter credentials.
> - Updated and fortified the documentation.
> - Added CONFIG_OVERLAY_FS_OVERRIDE_CREDS
>
> v2:
> - Forward port changed attr to stat, resulting in a build error.
> - altered commit message.
>
> Documentation/filesystems/overlayfs.txt | 17 +++++++++++++++++
> fs/overlayfs/copy_up.c | 2 +-
> fs/overlayfs/dir.c | 9 +++++----
> fs/overlayfs/inode.c | 16 ++++++++--------
> fs/overlayfs/namei.c | 6 +++---
> fs/overlayfs/overlayfs.h | 1 +
> fs/overlayfs/ovl_entry.h | 1 +
> fs/overlayfs/readdir.c | 4 ++--
> fs/overlayfs/super.c | 22 +++++++++++++++++++++-
> fs/overlayfs/util.c | 12 ++++++++++--
> 10 files changed, 69 insertions(+), 21 deletions(-)
>
> diff --git a/Documentation/filesystems/overlayfs.txt b/Documentation/filesystems/overlayfs.txt
> index eef7d9d259e8..5cc299df4436 100644
> --- a/Documentation/filesystems/overlayfs.txt
> +++ b/Documentation/filesystems/overlayfs.txt
> @@ -102,6 +102,23 @@ Only the lists of names from directories are merged. Other content
> such as metadata and extended attributes are reported for the upper
> directory only. These attributes of the lower directory are hidden.
>
> +credentials
> +-----------
> +
> +By default, all access to the upper, lower and work directories is the
> +recorded mounter's MAC and DAC credentials. The incoming accesses are
> +checked against the caller's credentials.
> +
> +override_creds mount flag turned off is reserved for when mounter and
> +caller MAC or DAC credentials do not overlap. Several unintended side
> +effects will occur. The caller with a lower privilege will not be
> +able to delete files or directories, create nodes, or search some
> +directories. The caller with higher privilege can perform unexpected
> +or unsecured operations. The uneven security model where upperdir
> +and workdir are opened at privilege, but accessed without, should only
> +be used with strict understanding of the side effects and of the
> +security policies.
> +
> whiteouts and opaque directories
> --------------------------------
>
> diff --git a/fs/overlayfs/copy_up.c b/fs/overlayfs/copy_up.c
> index 9e62dcf06fc4..dfab62ce7504 100644
> --- a/fs/overlayfs/copy_up.c
> +++ b/fs/overlayfs/copy_up.c
> @@ -860,7 +860,7 @@ int ovl_copy_up_flags(struct dentry *dentry, int flags)
> dput(parent);
> dput(next);
> }
> - revert_creds(old_cred);
> + ovl_revert_creds(old_cred);
>
> return err;
> }
> diff --git a/fs/overlayfs/dir.c b/fs/overlayfs/dir.c
> index c6289147c787..b7052e23c467 100644
> --- a/fs/overlayfs/dir.c
> +++ b/fs/overlayfs/dir.c
> @@ -566,7 +566,8 @@ static int ovl_create_or_link(struct dentry *dentry, struct inode *inode,
> override_cred->fsgid = inode->i_gid;
> if (!attr->hardlink) {
> err = security_dentry_create_files_as(dentry,
> - attr->mode, &dentry->d_name, old_cred,
> + attr->mode, &dentry->d_name,
> + old_cred ? old_cred : current_cred(),
> override_cred);
> if (err) {
> put_cred(override_cred);
> @@ -582,7 +583,7 @@ static int ovl_create_or_link(struct dentry *dentry, struct inode *inode,
> err = ovl_create_over_whiteout(dentry, inode, attr);
> }
> out_revert_creds:
> - revert_creds(old_cred);
> + ovl_revert_creds(old_cred);
> return err;
> }
>
> @@ -842,7 +843,7 @@ static int ovl_do_remove(struct dentry *dentry, bool is_dir)
> err = ovl_remove_upper(dentry, is_dir, &list);
> else
> err = ovl_remove_and_whiteout(dentry, &list);
> - revert_creds(old_cred);
> + ovl_revert_creds(old_cred);
> if (!err) {
> if (is_dir)
> clear_nlink(dentry->d_inode);
> @@ -1212,7 +1213,7 @@ static int ovl_rename(struct inode *olddir, struct dentry *old,
> out_unlock:
> unlock_rename(new_upperdir, old_upperdir);
> out_revert_creds:
> - revert_creds(old_cred);
> + ovl_revert_creds(old_cred);
> if (update_nlink)
> ovl_nlink_end(new);
> out_drop_write:
> diff --git a/fs/overlayfs/inode.c b/fs/overlayfs/inode.c
> index 6bcc9dedc342..192f5508ed45 100644
> --- a/fs/overlayfs/inode.c
> +++ b/fs/overlayfs/inode.c
> @@ -64,7 +64,7 @@ int ovl_setattr(struct dentry *dentry, struct iattr *attr)
> inode_lock(upperdentry->d_inode);
> old_cred = ovl_override_creds(dentry->d_sb);
> err = notify_change(upperdentry, attr, NULL);
> - revert_creds(old_cred);
> + ovl_revert_creds(old_cred);
> if (!err)
> ovl_copyattr(upperdentry->d_inode, dentry->d_inode);
> inode_unlock(upperdentry->d_inode);
> @@ -260,7 +260,7 @@ int ovl_getattr(const struct path *path, struct kstat *stat,
> stat->nlink = dentry->d_inode->i_nlink;
>
> out:
> - revert_creds(old_cred);
> + ovl_revert_creds(old_cred);
>
> return err;
> }
> @@ -303,7 +303,7 @@ int ovl_permission(struct inode *inode, int mask)
>
> old_cred = ovl_override_creds(inode->i_sb);
> err = inode_permission(realinode, mask);
> - revert_creds(old_cred);
> + ovl_revert_creds(old_cred);
>
> return err;
> }
> @@ -320,7 +320,7 @@ static const char *ovl_get_link(struct dentry *dentry,
>
> old_cred = ovl_override_creds(dentry->d_sb);
> p = vfs_get_link(ovl_dentry_real(dentry), done);
> - revert_creds(old_cred);
> + ovl_revert_creds(old_cred);
> return p;
> }
>
> @@ -363,7 +363,7 @@ int ovl_xattr_set(struct dentry *dentry, struct inode *inode, const char *name,
> WARN_ON(flags != XATTR_REPLACE);
> err = vfs_removexattr(realdentry, name);
> }
> - revert_creds(old_cred);
> + ovl_revert_creds(old_cred);
>
> /* copy c/mtime */
> ovl_copyattr(d_inode(realdentry), inode);
> @@ -384,7 +384,7 @@ int ovl_xattr_get(struct dentry *dentry, struct inode *inode, const char *name,
>
> old_cred = ovl_override_creds(dentry->d_sb);
> res = vfs_getxattr(realdentry, name, value, size);
> - revert_creds(old_cred);
> + ovl_revert_creds(old_cred);
> return res;
> }
>
> @@ -408,7 +408,7 @@ ssize_t ovl_listxattr(struct dentry *dentry, char *list, size_t size)
>
> old_cred = ovl_override_creds(dentry->d_sb);
> res = vfs_listxattr(realdentry, list, size);
> - revert_creds(old_cred);
> + ovl_revert_creds(old_cred);
> if (res <= 0 || size == 0)
> return res;
>
> @@ -443,7 +443,7 @@ struct posix_acl *ovl_get_acl(struct inode *inode, int type)
>
> old_cred = ovl_override_creds(inode->i_sb);
> acl = get_acl(realinode, type);
> - revert_creds(old_cred);
> + ovl_revert_creds(old_cred);
>
> return acl;
> }
> diff --git a/fs/overlayfs/namei.c b/fs/overlayfs/namei.c
> index 3ac9dc8f6cc0..c32fa8ed72e6 100644
> --- a/fs/overlayfs/namei.c
> +++ b/fs/overlayfs/namei.c
> @@ -1072,7 +1072,7 @@ struct dentry *ovl_lookup(struct inode *dir, struct dentry *dentry,
> goto out_free_oe;
> }
>
> - revert_creds(old_cred);
> + ovl_revert_creds(old_cred);
> if (origin_path) {
> dput(origin_path->dentry);
> kfree(origin_path);
> @@ -1099,7 +1099,7 @@ struct dentry *ovl_lookup(struct inode *dir, struct dentry *dentry,
> kfree(upperredirect);
> out:
> kfree(d.redirect);
> - revert_creds(old_cred);
> + ovl_revert_creds(old_cred);
> return ERR_PTR(err);
> }
>
> @@ -1153,7 +1153,7 @@ bool ovl_lower_positive(struct dentry *dentry)
> dput(this);
> }
> }
> - revert_creds(old_cred);
> + ovl_revert_creds(old_cred);
>
> return positive;
> }
> diff --git a/fs/overlayfs/overlayfs.h b/fs/overlayfs/overlayfs.h
> index 5e45cb3630a0..6f8b6f9ff357 100644
> --- a/fs/overlayfs/overlayfs.h
> +++ b/fs/overlayfs/overlayfs.h
> @@ -208,6 +208,7 @@ int ovl_want_write(struct dentry *dentry);
> void ovl_drop_write(struct dentry *dentry);
> struct dentry *ovl_workdir(struct dentry *dentry);
> const struct cred *ovl_override_creds(struct super_block *sb);
> +void ovl_revert_creds(const struct cred *oldcred);
> struct super_block *ovl_same_sb(struct super_block *sb);
> int ovl_can_decode_fh(struct super_block *sb);
> struct dentry *ovl_indexdir(struct super_block *sb);
> diff --git a/fs/overlayfs/ovl_entry.h b/fs/overlayfs/ovl_entry.h
> index ec237035333a..e38eea8104be 100644
> --- a/fs/overlayfs/ovl_entry.h
> +++ b/fs/overlayfs/ovl_entry.h
> @@ -20,6 +20,7 @@ struct ovl_config {
> bool nfs_export;
> int xino;
> bool metacopy;
> + bool override_creds;
> };
>
> struct ovl_sb {
> diff --git a/fs/overlayfs/readdir.c b/fs/overlayfs/readdir.c
> index cc8303a806b4..ec591b49e902 100644
> --- a/fs/overlayfs/readdir.c
> +++ b/fs/overlayfs/readdir.c
> @@ -289,7 +289,7 @@ static int ovl_check_whiteouts(struct dentry *dir, struct ovl_readdir_data *rdd)
> }
> inode_unlock(dir->d_inode);
> }
> - revert_creds(old_cred);
> + ovl_revert_creds(old_cred);
>
> return err;
> }
> @@ -921,7 +921,7 @@ int ovl_check_empty_dir(struct dentry *dentry, struct list_head *list)
>
> old_cred = ovl_override_creds(dentry->d_sb);
> err = ovl_dir_read_merged(dentry, list, &root);
> - revert_creds(old_cred);
> + ovl_revert_creds(old_cred);
> if (err)
> return err;
>
> diff --git a/fs/overlayfs/super.c b/fs/overlayfs/super.c
> index 0116735cc321..933829ca7d7d 100644
> --- a/fs/overlayfs/super.c
> +++ b/fs/overlayfs/super.c
> @@ -56,6 +56,11 @@ module_param_named(xino_auto, ovl_xino_auto_def, bool, 0644);
> MODULE_PARM_DESC(ovl_xino_auto_def,
> "Auto enable xino feature");
>
> +static bool __read_mostly ovl_override_creds_def = true;
> +module_param_named(override_creds, ovl_override_creds_def, bool, 0644);
> +MODULE_PARM_DESC(ovl_override_creds_def,
> + "Use mounter's credentials for accesses");
> +
> static void ovl_entry_stack_free(struct ovl_entry *oe)
> {
> unsigned int i;
> @@ -362,6 +367,9 @@ static int ovl_show_options(struct seq_file *m, struct dentry *dentry)
> if (ofs->config.metacopy != ovl_metacopy_def)
> seq_printf(m, ",metacopy=%s",
> ofs->config.metacopy ? "on" : "off");
> + if (ofs->config.override_creds != ovl_override_creds_def)
> + seq_show_option(m, "override_creds",
> + ofs->config.override_creds ? "on" : "off");
> return 0;
> }
>
> @@ -401,6 +409,8 @@ enum {
> OPT_XINO_AUTO,
> OPT_METACOPY_ON,
> OPT_METACOPY_OFF,
> + OPT_OVERRIDE_CREDS_ON,
> + OPT_OVERRIDE_CREDS_OFF,
> OPT_ERR,
> };
>
> @@ -419,6 +429,8 @@ static const match_table_t ovl_tokens = {
> {OPT_XINO_AUTO, "xino=auto"},
> {OPT_METACOPY_ON, "metacopy=on"},
> {OPT_METACOPY_OFF, "metacopy=off"},
> + {OPT_OVERRIDE_CREDS_ON, "override_creds=on"},
> + {OPT_OVERRIDE_CREDS_OFF, "override_creds=off"},
> {OPT_ERR, NULL}
> };
>
> @@ -477,6 +489,7 @@ static int ovl_parse_opt(char *opt, struct ovl_config *config)
> config->redirect_mode = kstrdup(ovl_redirect_mode_def(), GFP_KERNEL);
> if (!config->redirect_mode)
> return -ENOMEM;
> + config->override_creds = ovl_override_creds_def;
>
> while ((p = ovl_next_opt(&opt)) != NULL) {
> int token;
> @@ -557,6 +570,14 @@ static int ovl_parse_opt(char *opt, struct ovl_config *config)
> config->metacopy = false;
> break;
>
> + case OPT_OVERRIDE_CREDS_ON:
> + config->override_creds = true;
> + break;
> +
> + case OPT_OVERRIDE_CREDS_OFF:
> + config->override_creds = false;
> + break;
> +
> default:
> pr_err("overlayfs: unrecognized mount option \"%s\" or missing value\n", p);
> return -EINVAL;
> @@ -1549,7 +1570,6 @@ static int ovl_fill_super(struct super_block *sb, void *data, int silent)
> ovl_dentry_lower(root_dentry), NULL);
>
> sb->s_root = root_dentry;
> -
> return 0;
>
> out_free_oe:
> diff --git a/fs/overlayfs/util.c b/fs/overlayfs/util.c
> index 7c01327b1852..484d7f76ac9c 100644
> --- a/fs/overlayfs/util.c
> +++ b/fs/overlayfs/util.c
> @@ -40,9 +40,17 @@ const struct cred *ovl_override_creds(struct super_block *sb)
> {
> struct ovl_fs *ofs = sb->s_fs_info;
>
> + if (!ofs->config.override_creds)
> + return NULL;
> return override_creds(ofs->creator_cred);
> }
>
> +void ovl_revert_creds(const struct cred *old_cred)
> +{
> + if (old_cred)
> + revert_creds(old_cred);
> +}
> +
> struct super_block *ovl_same_sb(struct super_block *sb)
> {
> struct ovl_fs *ofs = sb->s_fs_info;
> @@ -782,7 +790,7 @@ int ovl_nlink_start(struct dentry *dentry)
> * value relative to the upper inode nlink in an upper inode xattr.
> */
> err = ovl_set_nlink_upper(dentry);
> - revert_creds(old_cred);
> + ovl_revert_creds(old_cred);
>
> out:
> if (err)
> @@ -800,7 +808,7 @@ void ovl_nlink_end(struct dentry *dentry)
>
> old_cred = ovl_override_creds(dentry->d_sb);
> ovl_cleanup_index(dentry);
> - revert_creds(old_cred);
> + ovl_revert_creds(old_cred);
> }
>
> ovl_inode_unlock(inode);
> --
> 2.19.1.930.g4563a0d9d0-goog
>