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

Some random things. Not sure what's the correct answer. It might not
even be a issue, just trying to think loud.

- ovl_permission() does not do the check for permission on underlying
inode if only MAY_EXEC is being asked for. This kind of sounds like
a problem. That means one can create an overlay mount with context=<foo>
and allow a process to execute a file which it could not execute
outside overlay mount. If this is an issue, it probably is an issue
both with override_creds=on/off.

- ovl_permission() does not check for permission on underlying inode
for special file. Is it a problem where one can not do an operation
on special device on host but can do it through overlay context
mount.

- What about creds for copy up. ovl_prep_cu_creds(). Looks like even
with override_creds=off, we will be switching to the creds as returned
by security_inode_copy_up(). This basically sets ->create_sid if
it is a context mount so that new inode gets created with same
label as context=<label>. I was worried about being able to create
files as context=label (while mounter itself probaly could not do
that). Dan Walsh mentioned that it might not be an issue because
even if ->create_sid has been set, selinux will still check whether
caller is allowed to create a file with that label in target dir
or not.

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@android.com>
> Cc: Miklos Szeredi <miklos@szeredi.hu>
> Cc: Jonathan Corbet <corbet@lwn.net>
> Cc: Vivek Goyal <vgoyal@redhat.com>
> Cc: Eric W. Biederman <ebiederm@xmission.com>
> Cc: Amir Goldstein <amir73il@gmail.com>
> Cc: Randy Dunlap <rdunlap@infradead.org>
> Cc: Stephen Smalley <sds@tycho.nsa.gov>
> Cc: linux-unionfs@vger.kernel.org
> Cc: linux-doc@vger.kernel.org
> Cc: linux-kernel@vger.kernel.org
> Cc: kernel-team@android.com
> ---
> 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
>

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