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Date: Thu, 25 Jul 2019 09:14:40 +0300
From: Amir Goldstein <amir73il@...il.com>
To: Mark Salyzyn <salyzyn@...roid.com>
Cc: linux-kernel <linux-kernel@...r.kernel.org>,
kernel-team@...roid.com, Miklos Szeredi <miklos@...redi.hu>,
Jonathan Corbet <corbet@....net>,
Vivek Goyal <vgoyal@...hat.com>,
"Eric W . Biederman" <ebiederm@...ssion.com>,
Randy Dunlap <rdunlap@...radead.org>,
Stephen Smalley <sds@...ho.nsa.gov>,
overlayfs <linux-unionfs@...r.kernel.org>,
linux-doc@...r.kernel.org
Subject: Re: [PATCH v10 5/5] overlayfs: override_creds=off option bypass creator_cred
On Wed, Jul 24, 2019 at 10:57 PM Mark Salyzyn <salyzyn@...roid.com> 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.
>
> 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 4.6 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). The caveats are not a problem in
> the Android usage model, however they should be fixed for
> completeness and for general use in time.
>
> Signed-off-by: Mark Salyzyn <salyzyn@...roid.com>
> Cc: Miklos Szeredi <miklos@...redi.hu>
> Cc: Jonathan Corbet <corbet@....net>
> Cc: Vivek Goyal <vgoyal@...hat.com>
> Cc: Eric W. Biederman <ebiederm@...ssion.com>
> Cc: Amir Goldstein <amir73il@...il.com>
> Cc: Randy Dunlap <rdunlap@...radead.org>
> Cc: Stephen Smalley <sds@...ho.nsa.gov>
> Cc: linux-unionfs@...r.kernel.org
> Cc: linux-doc@...r.kernel.org
> Cc: linux-kernel@...r.kernel.org
> Cc: kernel-team@...roid.com
> ---
> v10:
> - Rebase (and expand because of increased revert_cred usage)
>
> v9:
> - Add to the caveats
>
> 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.
>
> a
> ---
> Documentation/filesystems/overlayfs.txt | 23 +++++++++++++++++++++++
> fs/overlayfs/copy_up.c | 2 +-
> fs/overlayfs/dir.c | 11 ++++++-----
> fs/overlayfs/file.c | 20 ++++++++++----------
> fs/overlayfs/inode.c | 18 +++++++++---------
> 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 ++++++++++--
> 11 files changed, 87 insertions(+), 33 deletions(-)
>
> diff --git a/Documentation/filesystems/overlayfs.txt b/Documentation/filesystems/overlayfs.txt
> index 1da2f1668f08..d48125076602 100644
> --- a/Documentation/filesystems/overlayfs.txt
> +++ b/Documentation/filesystems/overlayfs.txt
> @@ -102,6 +102,29 @@ 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.
> +
> +In the case where caller MAC or DAC credentials do not overlap, a
> +use case available in older versions of the driver, the
> +override_creds mount flag can be turned off and help when the use
> +pattern has caller with legitimate credentials where the mounter
> +does not. Several unintended side effects will occur though. The
> +caller without certain key capabilities or lower privilege will not
> +always be able to delete files or directories, create nodes, or
> +search some restricted directories. The ability to search and read
> +a directory entry is spotty as a result of the cache mechanism not
> +retesting the credentials because of the assumption, a privileged
> +caller can fill cache, then a lower privilege can read the directory
> +cache. The uneven security model where cache, upperdir and workdir
> +are opened at privilege, but accessed without creating a form of
> +privilege escalation, 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 b801c6353100..1311ab4aea00 100644
> --- a/fs/overlayfs/copy_up.c
> +++ b/fs/overlayfs/copy_up.c
> @@ -886,7 +886,7 @@ 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 702aa63f6774..c4b061c3a6ef 100644
> --- a/fs/overlayfs/dir.c
> +++ b/fs/overlayfs/dir.c
> @@ -563,7 +563,8 @@ 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);
> @@ -579,7 +580,7 @@ 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;
> }
>
> @@ -655,7 +656,7 @@ static int ovl_set_link_redirect(struct dentry *dentry)
>
> old_cred = ovl_override_creds(dentry->d_sb);
> err = ovl_set_redirect(dentry, false);
> - revert_creds(old_cred);
> + ovl_revert_creds(old_cred);
>
> return err;
> }
> @@ -851,7 +852,7 @@ 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);
> @@ -1221,7 +1222,7 @@ 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/file.c b/fs/overlayfs/file.c
> index e235a635d9ec..39a50fad9f7f 100644
> --- a/fs/overlayfs/file.c
> +++ b/fs/overlayfs/file.c
> @@ -32,7 +32,7 @@ static struct file *ovl_open_realfile(const struct file *file,
> old_cred = ovl_override_creds(inode->i_sb);
> realfile = open_with_fake_path(&file->f_path, flags, realinode,
> current_cred());
> - revert_creds(old_cred);
> + ovl_revert_creds(old_cred);
>
> pr_debug("open(%p[%pD2/%c], 0%o) -> (%p, 0%o)\n",
> file, file, ovl_whatisit(inode, realinode), file->f_flags,
> @@ -176,7 +176,7 @@ static loff_t ovl_llseek(struct file *file, loff_t offset, int whence)
>
> old_cred = ovl_override_creds(inode->i_sb);
> ret = vfs_llseek(real.file, offset, whence);
> - revert_creds(old_cred);
> + ovl_revert_creds(old_cred);
>
> file->f_pos = real.file->f_pos;
> inode_unlock(inode);
> @@ -242,7 +242,7 @@ static ssize_t ovl_read_iter(struct kiocb *iocb, struct iov_iter *iter)
> old_cred = ovl_override_creds(file_inode(file)->i_sb);
> ret = vfs_iter_read(real.file, iter, &iocb->ki_pos,
> ovl_iocb_to_rwf(iocb));
> - revert_creds(old_cred);
> + ovl_revert_creds(old_cred);
>
> ovl_file_accessed(file);
>
> @@ -278,7 +278,7 @@ static ssize_t ovl_write_iter(struct kiocb *iocb, struct iov_iter *iter)
> ret = vfs_iter_write(real.file, iter, &iocb->ki_pos,
> ovl_iocb_to_rwf(iocb));
> file_end_write(real.file);
> - revert_creds(old_cred);
> + ovl_revert_creds(old_cred);
>
> /* Update size */
> ovl_copyattr(ovl_inode_real(inode), inode);
> @@ -305,7 +305,7 @@ static int ovl_fsync(struct file *file, loff_t start, loff_t end, int datasync)
> if (file_inode(real.file) == ovl_inode_upper(file_inode(file))) {
> old_cred = ovl_override_creds(file_inode(file)->i_sb);
> ret = vfs_fsync_range(real.file, start, end, datasync);
> - revert_creds(old_cred);
> + ovl_revert_creds(old_cred);
> }
>
> fdput(real);
> @@ -329,7 +329,7 @@ static int ovl_mmap(struct file *file, struct vm_area_struct *vma)
>
> old_cred = ovl_override_creds(file_inode(file)->i_sb);
> ret = call_mmap(vma->vm_file, vma);
> - revert_creds(old_cred);
> + ovl_revert_creds(old_cred);
>
> if (ret) {
> /* Drop reference count from new vm_file value */
> @@ -357,7 +357,7 @@ static long ovl_fallocate(struct file *file, int mode, loff_t offset, loff_t len
>
> old_cred = ovl_override_creds(file_inode(file)->i_sb);
> ret = vfs_fallocate(real.file, mode, offset, len);
> - revert_creds(old_cred);
> + ovl_revert_creds(old_cred);
>
> /* Update size */
> ovl_copyattr(ovl_inode_real(inode), inode);
> @@ -379,7 +379,7 @@ static int ovl_fadvise(struct file *file, loff_t offset, loff_t len, int advice)
>
> old_cred = ovl_override_creds(file_inode(file)->i_sb);
> ret = vfs_fadvise(real.file, offset, len, advice);
> - revert_creds(old_cred);
> + ovl_revert_creds(old_cred);
>
> fdput(real);
>
> @@ -399,7 +399,7 @@ static long ovl_real_ioctl(struct file *file, unsigned int cmd,
>
> old_cred = ovl_override_creds(file_inode(file)->i_sb);
> ret = vfs_ioctl(real.file, cmd, arg);
> - revert_creds(old_cred);
> + ovl_revert_creds(old_cred);
>
> fdput(real);
>
> @@ -589,7 +589,7 @@ static loff_t ovl_copyfile(struct file *file_in, loff_t pos_in,
> flags);
> break;
> }
> - revert_creds(old_cred);
> + ovl_revert_creds(old_cred);
>
> /* Update size */
> ovl_copyattr(ovl_inode_real(inode_out), inode_out);
> diff --git a/fs/overlayfs/inode.c b/fs/overlayfs/inode.c
> index d3b53849615c..6c11c7af5157 100644
> --- a/fs/overlayfs/inode.c
> +++ b/fs/overlayfs/inode.c
> @@ -61,7 +61,7 @@ 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);
> @@ -257,7 +257,7 @@ 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;
> }
> @@ -291,7 +291,7 @@ int ovl_permission(struct inode *inode, int mask)
> mask |= MAY_READ;
> }
> err = inode_permission(realinode, mask);
> - revert_creds(old_cred);
> + ovl_revert_creds(old_cred);
>
> return err;
> }
> @@ -308,7 +308,7 @@ 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;
> }
>
> @@ -351,7 +351,7 @@ 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);
> @@ -387,7 +387,7 @@ 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;
> }
>
> @@ -411,7 +411,7 @@ 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;
>
> @@ -446,7 +446,7 @@ 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;
> }
> @@ -484,7 +484,7 @@ static int ovl_fiemap(struct inode *inode, struct fiemap_extent_info *fieinfo,
> filemap_write_and_wait(realinode->i_mapping);
>
> err = realinode->i_op->fiemap(realinode, fieinfo, start, len);
> - revert_creds(old_cred);
> + ovl_revert_creds(old_cred);
>
> return err;
> }
> diff --git a/fs/overlayfs/namei.c b/fs/overlayfs/namei.c
> index fb6c0cd7b65f..12627018b00a 100644
> --- a/fs/overlayfs/namei.c
> +++ b/fs/overlayfs/namei.c
> @@ -1079,7 +1079,7 @@ 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);
> @@ -1106,7 +1106,7 @@ 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);
> }
>
> @@ -1160,7 +1160,7 @@ 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 82574684a9b6..cdbdb533d3bd 100644
> --- a/fs/overlayfs/overlayfs.h
> +++ b/fs/overlayfs/overlayfs.h
> @@ -205,6 +205,7 @@ 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);
> ssize_t ovl_vfs_getxattr(struct dentry *dentry, const char *name, void *buf,
> size_t size);
> struct super_block *ovl_same_sb(struct super_block *sb);
> diff --git a/fs/overlayfs/ovl_entry.h b/fs/overlayfs/ovl_entry.h
> index 28a2d12a1029..2637c5aadf7f 100644
> --- a/fs/overlayfs/ovl_entry.h
> +++ b/fs/overlayfs/ovl_entry.h
> @@ -17,6 +17,7 @@ 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 47a91c9733a5..f31ef39e5afa 100644
> --- a/fs/overlayfs/readdir.c
> +++ b/fs/overlayfs/readdir.c
> @@ -286,7 +286,7 @@ 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;
> }
> @@ -918,7 +918,7 @@ 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 82e1130de206..c2ddce5d488c 100644
> --- a/fs/overlayfs/super.c
> +++ b/fs/overlayfs/super.c
> @@ -53,6 +53,11 @@ module_param_named(xino_auto, ovl_xino_auto_def, bool, 0644);
> MODULE_PARM_DESC(xino_auto,
> "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;
> }
>
> @@ -402,6 +410,8 @@ enum {
> OPT_XINO_AUTO,
> OPT_METACOPY_ON,
> OPT_METACOPY_OFF,
> + OPT_OVERRIDE_CREDS_ON,
> + OPT_OVERRIDE_CREDS_OFF,
> OPT_ERR,
> };
>
> @@ -420,6 +430,8 @@ 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}
> };
>
> @@ -478,6 +490,7 @@ 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;
> @@ -558,6 +571,14 @@ 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;
> @@ -1690,7 +1711,6 @@ 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 672459c3cff7..320aad599bcd 100644
> --- a/fs/overlayfs/util.c
> +++ b/fs/overlayfs/util.c
> @@ -37,9 +37,17 @@ 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);
> +}
> +
Mark,
Not sure if you have seen my "shutdown" patches:
https://lore.kernel.org/linux-fsdevel/20190715133839.9878-4-amir73il@gmail.com/
I am fine with this patch, but would like to request that you add @sb arg
to the ovl_revert_creds() helper, so it is more useful for other things in the
future that scope the underlying layers access (like shutdown).
Thanks,
Amir.
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