ANDROID: overlayfs: override_creds=off option bypass creator_cred
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@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
---
---
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.
Signed-off-by: Mark Salyzyn <salyzyn@google.com>
(cherry picked from https://lore.kernel.org/patchwork/patch/1009299)
Bug: 109821005
Bug: 112955896
Bug: 126256072
Bug: 127298877
Change-Id: Ie43b0c7dfd64f4cfc27dfe5e1622ea01f3b000cf
diff --git a/Documentation/filesystems/overlayfs.txt b/Documentation/filesystems/overlayfs.txt
index 2809145..771bb22 100644
--- a/Documentation/filesystems/overlayfs.txt
+++ b/Documentation/filesystems/overlayfs.txt
@@ -82,6 +82,29 @@
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 64c5386..d49ac2a 100644
--- a/fs/overlayfs/copy_up.c
+++ b/fs/overlayfs/copy_up.c
@@ -357,7 +357,7 @@
}
out_unlock:
unlock_rename(workdir, upperdir);
- revert_creds(old_cred);
+ ovl_revert_creds(old_cred);
if (link)
free_page((unsigned long) link);
diff --git a/fs/overlayfs/dir.c b/fs/overlayfs/dir.c
index f8aa542..fd71597 100644
--- a/fs/overlayfs/dir.c
+++ b/fs/overlayfs/dir.c
@@ -414,7 +414,7 @@
err = ovl_create_over_whiteout(dentry, inode, &stat, link,
hardlink);
- revert_creds(old_cred);
+ ovl_revert_creds(old_cred);
}
if (!err)
@@ -648,7 +648,7 @@
err = ovl_remove_and_whiteout(dentry, is_dir);
- revert_creds(old_cred);
+ ovl_revert_creds(old_cred);
}
out_drop_write:
ovl_drop_write(dentry);
@@ -887,8 +887,7 @@
out_unlock:
unlock_rename(new_upperdir, old_upperdir);
out_revert_creds:
- if (old_opaque || new_opaque)
- revert_creds(old_cred);
+ ovl_revert_creds(old_cred);
out_drop_write:
ovl_drop_write(old);
out:
diff --git a/fs/overlayfs/overlayfs.h b/fs/overlayfs/overlayfs.h
index 27a4297..0723b8f 100644
--- a/fs/overlayfs/overlayfs.h
+++ b/fs/overlayfs/overlayfs.h
@@ -151,6 +151,7 @@
void ovl_dentry_set_opaque(struct dentry *dentry, bool opaque);
bool ovl_is_whiteout(struct dentry *dentry);
const struct cred *ovl_override_creds(struct super_block *sb);
+void ovl_revert_creds(const struct cred *oldcred);
void ovl_dentry_update(struct dentry *dentry, struct dentry *upperdentry);
struct dentry *ovl_lookup(struct inode *dir, struct dentry *dentry,
unsigned int flags);
diff --git a/fs/overlayfs/readdir.c b/fs/overlayfs/readdir.c
index da999e7..9ff8f81 100644
--- a/fs/overlayfs/readdir.c
+++ b/fs/overlayfs/readdir.c
@@ -223,7 +223,7 @@
}
mutex_unlock(&dir->d_inode->i_mutex);
}
- revert_creds(old_cred);
+ ovl_revert_creds(old_cred);
return err;
}
diff --git a/fs/overlayfs/super.c b/fs/overlayfs/super.c
index fa20c95..48f7c1a 100644
--- a/fs/overlayfs/super.c
+++ b/fs/overlayfs/super.c
@@ -31,6 +31,7 @@
char *lowerdir;
char *upperdir;
char *workdir;
+ bool override_creds;
};
/* private information held for overlayfs's superblock */
@@ -252,9 +253,17 @@
{
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);
+}
+
static bool ovl_is_opaquedir(struct dentry *dentry)
{
int res;
@@ -626,6 +635,11 @@
return err;
}
+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");
+
/**
* ovl_show_options
*
@@ -642,6 +656,9 @@
seq_show_option(m, "upperdir", ufs->config.upperdir);
seq_show_option(m, "workdir", ufs->config.workdir);
}
+ if (ufs->config.override_creds != ovl_override_creds_def)
+ seq_show_option(m, "override_creds",
+ ufs->config.override_creds ? "on" : "off");
return 0;
}
@@ -666,6 +683,8 @@
OPT_LOWERDIR,
OPT_UPPERDIR,
OPT_WORKDIR,
+ OPT_OVERRIDE_CREDS_ON,
+ OPT_OVERRIDE_CREDS_OFF,
OPT_ERR,
};
@@ -673,6 +692,8 @@
{OPT_LOWERDIR, "lowerdir=%s"},
{OPT_UPPERDIR, "upperdir=%s"},
{OPT_WORKDIR, "workdir=%s"},
+ {OPT_OVERRIDE_CREDS_ON, "override_creds=on"},
+ {OPT_OVERRIDE_CREDS_OFF, "override_creds=off"},
{OPT_ERR, NULL}
};
@@ -703,6 +724,7 @@
{
char *p;
+ config->override_creds = ovl_override_creds_def;
while ((p = ovl_next_opt(&opt)) != NULL) {
int token;
substring_t args[MAX_OPT_ARGS];
@@ -733,6 +755,14 @@
return -ENOMEM;
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;
