[v14,4/5] overlayfs: internal getxattr operations without sepolicy checking
diff mbox series

Message ID 20191022204453.97058-5-salyzyn@android.com
State New
Headers show
Series
  • Untitled series #191765
Related show

Commit Message

Mark Salyzyn Oct. 22, 2019, 8:44 p.m. UTC
Check impure, opaque, origin & meta xattr with no sepolicy audit
(using __vfs_getxattr) since these operations are internal to
overlayfs operations and do not disclose any data.  This became
an issue for credential override off since sys_admin would have
been required by the caller; whereas would have been inherently
present for the creator since it performed the mount.

This is a change in operations since we do not check in the new
ovl_do_vfs_getxattr function if the credential override is off or
not.  Reasoning is that the sepolicy check is unnecessary overhead,
especially since the check can be expensive.

Because for override credentials off, this affects _everyone_ that
underneath performs private xattr calls without the appropriate
sepolicy permissions and sys_admin capability.  Providing blanket
support for sys_admin would be bad for all possible callers.

For the override credentials on, this will affect only the mounter,
should it lack sepolicy permissions. Not considered a security
problem since mounting by definition has sys_admin capabilities,
but sepolicy contexts would still need to be crafted.

It should be noted that there is precedence, __vfs_getxattr is used
in other filesystems for their own internal trusted xattr management.

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
Cc: linux-security-module@vger.kernel.org

---
v14 - rebase to use xattr_gs_args.

v13 - rebase to use __vfs_getxattr flags option

v12 - rebase

v11 - switch name to ovl_do_vfs_getxattr, fortify comment

v10 - added to patch series

---
 fs/overlayfs/namei.c     | 12 +++++++-----
 fs/overlayfs/overlayfs.h |  2 ++
 fs/overlayfs/util.c      | 32 +++++++++++++++++++++++---------
 3 files changed, 32 insertions(+), 14 deletions(-)

Comments

Amir Goldstein Oct. 23, 2019, 6:39 a.m. UTC | #1
On Tue, Oct 22, 2019 at 11:46 PM Mark Salyzyn <salyzyn@android.com> wrote:
>
> Check impure, opaque, origin & meta xattr with no sepolicy audit
> (using __vfs_getxattr) since these operations are internal to
> overlayfs operations and do not disclose any data.  This became
> an issue for credential override off since sys_admin would have
> been required by the caller; whereas would have been inherently
> present for the creator since it performed the mount.
>
> This is a change in operations since we do not check in the new
> ovl_do_vfs_getxattr function if the credential override is off or
> not.  Reasoning is that the sepolicy check is unnecessary overhead,
> especially since the check can be expensive.
>
> Because for override credentials off, this affects _everyone_ that
> underneath performs private xattr calls without the appropriate
> sepolicy permissions and sys_admin capability.  Providing blanket
> support for sys_admin would be bad for all possible callers.
>
> For the override credentials on, this will affect only the mounter,
> should it lack sepolicy permissions. Not considered a security
> problem since mounting by definition has sys_admin capabilities,
> but sepolicy contexts would still need to be crafted.
>

It sounds reasonable to me, but I am not a "security person".

> It should be noted that there is precedence, __vfs_getxattr is used
> in other filesystems for their own internal trusted xattr management.
>

Urgh! "other" filesystems meaning ecryptfs_getxattr()?
That looks like a loop hole to read any trusted xattr without any
security checks. Not sure its a good example...

> 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
> Cc: linux-security-module@vger.kernel.org
>
> ---
> v14 - rebase to use xattr_gs_args.
>
> v13 - rebase to use __vfs_getxattr flags option
>
> v12 - rebase
>
> v11 - switch name to ovl_do_vfs_getxattr, fortify comment
>
> v10 - added to patch series
>
> ---
>  fs/overlayfs/namei.c     | 12 +++++++-----
>  fs/overlayfs/overlayfs.h |  2 ++
>  fs/overlayfs/util.c      | 32 +++++++++++++++++++++++---------
>  3 files changed, 32 insertions(+), 14 deletions(-)
>
> diff --git a/fs/overlayfs/namei.c b/fs/overlayfs/namei.c
> index 9702f0d5309d..a4a452c489fa 100644
> --- a/fs/overlayfs/namei.c
> +++ b/fs/overlayfs/namei.c
> @@ -106,10 +106,11 @@ int ovl_check_fh_len(struct ovl_fh *fh, int fh_len)
>
>  static struct ovl_fh *ovl_get_fh(struct dentry *dentry, const char *name)
>  {
> -       int res, err;
> +       ssize_t res;
> +       int err;
>         struct ovl_fh *fh = NULL;
>
> -       res = vfs_getxattr(dentry, name, NULL, 0);
> +       res = ovl_do_vfs_getxattr(dentry, name, NULL, 0);
>         if (res < 0) {
>                 if (res == -ENODATA || res == -EOPNOTSUPP)
>                         return NULL;
> @@ -123,7 +124,7 @@ static struct ovl_fh *ovl_get_fh(struct dentry *dentry, const char *name)
>         if (!fh)
>                 return ERR_PTR(-ENOMEM);
>
> -       res = vfs_getxattr(dentry, name, fh, res);
> +       res = ovl_do_vfs_getxattr(dentry, name, fh, res);
>         if (res < 0)
>                 goto fail;
>
> @@ -141,10 +142,11 @@ static struct ovl_fh *ovl_get_fh(struct dentry *dentry, const char *name)
>         return NULL;
>
>  fail:
> -       pr_warn_ratelimited("overlayfs: failed to get origin (%i)\n", res);
> +       pr_warn_ratelimited("overlayfs: failed to get origin (%zi)\n", res);
>         goto out;
>  invalid:
> -       pr_warn_ratelimited("overlayfs: invalid origin (%*phN)\n", res, fh);
> +       pr_warn_ratelimited("overlayfs: invalid origin (%*phN)\n",
> +                           (int)res, fh);
>         goto out;
>  }
>
> diff --git a/fs/overlayfs/overlayfs.h b/fs/overlayfs/overlayfs.h
> index c6a8ec049099..72762642b247 100644
> --- a/fs/overlayfs/overlayfs.h
> +++ b/fs/overlayfs/overlayfs.h
> @@ -205,6 +205,8 @@ 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);
> +ssize_t ovl_do_vfs_getxattr(struct dentry *dentry, const char *name, void *buf,
> +                           size_t size);
>  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/util.c b/fs/overlayfs/util.c
> index f5678a3f8350..bed12aed902c 100644
> --- a/fs/overlayfs/util.c
> +++ b/fs/overlayfs/util.c
> @@ -40,6 +40,20 @@ const struct cred *ovl_override_creds(struct super_block *sb)
>         return override_creds(ofs->creator_cred);
>  }
>
> +ssize_t ovl_do_vfs_getxattr(struct dentry *dentry, const char *name, void *buf,
> +                           size_t size)
> +{
> +       struct xattr_gs_args args = {};
> +
> +       args.dentry = dentry;
> +       args.inode = d_inode(dentry);
> +       args.name = name;
> +       args.buffer = buf;
> +       args.size = size;
> +       args.flags = XATTR_NOSECURITY;
> +       return __vfs_getxattr(&args);
> +}
> +

We do not understand each other.
I commented on this several times.
please put the wrapper helper ovl_do_getxattr() in overlayfs.h
next to the other ovl_do_ wrapper helpers and add pr_debug()
as all other wrappers have.

Thanks,
Amir.
Mark Salyzyn Nov. 4, 2019, 9:47 p.m. UTC | #2
On 10/22/19 11:39 PM, Amir Goldstein wrote:
> On Tue, Oct 22, 2019 at 11:46 PM Mark Salyzyn <salyzyn@android.com> wrote:
>> Check impure, opaque, origin & meta xattr with no sepolicy audit
>> (using __vfs_getxattr) since these operations are internal to
>> overlayfs operations and do not disclose any data.  This became
>> an issue for credential override off since sys_admin would have
>> been required by the caller; whereas would have been inherently
>> present for the creator since it performed the mount.
>>
>> This is a change in operations since we do not check in the new
>> ovl_do_vfs_getxattr function if the credential override is off or
>> not.  Reasoning is that the sepolicy check is unnecessary overhead,
>> especially since the check can be expensive.
>>
>> Because for override credentials off, this affects _everyone_ that
>> underneath performs private xattr calls without the appropriate
>> sepolicy permissions and sys_admin capability.  Providing blanket
>> support for sys_admin would be bad for all possible callers.
>>
>> For the override credentials on, this will affect only the mounter,
>> should it lack sepolicy permissions. Not considered a security
>> problem since mounting by definition has sys_admin capabilities,
>> but sepolicy contexts would still need to be crafted.
>>
> It sounds reasonable to me, but I am not a "security person".
>
>> It should be noted that there is precedence, __vfs_getxattr is used
>> in other filesystems for their own internal trusted xattr management.
>>
> Urgh! "other" filesystems meaning ecryptfs_getxattr()?
> That looks like a loop hole to read any trusted xattr without any
> security checks. Not sure its a good example...

Yes. But it also makes sense since ecryptfs_getxattr is performed inside 
a layer where the security check is done above by the filesystem that 
called it (AFAIK)? This is used by the filesystem, or the security 
layers to pull the actual sepolicy, rather than getting an EPERM and no 
data. The xattr 'data' is needed by the internal layers.

-- Mark

Patch
diff mbox series

diff --git a/fs/overlayfs/namei.c b/fs/overlayfs/namei.c
index 9702f0d5309d..a4a452c489fa 100644
--- a/fs/overlayfs/namei.c
+++ b/fs/overlayfs/namei.c
@@ -106,10 +106,11 @@  int ovl_check_fh_len(struct ovl_fh *fh, int fh_len)
 
 static struct ovl_fh *ovl_get_fh(struct dentry *dentry, const char *name)
 {
-	int res, err;
+	ssize_t res;
+	int err;
 	struct ovl_fh *fh = NULL;
 
-	res = vfs_getxattr(dentry, name, NULL, 0);
+	res = ovl_do_vfs_getxattr(dentry, name, NULL, 0);
 	if (res < 0) {
 		if (res == -ENODATA || res == -EOPNOTSUPP)
 			return NULL;
@@ -123,7 +124,7 @@  static struct ovl_fh *ovl_get_fh(struct dentry *dentry, const char *name)
 	if (!fh)
 		return ERR_PTR(-ENOMEM);
 
-	res = vfs_getxattr(dentry, name, fh, res);
+	res = ovl_do_vfs_getxattr(dentry, name, fh, res);
 	if (res < 0)
 		goto fail;
 
@@ -141,10 +142,11 @@  static struct ovl_fh *ovl_get_fh(struct dentry *dentry, const char *name)
 	return NULL;
 
 fail:
-	pr_warn_ratelimited("overlayfs: failed to get origin (%i)\n", res);
+	pr_warn_ratelimited("overlayfs: failed to get origin (%zi)\n", res);
 	goto out;
 invalid:
-	pr_warn_ratelimited("overlayfs: invalid origin (%*phN)\n", res, fh);
+	pr_warn_ratelimited("overlayfs: invalid origin (%*phN)\n",
+			    (int)res, fh);
 	goto out;
 }
 
diff --git a/fs/overlayfs/overlayfs.h b/fs/overlayfs/overlayfs.h
index c6a8ec049099..72762642b247 100644
--- a/fs/overlayfs/overlayfs.h
+++ b/fs/overlayfs/overlayfs.h
@@ -205,6 +205,8 @@  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);
+ssize_t ovl_do_vfs_getxattr(struct dentry *dentry, const char *name, void *buf,
+			    size_t size);
 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/util.c b/fs/overlayfs/util.c
index f5678a3f8350..bed12aed902c 100644
--- a/fs/overlayfs/util.c
+++ b/fs/overlayfs/util.c
@@ -40,6 +40,20 @@  const struct cred *ovl_override_creds(struct super_block *sb)
 	return override_creds(ofs->creator_cred);
 }
 
+ssize_t ovl_do_vfs_getxattr(struct dentry *dentry, const char *name, void *buf,
+			    size_t size)
+{
+	struct xattr_gs_args args = {};
+
+	args.dentry = dentry;
+	args.inode = d_inode(dentry);
+	args.name = name;
+	args.buffer = buf;
+	args.size = size;
+	args.flags = XATTR_NOSECURITY;
+	return __vfs_getxattr(&args);
+}
+
 struct super_block *ovl_same_sb(struct super_block *sb)
 {
 	struct ovl_fs *ofs = sb->s_fs_info;
@@ -537,9 +551,9 @@  void ovl_copy_up_end(struct dentry *dentry)
 
 bool ovl_check_origin_xattr(struct dentry *dentry)
 {
-	int res;
+	ssize_t res;
 
-	res = vfs_getxattr(dentry, OVL_XATTR_ORIGIN, NULL, 0);
+	res = ovl_do_vfs_getxattr(dentry, OVL_XATTR_ORIGIN, NULL, 0);
 
 	/* Zero size value means "copied up but origin unknown" */
 	if (res >= 0)
@@ -550,13 +564,13 @@  bool ovl_check_origin_xattr(struct dentry *dentry)
 
 bool ovl_check_dir_xattr(struct dentry *dentry, const char *name)
 {
-	int res;
+	ssize_t res;
 	char val;
 
 	if (!d_is_dir(dentry))
 		return false;
 
-	res = vfs_getxattr(dentry, name, &val, 1);
+	res = ovl_do_vfs_getxattr(dentry, name, &val, 1);
 	if (res == 1 && val == 'y')
 		return true;
 
@@ -837,13 +851,13 @@  int ovl_lock_rename_workdir(struct dentry *workdir, struct dentry *upperdir)
 /* err < 0, 0 if no metacopy xattr, 1 if metacopy xattr found */
 int ovl_check_metacopy_xattr(struct dentry *dentry)
 {
-	int res;
+	ssize_t res;
 
 	/* Only regular files can have metacopy xattr */
 	if (!S_ISREG(d_inode(dentry)->i_mode))
 		return 0;
 
-	res = vfs_getxattr(dentry, OVL_XATTR_METACOPY, NULL, 0);
+	res = ovl_do_vfs_getxattr(dentry, OVL_XATTR_METACOPY, NULL, 0);
 	if (res < 0) {
 		if (res == -ENODATA || res == -EOPNOTSUPP)
 			return 0;
@@ -852,7 +866,7 @@  int ovl_check_metacopy_xattr(struct dentry *dentry)
 
 	return 1;
 out:
-	pr_warn_ratelimited("overlayfs: failed to get metacopy (%i)\n", res);
+	pr_warn_ratelimited("overlayfs: failed to get metacopy (%zi)\n", res);
 	return res;
 }
 
@@ -878,7 +892,7 @@  ssize_t ovl_getxattr(struct dentry *dentry, char *name, char **value,
 	ssize_t res;
 	char *buf = NULL;
 
-	res = vfs_getxattr(dentry, name, NULL, 0);
+	res = ovl_do_vfs_getxattr(dentry, name, NULL, 0);
 	if (res < 0) {
 		if (res == -ENODATA || res == -EOPNOTSUPP)
 			return -ENODATA;
@@ -890,7 +904,7 @@  ssize_t ovl_getxattr(struct dentry *dentry, char *name, char **value,
 		if (!buf)
 			return -ENOMEM;
 
-		res = vfs_getxattr(dentry, name, buf, res);
+		res = ovl_do_vfs_getxattr(dentry, name, buf, res);
 		if (res < 0)
 			goto fail;
 	}