From 83a8b309e949df4b8f82f26085cd62284f9eeb87 Mon Sep 17 00:00:00 2001
From: Nicolas Lacasse <nlacasse@google.com>
Date: Wed, 26 Aug 2020 10:03:46 -0700
Subject: tmpfs: Allow xattrs in the trusted namespace if creds has
 CAP_SYS_ADMIN.

This is needed to support the overlay opaque attribute.

PiperOrigin-RevId: 328552985
---
 pkg/sentry/fsimpl/overlay/filesystem.go |  2 +-
 pkg/sentry/fsimpl/tmpfs/tmpfs.go        | 68 ++++++++++++++++++++-------------
 2 files changed, 43 insertions(+), 27 deletions(-)

(limited to 'pkg/sentry')

diff --git a/pkg/sentry/fsimpl/overlay/filesystem.go b/pkg/sentry/fsimpl/overlay/filesystem.go
index a3cee4047..e720bfb0b 100644
--- a/pkg/sentry/fsimpl/overlay/filesystem.go
+++ b/pkg/sentry/fsimpl/overlay/filesystem.go
@@ -30,7 +30,7 @@ import (
 // _OVL_XATTR_OPAQUE is an extended attribute key whose value is set to "y" for
 // opaque directories.
 // Linux: fs/overlayfs/overlayfs.h:OVL_XATTR_OPAQUE
-const _OVL_XATTR_OPAQUE = "trusted.overlay.opaque"
+const _OVL_XATTR_OPAQUE = linux.XATTR_TRUSTED_PREFIX + "overlay.opaque"
 
 func isWhiteout(stat *linux.Statx) bool {
 	return stat.Mode&linux.S_IFMT == linux.S_IFCHR && stat.RdevMajor == 0 && stat.RdevMinor == 0
diff --git a/pkg/sentry/fsimpl/tmpfs/tmpfs.go b/pkg/sentry/fsimpl/tmpfs/tmpfs.go
index a7fdf19ca..c4cec4130 100644
--- a/pkg/sentry/fsimpl/tmpfs/tmpfs.go
+++ b/pkg/sentry/fsimpl/tmpfs/tmpfs.go
@@ -631,49 +631,65 @@ func (i *inode) listxattr(size uint64) ([]string, error) {
 }
 
 func (i *inode) getxattr(creds *auth.Credentials, opts *vfs.GetxattrOptions) (string, error) {
-	if err := i.checkPermissions(creds, vfs.MayRead); err != nil {
+	if err := i.checkXattrPermissions(creds, opts.Name, vfs.MayRead); err != nil {
 		return "", err
 	}
-	if !strings.HasPrefix(opts.Name, linux.XATTR_USER_PREFIX) {
-		return "", syserror.EOPNOTSUPP
-	}
-	if !i.userXattrSupported() {
-		return "", syserror.ENODATA
-	}
 	return i.xattrs.Getxattr(opts)
 }
 
 func (i *inode) setxattr(creds *auth.Credentials, opts *vfs.SetxattrOptions) error {
-	if err := i.checkPermissions(creds, vfs.MayWrite); err != nil {
+	if err := i.checkXattrPermissions(creds, opts.Name, vfs.MayWrite); err != nil {
 		return err
 	}
-	if !strings.HasPrefix(opts.Name, linux.XATTR_USER_PREFIX) {
-		return syserror.EOPNOTSUPP
-	}
-	if !i.userXattrSupported() {
-		return syserror.EPERM
-	}
 	return i.xattrs.Setxattr(opts)
 }
 
 func (i *inode) removexattr(creds *auth.Credentials, name string) error {
-	if err := i.checkPermissions(creds, vfs.MayWrite); err != nil {
+	if err := i.checkXattrPermissions(creds, name, vfs.MayWrite); err != nil {
 		return err
 	}
-	if !strings.HasPrefix(name, linux.XATTR_USER_PREFIX) {
-		return syserror.EOPNOTSUPP
-	}
-	if !i.userXattrSupported() {
-		return syserror.EPERM
-	}
 	return i.xattrs.Removexattr(name)
 }
 
-// Extended attributes in the user.* namespace are only supported for regular
-// files and directories.
-func (i *inode) userXattrSupported() bool {
-	filetype := linux.S_IFMT & atomic.LoadUint32(&i.mode)
-	return filetype == linux.S_IFREG || filetype == linux.S_IFDIR
+func (i *inode) checkXattrPermissions(creds *auth.Credentials, name string, ats vfs.AccessTypes) error {
+	switch {
+	case ats&vfs.MayRead == vfs.MayRead:
+		if err := i.checkPermissions(creds, vfs.MayRead); err != nil {
+			return err
+		}
+	case ats&vfs.MayWrite == vfs.MayWrite:
+		if err := i.checkPermissions(creds, vfs.MayWrite); err != nil {
+			return err
+		}
+	default:
+		panic(fmt.Sprintf("checkXattrPermissions called with impossible AccessTypes: %v", ats))
+	}
+
+	switch {
+	case strings.HasPrefix(name, linux.XATTR_TRUSTED_PREFIX):
+		// The trusted.* namespace can only be accessed by privileged
+		// users.
+		if creds.HasCapability(linux.CAP_SYS_ADMIN) {
+			return nil
+		}
+		if ats&vfs.MayWrite == vfs.MayWrite {
+			return syserror.EPERM
+		}
+		return syserror.ENODATA
+	case strings.HasPrefix(name, linux.XATTR_USER_PREFIX):
+		// Extended attributes in the user.* namespace are only
+		// supported for regular files and directories.
+		filetype := linux.S_IFMT & atomic.LoadUint32(&i.mode)
+		if filetype == linux.S_IFREG || filetype == linux.S_IFDIR {
+			return nil
+		}
+		if ats&vfs.MayWrite == vfs.MayWrite {
+			return syserror.EPERM
+		}
+		return syserror.ENODATA
+
+	}
+	return syserror.EOPNOTSUPP
 }
 
 // fileDescription is embedded by tmpfs implementations of
-- 
cgit v1.2.3