From f33077c875523d5f33edf8c395300e29e7c9e609 Mon Sep 17 00:00:00 2001
From: Ayush Ranjan <ayushranjan@google.com>
Date: Wed, 2 Sep 2020 17:56:30 -0700
Subject: [vfs] Implement xattr for overlayfs.

PiperOrigin-RevId: 329825497
---
 pkg/sentry/vfs/permissions.go | 38 ++++++++++++++++++++++++++++++++++++++
 1 file changed, 38 insertions(+)

(limited to 'pkg/sentry/vfs')

diff --git a/pkg/sentry/vfs/permissions.go b/pkg/sentry/vfs/permissions.go
index 014b928ed..00eeb8842 100644
--- a/pkg/sentry/vfs/permissions.go
+++ b/pkg/sentry/vfs/permissions.go
@@ -16,6 +16,7 @@ package vfs
 
 import (
 	"math"
+	"strings"
 
 	"gvisor.dev/gvisor/pkg/abi/linux"
 	"gvisor.dev/gvisor/pkg/context"
@@ -284,3 +285,40 @@ func CheckLimit(ctx context.Context, offset, size int64) (int64, error) {
 	}
 	return size, nil
 }
+
+// CheckXattrPermissions checks permissions for extended attribute access.
+// This is analogous to fs/xattr.c:xattr_permission(). Some key differences:
+// * Does not check for read-only filesystem property.
+// * Does not check inode immutability or append only mode. In both cases EPERM
+//   must be returned by filesystem implementations.
+// * Does not do inode permission checks. Filesystem implementations should
+//   handle inode permission checks as they may differ across implementations.
+func CheckXattrPermissions(creds *auth.Credentials, ats AccessTypes, mode linux.FileMode, kuid auth.KUID, name string) error {
+	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.MayWrite() {
+			return syserror.EPERM
+		}
+		return syserror.ENODATA
+	case strings.HasPrefix(name, linux.XATTR_USER_PREFIX):
+		// In the user.* namespace, only regular files and directories can have
+		// extended attributes. For sticky directories, only the owner and
+		// privileged users can write attributes.
+		filetype := mode.FileType()
+		if filetype != linux.ModeRegular && filetype != linux.ModeDirectory {
+			if ats.MayWrite() {
+				return syserror.EPERM
+			}
+			return syserror.ENODATA
+		}
+		if filetype == linux.ModeDirectory && mode&linux.ModeSticky != 0 && ats.MayWrite() && !CanActAsOwner(creds, kuid) {
+			return syserror.EPERM
+		}
+	}
+	return nil
+}
-- 
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