fusefs: Implement default_permissions and allow_other mount options.

By default, fusefs defers node permission checks to the server. The
default_permissions mount option enables the usual unix permission
checks based on the node owner and mode bits. Previously fusefs was
incorrectly checking permissions unconditionally.

Additionally, fusefs should restrict filesystem access to processes
started by the mount owner to prevent the fuse daemon from gaining
priviledge over other processes. The allow_other mount option
overrides this behaviour. Previously fusefs was incorrectly skipping
this check.

Updates #3229

PiperOrigin-RevId: 362419092

3 files changed, 95 insertions, 29 deletions

diff --git a/Makefile b/Makefile
index de22509cd..e424308fa 100644
--- a/Makefile
+++ b/Makefile
@@ -143,6 +143,7 @@ dev: $(RUNTIME_BIN) ## Installs a set of local runtimes. Requires sudo.
 	@$(call configure_noreload,$(RUNTIME)-d,--net-raw --debug --strace --log-packets)
 	@$(call configure_noreload,$(RUNTIME)-p,--net-raw --profile)
 	@$(call configure_noreload,$(RUNTIME)-vfs2-d,--net-raw --debug --strace --log-packets --vfs2)
+	@$(call configure_noreload,$(RUNTIME)-vfs2-fuse-d,--net-raw --debug --strace --log-packets --vfs2 --fuse)
 	@$(call reload_docker)
 .PHONY: dev
 
diff --git a/pkg/sentry/fsimpl/fuse/fusefs.go b/pkg/sentry/fsimpl/fuse/fusefs.go
index fb0ba2c6d..fef857afb 100644
--- a/pkg/sentry/fsimpl/fuse/fusefs.go
+++ b/pkg/sentry/fsimpl/fuse/fusefs.go
@@ -50,19 +50,11 @@ type filesystemOptions struct {
 	// mopts contains the raw, unparsed mount options passed to this filesystem.
 	mopts string
 
-	// userID specifies the numeric uid of the mount owner.
-	// This option should not be specified by the filesystem owner.
-	// It is set by libfuse (or, if libfuse is not used, must be set
-	// by the filesystem itself). For more information, see man page
-	// for fuse(8)
-	userID uint32
-
-	// groupID specifies the numeric gid of the mount owner.
-	// This option should not be specified by the filesystem owner.
-	// It is set by libfuse (or, if libfuse is not used, must be set
-	// by the filesystem itself). For more information, see man page
-	// for fuse(8)
-	groupID uint32
+	// uid of the mount owner.
+	uid auth.KUID
+
+	// gid of the mount owner.
+	gid auth.KGID
 
 	// rootMode specifies the the file mode of the filesystem's root.
 	rootMode linux.FileMode
@@ -76,6 +68,19 @@ type filesystemOptions struct {
 	// specified as "max_read" in fs parameters.
 	// If not specified by user, use math.MaxUint32 as default value.
 	maxRead uint32
+
+	// defaultPermissions is the default_permissions mount option. It instructs
+	// the kernel to perform a standard unix permission checks based on
+	// ownership and mode bits, instead of deferring the check to the server.
+	//
+	// Immutable after mount.
+	defaultPermissions bool
+
+	// allowOther is the allow_other mount option. It allows processes that
+	// don't own the FUSE mount to call into it.
+	//
+	// Immutable after mount.
+	allowOther bool
 }
 
 // filesystem implements vfs.FilesystemImpl.
@@ -115,14 +120,14 @@ func (fsType FilesystemType) GetFilesystem(ctx context.Context, vfsObj *vfs.Virt
 	mopts := vfs.GenericParseMountOptions(opts.Data)
 	deviceDescriptorStr, ok := mopts["fd"]
 	if !ok {
-		log.Warningf("%s.GetFilesystem: communication file descriptor N (obtained by opening /dev/fuse) must be specified as 'fd=N'", fsType.Name())
+		ctx.Warningf("fusefs.FilesystemType.GetFilesystem: mandatory mount option fd missing")
 		return nil, nil, syserror.EINVAL
 	}
 	delete(mopts, "fd")
 
 	deviceDescriptor, err := strconv.ParseInt(deviceDescriptorStr, 10 /* base */, 32 /* bitSize */)
 	if err != nil {
-		log.Debugf("%s.GetFilesystem: device FD '%v' not parsable: %v", fsType.Name(), deviceDescriptorStr, err)
+		ctx.Debugf("fusefs.FilesystemType.GetFilesystem: invalid fd: %q (%v)", deviceDescriptorStr, err)
 		return nil, nil, syserror.EINVAL
 	}
 
@@ -144,38 +149,54 @@ func (fsType FilesystemType) GetFilesystem(ctx context.Context, vfsObj *vfs.Virt
 
 	// Parse and set all the other supported FUSE mount options.
 	// TODO(gVisor.dev/issue/3229): Expand the supported mount options.
-	if userIDStr, ok := mopts["user_id"]; ok {
+	if uidStr, ok := mopts["user_id"]; ok {
 		delete(mopts, "user_id")
-		userID, err := strconv.ParseUint(userIDStr, 10, 32)
+		uid, err := strconv.ParseUint(uidStr, 10, 32)
 		if err != nil {
-			log.Warningf("%s.GetFilesystem: invalid user_id: user_id=%s", fsType.Name(), userIDStr)
+			log.Warningf("%s.GetFilesystem: invalid user_id: user_id=%s", fsType.Name(), uidStr)
 			return nil, nil, syserror.EINVAL
 		}
-		fsopts.userID = uint32(userID)
+		kuid := creds.UserNamespace.MapToKUID(auth.UID(uid))
+		if !kuid.Ok() {
+			ctx.Warningf("fusefs.FilesystemType.GetFilesystem: unmapped uid: %d", uid)
+			return nil, nil, syserror.EINVAL
+		}
+		fsopts.uid = kuid
+	} else {
+		ctx.Warningf("fusefs.FilesystemType.GetFilesystem: mandatory mount option user_id missing")
+		return nil, nil, syserror.EINVAL
 	}
 
-	if groupIDStr, ok := mopts["group_id"]; ok {
+	if gidStr, ok := mopts["group_id"]; ok {
 		delete(mopts, "group_id")
-		groupID, err := strconv.ParseUint(groupIDStr, 10, 32)
+		gid, err := strconv.ParseUint(gidStr, 10, 32)
 		if err != nil {
-			log.Warningf("%s.GetFilesystem: invalid group_id: group_id=%s", fsType.Name(), groupIDStr)
+			log.Warningf("%s.GetFilesystem: invalid group_id: group_id=%s", fsType.Name(), gidStr)
+			return nil, nil, syserror.EINVAL
+		}
+		kgid := creds.UserNamespace.MapToKGID(auth.GID(gid))
+		if !kgid.Ok() {
+			ctx.Warningf("fusefs.FilesystemType.GetFilesystem: unmapped gid: %d", gid)
 			return nil, nil, syserror.EINVAL
 		}
-		fsopts.groupID = uint32(groupID)
+		fsopts.gid = kgid
+	} else {
+		ctx.Warningf("fusefs.FilesystemType.GetFilesystem: mandatory mount option group_id missing")
+		return nil, nil, syserror.EINVAL
 	}
 
-	rootMode := linux.FileMode(0777)
-	modeStr, ok := mopts["rootmode"]
-	if ok {
+	if modeStr, ok := mopts["rootmode"]; ok {
 		delete(mopts, "rootmode")
 		mode, err := strconv.ParseUint(modeStr, 8, 32)
 		if err != nil {
 			log.Warningf("%s.GetFilesystem: invalid mode: %q", fsType.Name(), modeStr)
 			return nil, nil, syserror.EINVAL
 		}
-		rootMode = linux.FileMode(mode)
+		fsopts.rootMode = linux.FileMode(mode)
+	} else {
+		ctx.Warningf("fusefs.FilesystemType.GetFilesystem: mandatory mount option rootmode missing")
+		return nil, nil, syserror.EINVAL
 	}
-	fsopts.rootMode = rootMode
 
 	// Set the maxInFlightRequests option.
 	fsopts.maxActiveRequests = maxActiveRequestsDefault
@@ -195,6 +216,16 @@ func (fsType FilesystemType) GetFilesystem(ctx context.Context, vfsObj *vfs.Virt
 		fsopts.maxRead = math.MaxUint32
 	}
 
+	if _, ok := mopts["default_permissions"]; ok {
+		delete(mopts, "default_permissions")
+		fsopts.defaultPermissions = true
+	}
+
+	if _, ok := mopts["allow_other"]; ok {
+		delete(mopts, "allow_other")
+		fsopts.allowOther = true
+	}
+
 	// Check for unparsed options.
 	if len(mopts) != 0 {
 		log.Warningf("%s.GetFilesystem: unsupported or unknown options: %v", fsType.Name(), mopts)
@@ -326,6 +357,37 @@ func (fs *filesystem) newInode(ctx context.Context, nodeID uint64, attr linux.FU
 	return i
 }
 
+// CheckPermissions implements kernfs.Inode.CheckPermissions.
+func (i *inode) CheckPermissions(ctx context.Context, creds *auth.Credentials, ats vfs.AccessTypes) error {
+	// Since FUSE operations are ultimately backed by a userspace process (the
+	// fuse daemon), allowing a process to call into fusefs grants the daemon
+	// ptrace-like capabilities over the calling process. Because of this, by
+	// default FUSE only allows the mount owner to interact with the
+	// filesystem. This explicitly excludes setuid/setgid processes.
+	//
+	// This behaviour can be overriden with the 'allow_other' mount option.
+	//
+	// See fs/fuse/dir.c:fuse_allow_current_process() in Linux.
+	if !i.fs.opts.allowOther {
+		if creds.RealKUID != i.fs.opts.uid ||
+			creds.EffectiveKUID != i.fs.opts.uid ||
+			creds.SavedKUID != i.fs.opts.uid ||
+			creds.RealKGID != i.fs.opts.gid ||
+			creds.EffectiveKGID != i.fs.opts.gid ||
+			creds.SavedKGID != i.fs.opts.gid {
+			return syserror.EACCES
+		}
+	}
+
+	// By default, fusefs delegates all permission checks to the server.
+	// However, standard unix permission checks can be enabled with the
+	// default_permissions mount option.
+	if i.fs.opts.defaultPermissions {
+		return i.InodeAttrs.CheckPermissions(ctx, creds, ats)
+	}
+	return nil
+}
+
 // Open implements kernfs.Inode.Open.
 func (i *inode) Open(ctx context.Context, rp *vfs.ResolvingPath, d *kernfs.Dentry, opts vfs.OpenOptions) (*vfs.FileDescription, error) {
 	isDir := i.InodeAttrs.Mode().IsDir()
diff --git a/test/fuse/linux/mount_test.cc b/test/fuse/linux/mount_test.cc
index 8a5478116..276f842ea 100644
--- a/test/fuse/linux/mount_test.cc
+++ b/test/fuse/linux/mount_test.cc
@@ -15,6 +15,7 @@
 #include <errno.h>
 #include <fcntl.h>
 #include <sys/mount.h>
+#include <unistd.h>
 
 #include "gtest/gtest.h"
 #include "test/util/mount_util.h"
@@ -29,7 +30,9 @@ namespace {
 TEST(FuseMount, Success) {
   const FileDescriptor fd =
       ASSERT_NO_ERRNO_AND_VALUE(Open("/dev/fuse", O_WRONLY));
-  std::string mopts = absl::StrCat("fd=", std::to_string(fd.get()));
+  std::string mopts =
+      absl::StrFormat("fd=%d,user_id=%d,group_id=%d,rootmode=0777", fd.get(),
+                      getuid(), getgid());
 
   const auto dir = ASSERT_NO_ERRNO_AND_VALUE(TempPath::CreateDir());