From fba479b3c78621cb122af20d1d677fe9193a971c Mon Sep 17 00:00:00 2001
From: Adin Scannell <ascannell@google.com>
Date: Tue, 25 Feb 2020 19:03:23 -0800
Subject: [PATCH] Fix DATA RACE in fs.MayDelete.

MayDelete must lock the directory also, otherwise concurrent renames may
race. Note that this also changes the methods to be aligned with the actual
Remove and RemoveDirectory methods to minimize confusion when reading the
code. (It was hard to see that resolution was correct.)

PiperOrigin-RevId: 297258304
---
 pkg/sentry/fs/dirent.go               | 25 ++++++++++++++-----------
 pkg/sentry/syscalls/linux/sys_file.go |  4 ++--
 2 files changed, 16 insertions(+), 13 deletions(-)

diff --git a/pkg/sentry/fs/dirent.go b/pkg/sentry/fs/dirent.go
index acab0411a..e0b32e1c1 100644
--- a/pkg/sentry/fs/dirent.go
+++ b/pkg/sentry/fs/dirent.go
@@ -1438,8 +1438,8 @@ func lockForRename(oldParent *Dirent, oldName string, newParent *Dirent, newName
 	}, nil
 }
 
-func checkSticky(ctx context.Context, dir *Dirent, victim *Dirent) error {
-	uattr, err := dir.Inode.UnstableAttr(ctx)
+func (d *Dirent) checkSticky(ctx context.Context, victim *Dirent) error {
+	uattr, err := d.Inode.UnstableAttr(ctx)
 	if err != nil {
 		return syserror.EPERM
 	}
@@ -1465,30 +1465,33 @@ func checkSticky(ctx context.Context, dir *Dirent, victim *Dirent) error {
 	return syserror.EPERM
 }
 
-// MayDelete determines whether `name`, a child of `dir`, can be deleted or
+// MayDelete determines whether `name`, a child of `d`, can be deleted or
 // renamed by `ctx`.
 //
 // Compare Linux kernel fs/namei.c:may_delete.
-func MayDelete(ctx context.Context, root, dir *Dirent, name string) error {
-	if err := dir.Inode.CheckPermission(ctx, PermMask{Write: true, Execute: true}); err != nil {
+func (d *Dirent) MayDelete(ctx context.Context, root *Dirent, name string) error {
+	if err := d.Inode.CheckPermission(ctx, PermMask{Write: true, Execute: true}); err != nil {
 		return err
 	}
 
-	victim, err := dir.Walk(ctx, root, name)
+	unlock := d.lockDirectory()
+	defer unlock()
+
+	victim, err := d.walk(ctx, root, name, true /* may unlock */)
 	if err != nil {
 		return err
 	}
 	defer victim.DecRef()
 
-	return mayDelete(ctx, dir, victim)
+	return d.mayDelete(ctx, victim)
 }
 
 // mayDelete determines whether `victim`, a child of `dir`, can be deleted or
 // renamed by `ctx`.
 //
 // Preconditions: `dir` is writable and executable by `ctx`.
-func mayDelete(ctx context.Context, dir, victim *Dirent) error {
-	if err := checkSticky(ctx, dir, victim); err != nil {
+func (d *Dirent) mayDelete(ctx context.Context, victim *Dirent) error {
+	if err := d.checkSticky(ctx, victim); err != nil {
 		return err
 	}
 
@@ -1542,7 +1545,7 @@ func Rename(ctx context.Context, root *Dirent, oldParent *Dirent, oldName string
 	defer renamed.DecRef()
 
 	// Check that the renamed dirent is deletable.
-	if err := mayDelete(ctx, oldParent, renamed); err != nil {
+	if err := oldParent.mayDelete(ctx, renamed); err != nil {
 		return err
 	}
 
@@ -1580,7 +1583,7 @@ func Rename(ctx context.Context, root *Dirent, oldParent *Dirent, oldName string
 		// across the Rename, so must call DecRef manually (no defer).
 
 		// Check that we can delete replaced.
-		if err := mayDelete(ctx, newParent, replaced); err != nil {
+		if err := newParent.mayDelete(ctx, replaced); err != nil {
 			replaced.DecRef()
 			return err
 		}
diff --git a/pkg/sentry/syscalls/linux/sys_file.go b/pkg/sentry/syscalls/linux/sys_file.go
index c21f14dc0..d10a9bed8 100644
--- a/pkg/sentry/syscalls/linux/sys_file.go
+++ b/pkg/sentry/syscalls/linux/sys_file.go
@@ -1236,7 +1236,7 @@ func rmdirAt(t *kernel.Task, dirFD int32, addr usermem.Addr) error {
 			return syserror.ENOTEMPTY
 		}
 
-		if err := fs.MayDelete(t, root, d, name); err != nil {
+		if err := d.MayDelete(t, root, name); err != nil {
 			return err
 		}
 
@@ -1517,7 +1517,7 @@ func unlinkAt(t *kernel.Task, dirFD int32, addr usermem.Addr) error {
 			return syserror.ENOTDIR
 		}
 
-		if err := fs.MayDelete(t, root, d, name); err != nil {
+		if err := d.MayDelete(t, root, name); err != nil {
 			return err
 		}