From 5accb135178325878840c6e36fc3e640ae582dea Mon Sep 17 00:00:00 2001
From: Jeff Sharkey <jsharkey@android.com>
Date: Thu, 7 Jan 2016 14:15:59 -0700
Subject: Use resolved path for both checking and opening.

This avoids a race condition where someone can change a symlink
target after the security checks have passed.

Bug: 26211054
Change-Id: I5842aaecc7b7d417a3b1902957b59b8a1f3c1ccb
---
 .../providers/downloads/DownloadProvider.java      | 12 ++++-
 src/com/android/providers/downloads/Helpers.java   | 62 ++++++++++++++++++----
 2 files changed, 62 insertions(+), 12 deletions(-)

diff --git a/src/com/android/providers/downloads/DownloadProvider.java b/src/com/android/providers/downloads/DownloadProvider.java
index ad3cf7ac..1595dfa9 100644
--- a/src/com/android/providers/downloads/DownloadProvider.java
+++ b/src/com/android/providers/downloads/DownloadProvider.java
@@ -1205,8 +1205,16 @@ public final class DownloadProvider extends ContentProvider {
         if (path == null) {
             throw new FileNotFoundException("No filename found.");
         }
-        if (!Helpers.isFilenameValid(path, mDownloadsDataDir)) {
-            throw new FileNotFoundException("Invalid filename: " + path);
+
+        final File file;
+        try {
+            file = new File(path).getCanonicalFile();
+        } catch (IOException e) {
+            throw new FileNotFoundException(e.getMessage());
+        }
+
+        if (!Helpers.isFilenameValid(getContext(), file)) {
+            throw new FileNotFoundException("Invalid file path: " + file);
         }
 
         final File file = new File(path);
diff --git a/src/com/android/providers/downloads/Helpers.java b/src/com/android/providers/downloads/Helpers.java
index 013faf27..61a49a2a 100644
--- a/src/com/android/providers/downloads/Helpers.java
+++ b/src/com/android/providers/downloads/Helpers.java
@@ -341,24 +341,25 @@ public class Helpers {
     }
 
     /**
-     * Checks whether the filename looks legitimate
+     * Checks whether the filename looks legitimate for security purposes. This
+     * prevents us from opening files that aren't actually downloads.
      */
-    static boolean isFilenameValid(String filename, File downloadsDataDir) {
-        final String[] whitelist;
+    static boolean isFilenameValid(Context context, File file) {
+        final File[] whitelist;
         try {
-            filename = new File(filename).getCanonicalPath();
-            whitelist = new String[] {
-                    downloadsDataDir.getCanonicalPath(),
-                    Environment.getDownloadCacheDirectory().getCanonicalPath(),
-                    Environment.getExternalStorageDirectory().getCanonicalPath(),
+            whitelist = new File[] {
+                    context.getFilesDir().getCanonicalFile(),
+                    context.getCacheDir().getCanonicalFile(),
+                    Environment.getDownloadCacheDirectory().getCanonicalFile(),
+                    Environment.getExternalStorageDirectory().getCanonicalFile(),
             };
         } catch (IOException e) {
             Log.w(TAG, "Failed to resolve canonical path: " + e);
             return false;
         }
 
-        for (String test : whitelist) {
-            if (filename.startsWith(test)) {
+        for (File testDir : whitelist) {
+            if (contains(testDir, file)) {
                 return true;
             }
         }
@@ -366,6 +367,47 @@ public class Helpers {
         return false;
     }
 
+    /**
+     * Test if a file lives under the given directory, either as a direct child
+     * or a distant grandchild.
+     * <p>
+     * Both files <em>must</em> have been resolved using
+     * {@link File#getCanonicalFile()} to avoid symlink or path traversal
+     * attacks.
+     */
+    public static boolean contains(File[] dirs, File file) {
+        for (File dir : dirs) {
+            if (contains(dir, file)) {
+                return true;
+            }
+        }
+        return false;
+    }
+
+    /**
+     * Test if a file lives under the given directory, either as a direct child
+     * or a distant grandchild.
+     * <p>
+     * Both files <em>must</em> have been resolved using
+     * {@link File#getCanonicalFile()} to avoid symlink or path traversal
+     * attacks.
+     */
+    public static boolean contains(File dir, File file) {
+        if (dir == null || file == null) return false;
+
+        String dirPath = dir.getAbsolutePath();
+        String filePath = file.getAbsolutePath();
+
+        if (dirPath.equals(filePath)) {
+            return true;
+        }
+
+        if (!dirPath.endsWith("/")) {
+            dirPath += "/";
+        }
+        return filePath.startsWith(dirPath);
+    }
+
     /**
      * Checks whether this looks like a legitimate selection parameter
      */
-- 
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