filename: Add safe_relative_path/1

Add safe_relative_path/1 to guard against directory traversal
attacks. It either returns a shorter path without any ".." or
"." components, or 'unsafe' if an ".." component would climb
up above the root of the relative path. Here are a few
examples:

  safe_relative_path("a/b/..") => "a"
  safe_relative_path("a/..") => ""
  safe_relative_path("a/../..") => unsafe
  safe_relative_path("/absolute/path") => unsafe

The returned path can be used directly or combined with an
absolute path using filename:join/2.

3 files changed, 130 insertions, 2 deletions

diff --git a/lib/stdlib/doc/src/filename.xml b/lib/stdlib/doc/src/filename.xml
index 2a413835d0..f7f3f7b504 100644
--- a/lib/stdlib/doc/src/filename.xml
+++ b/lib/stdlib/doc/src/filename.xml
@@ -511,6 +511,33 @@ true
     </func>
 
     <func>
+      <name name="safe_relative_path" arity="1"/>
+      <fsummary>Sanitize a relative path to avoid directory traversal attacks.</fsummary>
+      <desc>
+        <p>Sanitizes the relative path by eliminating ".." and "."
+        components to protect against directory traversal attacks.
+        Either returns the sanitized path name, or the atom
+        <c>unsafe</c> if the path is unsafe.
+        The path is considered unsafe in the following circumstances:</p>
+        <list type="bulleted">
+          <item><p>The path is not relative.</p></item>
+          <item><p>A ".." component would climb up above the root of
+          the relative path.</p></item>
+        </list>
+        <p><em>Examples:</em></p>
+        <pre>
+1> <input>filename:safe_relative_path("dir/sub_dir/..").</input>
+"dir"
+2> <input>filename:safe_relative_path("dir/..").</input>
+[]
+3> <input>filename:safe_relative_path("dir/../..").</input>
+unsafe
+4> <input>filename:safe_relative_path("/abs/path").</input>
+unsafe</pre>
+       </desc>
+    </func>
+
+    <func>
       <name name="split" arity="1"/>
       <fsummary>Split a filename into its path components.</fsummary>
       <desc>
diff --git a/lib/stdlib/src/filename.erl b/lib/stdlib/src/filename.erl
index c4586171ca..5d60b3837e 100644
--- a/lib/stdlib/src/filename.erl
+++ b/lib/stdlib/src/filename.erl
@@ -34,7 +34,8 @@
 -export([absname/1, absname/2, absname_join/2, 
 	 basename/1, basename/2, dirname/1,
 	 extension/1, join/1, join/2, pathtype/1,
-	 rootname/1, rootname/2, split/1, nativename/1]).
+         rootname/1, rootname/2, split/1, nativename/1,
+         safe_relative_path/1]).
 -export([find_src/1, find_src/2, flatten/1]).
 -export([basedir/2, basedir/3]).
 
@@ -750,6 +751,39 @@ separators() ->
 	_ -> {false, false}
     end.
 
+-spec safe_relative_path(Filename) -> 'unsafe' | SafeFilename when
+      Filename :: file:name_all(),
+      SafeFilename :: file:name_all().
+
+safe_relative_path(Path) ->
+    case pathtype(Path) of
+        relative ->
+            Cs0 = split(Path),
+            safe_relative_path_1(Cs0, []);
+        _ ->
+            unsafe
+    end.
+
+safe_relative_path_1(["."|T], Acc) ->
+    safe_relative_path_1(T, Acc);
+safe_relative_path_1([<<".">>|T], Acc) ->
+    safe_relative_path_1(T, Acc);
+safe_relative_path_1([".."|T], Acc) ->
+    climb(T, Acc);
+safe_relative_path_1([<<"..">>|T], Acc) ->
+    climb(T, Acc);
+safe_relative_path_1([H|T], Acc) ->
+    safe_relative_path_1(T, [H|Acc]);
+safe_relative_path_1([], []) ->
+    [];
+safe_relative_path_1([], Acc) ->
+    join(lists:reverse(Acc)).
+
+climb(_, []) ->
+    unsafe;
+climb(T, [_|Acc]) ->
+    safe_relative_path_1(T, Acc).
+
 
 
 %% find_src(Module) --
diff --git a/lib/stdlib/test/filename_SUITE.erl b/lib/stdlib/test/filename_SUITE.erl
index b7c4d3a6e5..f64ec6acb7 100644
--- a/lib/stdlib/test/filename_SUITE.erl
+++ b/lib/stdlib/test/filename_SUITE.erl
@@ -29,6 +29,7 @@
 	 dirname_bin/1, extension_bin/1, join_bin/1, t_nativename_bin/1]).
 -export([pathtype_bin/1,rootname_bin/1,split_bin/1]).
 -export([t_basedir_api/1, t_basedir_xdg/1, t_basedir_windows/1]).
+-export([safe_relative_path/1]).
 
 -include_lib("common_test/include/ct.hrl").
 
@@ -41,7 +42,8 @@ all() ->
      find_src,
      absname_bin, absname_bin_2,
      {group,p},
-     t_basedir_xdg, t_basedir_windows].
+     t_basedir_xdg, t_basedir_windows,
+     safe_relative_path].
 
 groups() -> 
     [{p, [parallel],
@@ -768,6 +770,71 @@ t_nativename_bin(Config) when is_list(Config) ->
                 filename:nativename(<<"/usr/tmp//arne/">>)
     end.
 
+safe_relative_path(Config) ->
+    PrivDir = proplists:get_value(priv_dir, Config),
+    Root = filename:join(PrivDir, ?FUNCTION_NAME),
+    ok = file:make_dir(Root),
+    ok = file:set_cwd(Root),
+
+    ok = file:make_dir("a"),
+    ok = file:set_cwd("a"),
+    ok = file:make_dir("b"),
+    ok = file:set_cwd("b"),
+    ok = file:make_dir("c"),
+
+    ok = file:set_cwd(Root),
+
+    "a" = test_srp("a"),
+    "a/b" = test_srp("a/b"),
+    "a/b" = test_srp("a/./b"),
+    "a/b" = test_srp("a/./b/."),
+
+    "" = test_srp("a/.."),
+    "" = test_srp("a/./.."),
+    "" = test_srp("a/../."),
+    "a" = test_srp("a/b/.."),
+    "a" = test_srp("a/../a"),
+    "a" = test_srp("a/../a/../a"),
+    "a/b/c" = test_srp("a/../a/b/c"),
+
+    unsafe = test_srp("a/../.."),
+    unsafe = test_srp("a/../../.."),
+    unsafe = test_srp("a/./../.."),
+    unsafe = test_srp("a/././../../.."),
+    unsafe = test_srp("a/b/././../../.."),
+
+    unsafe = test_srp(PrivDir),                 %Absolute path.
+
+    ok.
+
+test_srp(RelPath) ->
+    Res = do_test_srp(RelPath),
+    Res = case do_test_srp(list_to_binary(RelPath)) of
+              Bin when is_binary(Bin) ->
+                  binary_to_list(Bin);
+              Other ->
+                  Other
+          end.
+
+do_test_srp(RelPath) ->
+    {ok,Root} = file:get_cwd(),
+    ok = file:set_cwd(RelPath),
+    {ok,Cwd} = file:get_cwd(),
+    ok = file:set_cwd(Root),
+    case filename:safe_relative_path(RelPath) of
+        unsafe ->
+            true = length(Cwd) < length(Root),
+            unsafe;
+        "" ->
+            "";
+        SafeRelPath ->
+            ok = file:set_cwd(SafeRelPath),
+            {ok,Cwd} = file:get_cwd(),
+            true = length(Cwd) >= length(Root),
+            ok = file:set_cwd(Root),
+            SafeRelPath
+    end.
+
 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
 %% basedirs
 t_basedir_api(Config) when is_list(Config) ->