Tighten key parser to not accept keys containing a non-numeric fields, which could be used to embed data useful for a SHA1 attack against git.

Also todo about why this is important, and with some further hardening to
add.

This commit was sponsored by Ignacio on Patreon.

CHANGELOG

@@ -33,6 +33,9 @@ git-annex (6.20170215) UNRELEASED; urgency=medium
     to wget, since curl is able to display only errors to stderr, unlike
     wget.
   * status: Pass --ignore-submodules=when option on to git status.
+  * Tighten key parser to not accept keys containing a non-numeric
+    fields, which could be used to embed data useful for a SHA1
+    attack against git.
 
  -- Joey Hess <id@joeyh.name>  Tue, 14 Feb 2017 15:54:25 -0400
 

Types/Key.hs

@@ -22,6 +22,7 @@ module Types.Key (
 
 import System.Posix.Types
 import Data.Aeson
+import Data.Char
 import qualified Data.Text as T
 
 import Common
@@ -108,6 +109,16 @@ file2key s
 	findfields _ v = v
 
 	addbackend k v = Just k { keyBackendName = v }
+
+	-- This is a strict parser for security reasons; a key
+	-- can contain only 4 fields, which all consist only of numbers.
+	-- Any key containing other fields, or non-numeric data is
+	-- rejected with Nothing.
+	--
+	-- If a key contained non-numeric fields, they could be used to
+	-- embed data used in a SHA1 collision attack, which would be a
+	-- problem since the keys are committed to git.
+	addfield _ _ v | not (all isDigit v) = Nothing
 	addfield 's' k v = do
 		sz <- readish v
 		return $ k { keySize = Just sz }
@@ -120,7 +131,7 @@ file2key s
 	addfield 'C' k v = case readish v of
 		Just chunknum | chunknum > 0 ->
 			return $ k { keyChunkNum = Just chunknum }
-		_ -> return k
+		_ -> Nothing
 	addfield _ _ _ = Nothing
 
 instance ToJSON Key where

doc/todo/sha1_collision_embedding_in_git-annex_keys.mdwn

@@ -0,0 +1,36 @@
+Some git-annex backends allow embedding enough data in the names of keys
+that it could be used for a SHA1 collision attack. So, a signed git commit
+could point to a tree with such a key in it, and the blob for the key could
+have two versions with the same SHA1.
+
+Users who want to use git-annex with signed commits to mitigate git's own
+SHA1 insecurities would like at least a way to disable the insecure
+git-annex backends:
+
+* WORM can contain fairly arbitrary data in a key name
+* URL too (also, of course, URLs download arbitrary data from the web,
+  so a signed git commit pointing at URL keys doesn't have any security
+  even w/o SHA1 collisions)
+* SHA1 and MD5 backends are insecure because there can be colliding
+  versions of the data they point to.
+
+A config setting to prevent git-annex from using insecure backends would be
+useful.
+
+(git-annex might suggest enabling that configuration if commit.gpgSign
+is enabled)
+
+A few other potential problems:
+
+* `*E` backends could embed sha1 collision data in a long filename
+  extension. That this is much harder to exploit because git-annex
+  checks the hash of the data when it enters the repository, and git-annex
+  fsck also verifies it. It still might be worth limiting the length
+  of an extension in such a key to the longest such extension git-annex has
+  ever supported (probably < 20 bytes or so), which would be less than the
+  size of the data needed for current SHA1 collision attacks.
+* It might be possible to embed colliding data in a specially constructed
+  key name with an extra field in it, eg "SHA256-cXXXXXXXXXXXXXXX-...".
+  Need to review the code and see if such extra fields are allowed.  
+  Update: All fields are numeric, but could contain arbitrary data
+  after the number. (fixed now)