verify_path(): disallow symlinks in .gitattributes and .gitignore

In commit 10ecfa76 (verify_path: disallow symlinks in .gitmodules,
2018-05-04) we made it impossible to load a .gitmodules file that's a
symlink into the index. The security reasons for doing so are described
there. We also discussed forbidding symlinks of other .git files as part
of that fix, but the tradeoff was less compelling:

  1. Unlike .gitmodules, the other files don't have content-level fsck
     checks. So an attacker using symlinks to evade those checks isn't a
     problem.

  2. Unlike .gitmodules, Git will never write .gitignore or
     .gitattributes itself, making it much less likely to use them to
     write outside the repo. They could be used for out-of-repo reads,
     however.

  3. The .gitmodules change was part of a critical bug-fix that was
     not publicly disclosed until it was released. Changing the other
     files was not needed for the minimal fix.

However, it's still a reasonable idea to forbid symlinks for these
files:

  - As noted, they can still be used to read out-of-repo files (which is
    fairly restricted, but in some circumstances you can probe file
    content by speculatively creating files and seeing if they get
    ignored)

  - They don't currently behave well in all cases. We sometimes read
    these files from the index, where we _don't_ follow symlinks (we'd
    just treat the symlink target as the .gitignore or .gitattributes
    content, which is actively wrong).

This patch forbids symlinked versions of these files from entering the
index. We already have helpers for obscured forms of the names from
e7cb0b44 (is_ntfs_dotgit: match other .git files, 2018-05-11) and
0fc333ba (is_hfs_dotgit: match other .git files, 2018-05-02), which
were done as part of the series touching .gitmodules.

No tests yet, as we'll add them in a subsequent patch once we have fsck
support, too.
Signed-off-by: NJeff King <peff@peff.net>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

read-cache.c

@@ -945,7 +945,9 @@ static int verify_dotfile(const char *rest, unsigned mode)
 			return 0;
 		if (S_ISLNK(mode)) {
 			rest += 3;
-			if (skip_iprefix(rest, "modules", &rest) &&
+			if ((skip_iprefix(rest, "modules", &rest) ||
+			     skip_iprefix(rest, "ignore", &rest) ||
+			     skip_iprefix(rest, "attributes", &rest)) &&
 			    (*rest == '\0' || is_dir_sep(*rest)))
 				return 0;
 		}
@@ -978,7 +980,9 @@ int verify_path(const char *path, unsigned mode)
 				if (is_hfs_dotgit(path))
 					return 0;
 				if (S_ISLNK(mode)) {
-					if (is_hfs_dotgitmodules(path))
+					if (is_hfs_dotgitmodules(path) ||
+					    is_hfs_dotgitignore(path) ||
+					    is_hfs_dotgitattributes(path))
 						return 0;
 				}
 			}
@@ -990,7 +994,9 @@ int verify_path(const char *path, unsigned mode)
 				if (is_ntfs_dotgit(path))
 					return 0;
 				if (S_ISLNK(mode)) {
-					if (is_ntfs_dotgitmodules(path))
+					if (is_ntfs_dotgitmodules(path) ||
+					    is_ntfs_dotgitignore(path) ||
+					    is_ntfs_dotgitattributes(path))
 						return 0;
 				}
 			}