Allow disabling symbolic link checking

This turns off all checks for symbolic links, returning to the R4.1
behavior.  This is unsafe, but it might be necessary in some cases.
However, it will not be included by either qubes.Filecopy or
qubes.UnsafeFileCopy.

(cherry picked from commit aa0b85e)

qrexec-lib/libqubes-rpc-filecopy.h

@@ -69,6 +69,7 @@ enum copy_flags {
     COPY_ALLOW_DIRECTORIES = (1 << 1),
     COPY_ALLOW_UNSAFE_CHARACTERS = (1 << 2),
     COPY_ALLOW_NON_CANONICAL_SYMLINKS = (1 << 3),
+    COPY_ALLOW_UNSAFE_SYMLINKS = (1 << 4),
 };
 
 /* feedback handling */

qrexec-lib/pure.h

@@ -260,6 +260,8 @@ enum QubesFilenameValidationFlags {
     /// to be canonial, as they are assumed to come from a filesystem
     /// traversal, which will always produce canonical paths.
     QUBES_PURE_ALLOW_NON_CANONICAL_SYMLINKS = (1 << 1),
+    /// Do not check symbolic links at all.
+    QUBES_PURE_ALLOW_UNSAFE_SYMLINKS = (1 << 2),
     /// Allow all paths to be non-canonical, including symlinks.
     QUBES_PURE_ALLOW_NON_CANONICAL_PATHS = (1 << 3),
 };

qrexec-lib/unicode.c

@@ -223,7 +223,8 @@ static bool flag_check(const uint32_t flags)
 {
     int const allowed = (QUBES_PURE_ALLOW_UNSAFE_CHARACTERS |
                          QUBES_PURE_ALLOW_NON_CANONICAL_SYMLINKS |
-                         QUBES_PURE_ALLOW_NON_CANONICAL_PATHS);
+                         QUBES_PURE_ALLOW_NON_CANONICAL_PATHS |
+                         QUBES_PURE_ALLOW_UNSAFE_SYMLINKS);
     return (flags & ~(__typeof__(flags))allowed) == 0;
 }
 
@@ -255,6 +256,8 @@ qubes_pure_validate_symbolic_link_v2(const uint8_t *untrusted_name,
     ssize_t depth = validate_path(untrusted_name, 0, flags);
     if (depth < 0)
         return -EILSEQ; // -ENOLINK is only for symlinks
+    if ((flags & QUBES_PURE_ALLOW_UNSAFE_SYMLINKS) != 0)
+        return depth > 0 ? 0 : -ENOLINK;
     if ((flags & QUBES_PURE_ALLOW_NON_CANONICAL_SYMLINKS) != 0)
         flags |= QUBES_PURE_ALLOW_NON_CANONICAL_PATHS;
     // Symlink paths must have at least 2 components: "a/b" is okay

qrexec-lib/unpack.c

@@ -346,7 +346,7 @@ static void process_one_file(struct file_header *untrusted_hdr, int flags)
     // Never set QUBES_PURE_ALLOW_NON_CANONICAL_PATHS -- paths from qfile-agent
     // will always be canonical.
     uint32_t validate_flags = ((uint32_t)flags >> 2) &
-        (QUBES_PURE_ALLOW_UNSAFE_CHARACTERS |
+        (QUBES_PURE_ALLOW_UNSAFE_CHARACTERS | QUBES_PURE_ALLOW_UNSAFE_SYMLINKS |
          QUBES_PURE_ALLOW_NON_CANONICAL_SYMLINKS);
     if (!read_all_with_crc(0, untrusted_namebuf, namelen))
         do_exit(LEGAL_EOF, NULL); // hopefully remote has produced error message

qrexec-lib/validator-test.c

@@ -172,6 +172,12 @@ int main(int argc, char **argv)
         TEST("a/b/c", "a/", 0, true),
         // Invalid paths are rejected...
         TEST("..", "a/", 0, false),
+        // ...even with QUBES_PURE_ALLOW_UNSAFE_SYMLINKS.
+        TEST("..", "a/", QUBES_PURE_ALLOW_UNSAFE_SYMLINKS, false),
+        // but QUBES_PURE_ALLOW_UNSAFE_SYMLINKS allows bad symlinks
+        TEST("a", "/home/user/.bashrc", QUBES_PURE_ALLOW_UNSAFE_SYMLINKS, true),
+        // that are otherwise rejected
+        TEST("a", "/home/user/.bashrc", 0, false),
         // QUBES_PURE_ALLOW_NON_CANONICAL_PATHS allows non-canonical symlinks...
         TEST("a/b", "b//c", QUBES_PURE_ALLOW_NON_CANONICAL_PATHS, true),
         // ...and non-canonical paths.