tac: avoid double free

* src/tac.c (main): Reading a line longer than 16KiB would cause
tac to realloc its primary buffer.  Then, just before exit, tac
would mistakenly free the original (now free'd) buffer.
This bug was introduced by commit be6c13e7, "maint: always free a
buffer, to avoid even semblance of a leak".
* NEWS (Bug fixes): Mention it.
* tests/misc/tac (double-free): New test, to exercise this.
Reported by Salvo Tomaselli in <http://bugs.debian.org/594666>.

3 files changed, 13 insertions, 2 deletions

diff --git a/NEWS b/NEWS
index 93a1f96b4..3eb28b1e3 100644
--- a/NEWS
+++ b/NEWS
@@ -11,6 +11,9 @@ GNU coreutils NEWS                                    -*- outline -*-
   du -H and -L now consistently count pointed-to files instead of
   symbolic links, and correctly diagnose dangling symlinks.
 
+  tac would perform a double-free when given an input line longer than 16KiB.
+  [bug introduced in coreutils-8.3]
+
 ** New features
 
   cp now accepts the --attributes-only option to not copy file data,
diff --git a/src/tac.c b/src/tac.c
index cec973601..859e0067a 100644
--- a/src/tac.c
+++ b/src/tac.c
@@ -633,7 +633,6 @@ main (int argc, char **argv)
   if (! (read_size < half_buffer_size && half_buffer_size < G_buffer_size))
     xalloc_die ();
   G_buffer = xmalloc (G_buffer_size);
-  void *buf = G_buffer;
   if (sentinel_length)
     {
       strcpy (G_buffer, separator);
@@ -666,6 +665,9 @@ main (int argc, char **argv)
       error (0, errno, "-");
       ok = false;
     }
-  free (buf);
+
+  size_t offset = sentinel_length ? sentinel_length : 1;
+  free (G_buffer - offset);
+
   exit (ok ? EXIT_SUCCESS : EXIT_FAILURE);
 }
diff --git a/tests/misc/tac b/tests/misc/tac
index 763104993..4130c004d 100755
--- a/tests/misc/tac
+++ b/tests/misc/tac
@@ -24,6 +24,9 @@ my $prog = 'tac';
 
 my $bad_dir = 'no/such/dir';
 
+# This must be longer than 16KiB to trigger the double free in coreutils-8.5.
+my $long_line = 'o' x (16 * 1024 + 1);
+
 my @Tests =
 (
   ['segfault', '-r', {IN=>"a\n"}, {IN=>"b\n"}, {OUT=>"a\nb\n"}],
@@ -67,6 +70,9 @@ my @Tests =
    {ERR_SUBST => "s,`$bad_dir': .*,...,"},
    {ERR => "$prog: cannot create temporary file in ...\n"},
    {EXIT => 1}],
+
+  # coreutils-8.5's tac would double-free its primary buffer.
+  ['double-free', {IN=>$long_line}, {OUT=>$long_line}],
 );
 
 @Tests = triple_test \@Tests;