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Re: Fix p_secstodate overflow handling (bug 22463)

On Wed, 22 Nov 2017, Florian Weimer wrote:

> I think we can change the function to always return NULL (or abort the process
> with an error message) and turn it into into a compat symbol.  I very much

That seems a bit too incompatible for me.  This patch adds a limit on the 
times it translates of 0x7fffffff, but still translates those to strings 
as before while using <overflow> as the output for larger time values.  
That seems to me to be reasonable semantics for a compat symbol to follow 
(however, this patch does not make it into a compat symbol).

Fix p_secstodate overflow handling (bug 22463).

The resolv/res_debug.c function p_secstodate (which is a public
function exported from libresolv, taking an unsigned long argument)

        struct tm timebuf;
        time = __gmtime_r(&clock, &timebuf);
        time->tm_year += 1900;
        time->tm_mon += 1;
        sprintf(output, "%04d%02d%02d%02d%02d%02d",
                time->tm_year, time->tm_mon, time->tm_mday,
                time->tm_hour, time->tm_min, time->tm_sec);

If __gmtime_r returns NULL (because the year overflows the range of
int), this will dereference a null pointer.  Otherwise, if the
computed year does not fit in four characters, this will cause a
buffer overrun of the fixed-size 15-byte buffer.  With current GCC
mainline, there is a compilation failure because of the possible
buffer overrun.

I couldn't find a specification for how this function is meant to
behave, but Paul pointed to RFC 4034 as relevant to the cases where
this function is called from within glibc.  The function's interface
is inherently problematic when dates beyond Y2038 might be involved,
because of the ambiguity in how to interpret 32-bit timestamps as such
dates (the RFC suggests interpreting times as being within 68 years of
the present date, which would mean some kind of interface whose
behavior depends on the present date).

This patch works on the basis of making a minimal fix in preparation
for obsoleting the function.  The function is made to handle times in
the interval [0, 0x7fffffff] only, on all platforms, with <overflow>
used as the output string in other cases (and errno set to EOVERFLOW
in such cases).  This seems to be a reasonable state for the function
to be in when made a compat symbol by a future patch, being compatible
with any existing uses for existing timestamps without trying to work
for later timestamps.  Results independent of the range of time_t also
simplify the testcase.

I couldn't persuade GCC to recognize the ranges of the struct tm
fields by adding explicit range checks with a call to
__builtin_unreachable if outside the range (this looks similar to
<>), so having added
a range check on the input, this patch then disables the
-Wformat-overflow= warning for the sprintf call (I prefer that to the
use of strftime, as being more transparently correct without knowing
what each of %m and %M etc. is).

I do not know why this build failure should be new with mainline GCC
(that is, I don't know what GCC change might have introduced it, when
the basic functionality for such warnings was already in GCC 7).

I do not know if this is a security issue (that is, if there are
plausible ways in which a date before -999 or after 9999 from an
untrusted source might end up in this function).  The system clock is
arguably an untrusted source (in that e.g. NTP is insecure), but
probably not to that extent (NTP can't communicate such wild
timestamps), and uses from within glibc are limited to 32-bit inputs.

Tested with that this restores the build for arm
with yesterday's mainline GCC (other build failures have been
introduced since then).  Also tested for x86_64 and x86.

2017-11-22  Joseph Myers  <>

	[BZ #22463]
	* resolv/res_debug.c: Include <libc-diag.h>.
	(p_secstodate): Assert time_t at least as wide as u_long.  On
	overflow, use integer seconds since the epoch as output, or use
	"<overflow>" as output and set errno to EOVERFLOW if integer
	seconds since the epoch would be 14 or more characters.
	(p_secstodate) [__GNUC_PREREQ (7, 0)]: Disable -Wformat-overflow=
	for sprintf call.
	* resolv/tst-p_secstodate.c: New file.
	* resolv/Makefile (tests): Add tst-p_secstodate.
	($(objpfx)tst-p_secstodate): Depend on $(objpfx)

diff --git a/resolv/Makefile b/resolv/Makefile
index 0323496..aff6710 100644
--- a/resolv/Makefile
+++ b/resolv/Makefile
@@ -55,6 +55,7 @@ tests += \
   tst-resolv-network \
   tst-resolv-res_init-multi \
   tst-resolv-search \
+  tst-p_secstodate \
 # These tests need libdl.
 ifeq (yes,$(build-shared))
@@ -176,6 +177,7 @@ $(objpfx)tst-ns_name.out:
 $(objpfx)tst-ns_name_compress: $(objpfx)
 $(objpfx)tst-ns_name_pton: $(objpfx)
 $(objpfx)tst-res_hnok: $(objpfx)
+$(objpfx)tst-p_secstodate: $(objpfx)
 # This test case uses the deprecated RES_USE_INET6 resolver option.
diff --git a/resolv/res_debug.c b/resolv/res_debug.c
index 4114c2d..e319774 100644
--- a/resolv/res_debug.c
+++ b/resolv/res_debug.c
@@ -107,6 +107,7 @@
 #include <string.h>
 #include <time.h>
 #include <shlib-compat.h>
+#include <libc-diag.h>
 # define SPRINTF(x) strlen(sprintf/**/x)
@@ -1054,6 +1055,8 @@ libresolv_hidden_def (__dn_count_labels)
  * Make dates expressed in seconds-since-Jan-1-1970 easy to read.
  * SIG records are required to be printed like this, by the Secure DNS RFC.
+ * This is an obsolescent function and does not handle dates outside the
+ * signed 32-bit range.
 char *
 p_secstodate (u_long secs) {
@@ -1063,12 +1066,27 @@ p_secstodate (u_long secs) {
 	struct tm *time;
 	struct tm timebuf;
-	time = __gmtime_r(&clock, &timebuf);
+	if (secs > 0x7fffffff
+	    || (time = __gmtime_r (&clock, &timebuf)) == NULL) {
+		strcpy (output, "<overflow>");
+		__set_errno (EOVERFLOW);
+		return output;
+	}
 	time->tm_year += 1900;
 	time->tm_mon += 1;
+	/* The struct tm fields, given the above range check,
+	   must have values that mean this sprintf exactly fills the
+	   buffer.  But as of GCC 8 of 2017-11-21, GCC cannot tell
+	   that, even given range checks on all fields with
+	   __builtin_unreachable called for out-of-range values.  */
+#if __GNUC_PREREQ (7, 0)
+	DIAG_IGNORE_NEEDS_COMMENT (8, "-Wformat-overflow=");
 	sprintf(output, "%04d%02d%02d%02d%02d%02d",
 		time->tm_year, time->tm_mon, time->tm_mday,
 		time->tm_hour, time->tm_min, time->tm_sec);
 	return (output);
 libresolv_hidden_def (__p_secstodate)
diff --git a/resolv/tst-p_secstodate.c b/resolv/tst-p_secstodate.c
new file mode 100644
index 0000000..9dac1ad
--- /dev/null
+++ b/resolv/tst-p_secstodate.c
@@ -0,0 +1,67 @@
+/* Test p_secstodate.
+   Copyright (C) 2017 Free Software Foundation, Inc.
+   This file is part of the GNU C Library.
+   The GNU C Library is free software; you can redistribute it and/or
+   modify it under the terms of the GNU Lesser General Public
+   License as published by the Free Software Foundation; either
+   version 2.1 of the License, or (at your option) any later version.
+   The GNU C Library is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   Lesser General Public License for more details.
+   You should have received a copy of the GNU Lesser General Public
+   License along with the GNU C Library; if not, see
+   <>.  */
+#include <array_length.h>
+#include <limits.h>
+#include <resolv.h>
+#include <stdbool.h>
+#include <stdio.h>
+#include <string.h>
+struct test
+  /* Argument to p_secstodate.  */
+  unsigned long int in;
+  /* Expected output.  */
+  const char *out;
+static const struct test tests[] =
+  {
+    { 0UL, "19700101000000" },
+    { 12345UL, "19700101032545" },
+    { 999999999UL, "20010909014639" },
+    { 2147483647UL, "20380119031407" },
+    { 2147483648UL, "<overflow>" },
+    { 4294967295UL, "<overflow>" },
+#if ULONG_MAX > 0xffffffffUL
+    { 4294967296UL, "<overflow>" },
+    { 9999999999UL, "<overflow>" },
+    { LONG_MAX, "<overflow>" },
+    { ULONG_MAX, "<overflow>" },
+  };
+static int
+do_test (void)
+  int ret = 0;
+  for (size_t i = 0; i < array_length (tests); i++)
+    {
+      char *p = p_secstodate (tests[i].in);
+      printf ("Test %zu: %lu -> %s\n", i, tests[i].in, p);
+      if (strcmp (p, tests[i].out) != 0)
+	{
+	  printf ("test %zu failed", i);
+	  ret = 1;
+	}
+    }
+  return ret;
+#include <support/test-driver.c>

Joseph S. Myers

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