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From 681900d29683722b1cb0a8e565a0585846ec5a61 Mon Sep 17 00:00:00 2001
From: Florian Weimer <fweimer@redhat.com>
Date: Tue, 22 Sep 2020 19:07:48 +0200
Subject: [PATCH] x86: Harden printf against non-normal long double values (bug
26649)
The behavior of isnan/__builtin_isnan on bit patterns that do not
correspond to something that the CPU would produce from valid inputs
is currently under-defined in the toolchain. (The GCC built-in and
glibc disagree.)
The isnan check in PRINTF_FP_FETCH in stdio-common/printf_fp.c
assumes the GCC behavior that returns true for non-normal numbers
which are not specified as NaN. (The glibc implementation returns
false for such numbers.)
At present, passing non-normal numbers to __mpn_extract_long_double
causes this function to produce irregularly shaped multi-precision
integers, triggering undefined behavior in __printf_fp_l.
With GCC 10 and glibc 2.32, this behavior is not visible because
__builtin_isnan is used, which avoids calling
__mpn_extract_long_double in this case. This commit updates the
implementation of __mpn_extract_long_double so that regularly shaped
multi-precision integers are produced in this case, avoiding
undefined behavior in __printf_fp_l.
Upstream-Status: Backport [git://sourceware.org/git/glibc.git]
CVE: CVE-2020-29573
Signed-off-By: Armin Kuster <akuster@mvista.com>
---
sysdeps/x86/Makefile | 4 ++
sysdeps/x86/ldbl2mpn.c | 8 ++++
sysdeps/x86/tst-ldbl-nonnormal-printf.c | 52 +++++++++++++++++++++++++
3 files changed, 64 insertions(+)
create mode 100644 sysdeps/x86/tst-ldbl-nonnormal-printf.c
Index: git/sysdeps/x86/Makefile
===================================================================
--- git.orig/sysdeps/x86/Makefile
+++ git/sysdeps/x86/Makefile
@@ -9,6 +9,10 @@ tests += tst-get-cpu-features tst-get-cp
tests-static += tst-get-cpu-features-static
endif
+ifeq ($(subdir),math)
+tests += tst-ldbl-nonnormal-printf
+endif # $(subdir) == math
+
ifeq ($(subdir),setjmp)
gen-as-const-headers += jmp_buf-ssp.sym
sysdep_routines += __longjmp_cancel
Index: git/sysdeps/x86/tst-ldbl-nonnormal-printf.c
===================================================================
--- /dev/null
+++ git/sysdeps/x86/tst-ldbl-nonnormal-printf.c
@@ -0,0 +1,52 @@
+/* Test printf with x86-specific non-normal long double value.
+ Copyright (C) 2020 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
+ MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
+ 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
+ <https://www.gnu.org/licenses/>. */
+
+#include <stdio.h>
+#include <string.h>
+#include <support/check.h>
+
+/* Fill the stack with non-zero values. This makes a crash in
+ snprintf more likely. */
+static void __attribute__ ((noinline, noclone))
+fill_stack (void)
+{
+ char buffer[65536];
+ memset (buffer, 0xc0, sizeof (buffer));
+ asm ("" ::: "memory");
+}
+
+static int
+do_test (void)
+{
+ fill_stack ();
+
+ long double value;
+ memcpy (&value, "\x00\x04\x00\x00\x00\x00\x00\x00\x00\x04", 10);
+
+ char buf[30];
+ int ret = snprintf (buf, sizeof (buf), "%Lg", value);
+ TEST_COMPARE (ret, strlen (buf));
+ if (strcmp (buf, "nan") != 0)
+ /* If snprintf does not recognize the non-normal number as a NaN,
+ it has added the missing explicit MSB. */
+ TEST_COMPARE_STRING (buf, "3.02201e-4624");
+ return 0;
+}
+
+#include <support/test-driver.c>
Index: git/sysdeps/i386/ldbl2mpn.c
===================================================================
--- git.orig/sysdeps/i386/ldbl2mpn.c
+++ git/sysdeps/i386/ldbl2mpn.c
@@ -115,6 +115,12 @@ __mpn_extract_long_double (mp_ptr res_pt
&& res_ptr[N - 1] == 0)
/* Pseudo zero. */
*expt = 0;
-
+ else
+ /* The sign bit is explicit, but add it in case it is missing in
+ the input. Otherwise, callers will not be able to produce the
+ expected multi-precision integer layout by shifting the sign
+ bit into the MSB. */
+ res_ptr[N - 1] |= (mp_limb_t) 1 << (LDBL_MANT_DIG - 1
+ - ((N - 1) * BITS_PER_MP_LIMB));
return N;
}
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