x86: Harden printf against non-normal long double values (bug 26649)

Message ID 87eemtyjam.fsf@oldenburg2.str.redhat.com
State New
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Series
  • x86: Harden printf against non-normal long double values (bug 26649)
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Commit Message

H.J. Lu via Libc-alpha Sept. 22, 2020, 2:31 p.m.
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.

---
 sysdeps/i386/ldbl2mpn.c                 |  7 +++++
 sysdeps/x86/Makefile                    |  4 +++
 sysdeps/x86/tst-ldbl-nonnormal-printf.c | 52 +++++++++++++++++++++++++++++++++
 3 files changed, 63 insertions(+)


-- 
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Commercial register: Amtsgericht Muenchen, HRB 153243,
Managing Directors: Charles Cachera, Brian Klemm, Laurie Krebs, Michael O'Neill

Comments

H.J. Lu via Libc-alpha Sept. 22, 2020, 2:58 p.m. | #1
On Tue, Sep 22, 2020 at 7:33 AM Florian Weimer via Libc-alpha
<libc-alpha@sourceware.org> wrote:
>

> 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.

>

> ---

>  sysdeps/i386/ldbl2mpn.c                 |  7 +++++

>  sysdeps/x86/Makefile                    |  4 +++

>  sysdeps/x86/tst-ldbl-nonnormal-printf.c | 52 +++++++++++++++++++++++++++++++++

>  3 files changed, 63 insertions(+)

>

> diff --git a/sysdeps/i386/ldbl2mpn.c b/sysdeps/i386/ldbl2mpn.c

> index ec8464eef7..867c66ef8d 100644

> --- a/sysdeps/i386/ldbl2mpn.c

> +++ b/sysdeps/i386/ldbl2mpn.c

> @@ -115,6 +115,13 @@ __mpn_extract_long_double (mp_ptr res_ptr, mp_size_t size,

>            && 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;

>  }

> diff --git a/sysdeps/x86/Makefile b/sysdeps/x86/Makefile

> index c369faf00d..081cc72e93 100644

> --- a/sysdeps/x86/Makefile

> +++ b/sysdeps/x86/Makefile

> @@ -11,6 +11,10 @@ tests += tst-get-cpu-features tst-get-cpu-features-static \

>  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

> diff --git a/sysdeps/x86/tst-ldbl-nonnormal-printf.c b/sysdeps/x86/tst-ldbl-nonnormal-printf.c

> new file mode 100644

> index 0000000000..72797fc33b

> --- /dev/null

> +++ b/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 sign bit.  */

> +    TEST_COMPARE_STRING (buf, "3.02201e-4624");

> +  return 0;

> +}

> +

> +#include <support/test-driver.c>

>

> --

> Red Hat GmbH, https://de.redhat.com/ , Registered seat: Grasbrunn,

> Commercial register: Amtsgericht Muenchen, HRB 153243,

> Managing Directors: Charles Cachera, Brian Klemm, Laurie Krebs, Michael O'Neill

>


Please create a patch to move i386/ldbl2mpn.c to x86 and remove
x86_64/ldbl2mpn.c.
And rebase your patch.

Thanks.

-- 
H.J.

Patch

diff --git a/sysdeps/i386/ldbl2mpn.c b/sysdeps/i386/ldbl2mpn.c
index ec8464eef7..867c66ef8d 100644
--- a/sysdeps/i386/ldbl2mpn.c
+++ b/sysdeps/i386/ldbl2mpn.c
@@ -115,6 +115,13 @@  __mpn_extract_long_double (mp_ptr res_ptr, mp_size_t size,
 	   && 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;
 }
diff --git a/sysdeps/x86/Makefile b/sysdeps/x86/Makefile
index c369faf00d..081cc72e93 100644
--- a/sysdeps/x86/Makefile
+++ b/sysdeps/x86/Makefile
@@ -11,6 +11,10 @@  tests += tst-get-cpu-features tst-get-cpu-features-static \
 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
diff --git a/sysdeps/x86/tst-ldbl-nonnormal-printf.c b/sysdeps/x86/tst-ldbl-nonnormal-printf.c
new file mode 100644
index 0000000000..72797fc33b
--- /dev/null
+++ b/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 sign bit.  */
+    TEST_COMPARE_STRING (buf, "3.02201e-4624");
+  return 0;
+}
+
+#include <support/test-driver.c>