From 9333498794cde1d5cca518badf79533a24114b6f Mon Sep 17 00:00:00 2001
From: Joseph Myers
Date: Wed, 12 Feb 2020 23:31:56 +0000
Subject: [PATCH] Avoid ldbl-96 stack corruption from range reduction of
pseudo-zero (bug 25487).
Bug 25487 reports stack corruption in ldbl-96 sinl on a pseudo-zero
argument (an representation where all the significand bits, including
the explicit high bit, are zero, but the exponent is not zero, which
is not a valid representation for the long double type).
Although this is not a valid long double representation, existing
practice in this area (see bug 4586, originally marked invalid but
subsequently fixed) is that we still seek to avoid invalid memory
accesses as a result, in case of programs that treat arbitrary binary
data as long double representations, although the invalid
representations of the ldbl-96 format do not need to be consistently
handled the same as any particular valid representation.
This patch makes the range reduction detect pseudo-zero and unnormal
representations that would otherwise go to __kernel_rem_pio2, and
returns a NaN for them instead of continuing with the range reduction
process. (Pseudo-zero and unnormal representations whose unbiased
exponent is less than -1 have already been safely returned from the
function before this point without going through the rest of range
reduction.) Pseudo-zero representations would previously result in
the value passed to __kernel_rem_pio2 being all-zero, which is
definitely unsafe; unnormal representations would previously result in
a value passed whose high bit is zero, which might well be unsafe
since that is not a form of input expected by __kernel_rem_pio2.
Tested for x86_64.
Index: glibc-2.22/sysdeps/ieee754/ldbl-96/e_rem_pio2l.c
===================================================================
--- glibc-2.22.orig/sysdeps/ieee754/ldbl-96/e_rem_pio2l.c
+++ glibc-2.22/sysdeps/ieee754/ldbl-96/e_rem_pio2l.c
@@ -210,6 +210,18 @@ __ieee754_rem_pio2l (long double x, long
return 0;
}
+ if ((i0 & 0x80000000) == 0)
+ {
+ /* Pseudo-zero and unnormal representations are not valid
+ representations of long double. We need to avoid stack
+ corruption in __kernel_rem_pio2, which expects input in a
+ particular normal form, but those representations do not need
+ to be consistently handled like any particular floating-point
+ value. */
+ y[1] = y[0] = __builtin_nanl ("");
+ return 0;
+ }
+
/* Split the 64 bits of the mantissa into three 24-bit integers
stored in a double array. */
exp = j0 - 23;