oss-security - possible flaw in widely used strtod.c implementation

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Message-ID: <AANLkTikvhcN5Fe9ik=1_fiMk+Rw+qPuO_psax1jLMaqX@mail.gmail.com>
Date: Wed, 5 Jan 2011 09:14:27 +0100
From: Pierre Joye <pierre.php@...il.com>
To: oss-security@...ts.openwall.com
Subject: possible flaw in widely used strtod.c implementation

hi,

Referring to: http://bugs.php.net/53632

This bug affects PHP and can be remotely triggered if someone actually
process an input as double (p.php?id=... and then $d
= $id +1 for example). However this issue could also affect any
software relying on the "strtod for IEEE-, VAX-, and IBM-arithmetic
machines." implementation (quite a lot actually do, according to
codesearch&co). See a non exhaustive list here:

http://www.google.com/codesearch?as_q=strtod+for+IEEE-,+VAX-,+and+IBM-arithmetic+machines.&btnG=Search+Code&hl=en&as_package=&as_lang=&as_filename=&as_class=&as_function=&as_license=&as_case=

Whether the bug exists in the respective builds of each of these
softwares may depend on how they are built (options, arch, etc.).

A fix is already in php's svn:
http://svn.php.net/viewvc?view=revision&revision=307095

A good explanation about this issue is in the gcc bug tracker (thanks
Rasmus for the pointer):

It is a design flaw in the x87 fpu registers, so keeping the float out
of those registers circumvents the problem.  It is
one of the suggested ways of fixing this that is mentioned in the famous
gcc bug 323 report:

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=323

See Comment 87:

 bruno 2006-12-21 15:08:57 UTC
 The option -ffloat-store, recommended by Richard Henderson, has
 the effect of decreasing the performance of floating-point
 operations for the entire compilation unit. If you want a minimal
 fix that does not affect other functions in the same compilation
 unit, you can use 'volatile double' instead of 'double'. It's
 like a one-shot -ffloat-store. Example:

 #include <stdio.h>

 void test(double x, double y) {
   const volatile double y2 = x + 1.0;
   if (y != y2) printf("error\n");
 }

 void main() {
   const double x = .012;
   const double y = x + 1.0;

   test(x, y);
 }

On windows it is slightly more complicated as it seems to do some more
under the wood work. I was able to reproduce the problem on certain
CPUs (i7) and not on other  (xeon) using the exact same binaries. I
still have to verify what is done exactly.

About getting a CVE #, I'm not sure it should be categorized only for
php or more generally about this strtod.c (newest version has the same
problem btw). Ideas? Comments?

Cheers,
--
Pierre

@pierrejoye | http://blog.thepimp.net | http://www.libgd.org

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