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Date: Thu, 22 Sep 2016 21:39:41 -0400
From: Jeffrey Walton <noloader@...il.com>
To: oss-security@...ts.openwall.com
Cc: John Byrd <jbyrd@...anticsoftware.com>
Subject: CVE Assignment for Crypto++ and "AES and incorrect argument to
_freea() under Microsoft compilers"
CVE assignment, please.
=== INTRODUCTION ===
John Byrd reported a crash in Crypto++'s AES implementation under
Microsoft compilers due to use of `_malloca`, `AliasedWithTables` and
`_freea`. Crypto++ can potentially free the wrong pointer when the
pointer is adjusted to flush cache lines to avoid Bernstein's cache
timing attacks.
=== SCOPE ===
The bug only affects Crypto++ 5.6.4 and was introduced at
http://github.com/weidai11/cryptopp/commit/823bc93357da32a3a4a2b71b9915a4e124839d18.
The bug is specific to Windows and Microsoft compilers because its
guarded by `_MSC_VER`.
The bug does not affect Unix and Linux; and does not affect
non-Microsoft compilers on Windows, like ICC and Borland.
=== DETAILS ===
The code in question asks `_malloca` for a block of memory. The code
also over-commits the size and adjust the pointer to a 256-byte
boundary. The pointer is sent `AliasedWithTables` to flush cache
lines. If `AliasedWithTables` cannot perform the flush, then it
returns false. Upon the false return, the code reallocates, adjusts
the pointer and calls `AliasedWithTables` again. Eventually the call
succeeds. After the code in question completes, it frees the adjusted
pointer and not the original pointer using `_freea`.
`alloca` is on Microsoft's [SDLC banned function list
(http://msdn.microsoft.com/en-us/library/bb288454.aspx). The code was
migrated from `alloca` to `_malloca` for C&A purposes.
`_malloca` and `_freea` are Microsoft SDLC functions. Microsoft
sometimes uses the heap rather than the stack for `_malloca`, and
that's the reason `_freea` is needed.
=== CORRECTION ===
The issue is being tracked at https://github.com/weidai11/cryptopp/issues/302.
The code will be fixed by using the correct pointer. The fix will be
pushed within about two weeks along with CVE-2016-7420.
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