Merge pull request #1367 from gilles-peskine-arm/aesni_has_support-volatile-3.6
Backport 3.6: Fix race condition in mbedtls_aesni_has_support
diff --git a/ChangeLog.d/aesni_has_support.txt b/ChangeLog.d/aesni_has_support.txt
new file mode 100644
index 0000000..26b7c2c
--- /dev/null
+++ b/ChangeLog.d/aesni_has_support.txt
@@ -0,0 +1,17 @@
+Bugfix
+ * Fix a race condition on x86/amd64 platforms in AESNI support detection
+ that could lead to using software AES in some threads at the very
+ beginning of a multithreaded program. Reported by Solar Designer.
+ Fixes #9840.
+
+Security
+ * On x86/amd64 platforms, with some compilers, when the library is
+ compiled with support for both AESNI and software AES and AESNI is
+ available in hardware, an adversary with fine control over which
+ threads make progress in a multithreaded program could force software
+ AES to be used for some time when the program starts. This could allow
+ the adversary to conduct timing attacks and potentially recover the
+ key. In particular, this attacker model may be possible against an SGX
+ enclave.
+ The same vulnerability affects GCM acceleration, which could allow
+ a similarly powerful adversary to craft GCM forgeries.
diff --git a/library/aesni.c b/library/aesni.c
index 4fc1cb9..2857068 100644
--- a/library/aesni.c
+++ b/library/aesni.c
@@ -48,8 +48,19 @@
*/
int mbedtls_aesni_has_support(unsigned int what)
{
- static int done = 0;
- static unsigned int c = 0;
+ /* To avoid a race condition, tell the compiler that the assignment
+ * `done = 1` and the assignment to `c` may not be reordered.
+ * https://github.com/Mbed-TLS/mbedtls/issues/9840
+ *
+ * Note that we may also be worried about memory access reordering,
+ * but fortunately the x86 memory model is not too wild: stores
+ * from the same thread are observed consistently by other threads.
+ * (See example 8-1 in Sewell et al., "x86-TSO: A Rigorous and Usable
+ * Programmer’s Model for x86 Multiprocessors", CACM, 2010,
+ * https://www.cl.cam.ac.uk/~pes20/weakmemory/cacm.pdf)
+ */
+ static volatile int done = 0;
+ static volatile unsigned int c = 0;
if (!done) {
#if MBEDTLS_AESNI_HAVE_CODE == 2