Headline
CVE-2023-37479: Security fix for AC flag poisoning and MCDT · openenclave/openenclave@ca54623
Open Enclave is a hardware-agnostic open source library for developing applications that utilize Hardware-based Trusted Execution Environments, also known as Enclaves. There are two issues that are mitigated in version 0.19.3. First, Open Enclave SDK does not properly sanitize the MXCSR register on enclave entry. This makes applications vulnerable to MXCSR Configuration Dependent Timing (MCDT) attacks, where incorrect MXCSR values can impact instruction retirement by at most one cycle, depending on the (secret) data operand value. Please find more details in the guidance from Intel in the references. Second, Open Enclave SDK does not sanitize x86’s alignment check flag RFLAGS.AC on enclave entry. This opens up the possibility for a side-channel attacker to be notified for every unaligned memory access performed by the enclave. The issue has been addressed in version 0.19.3 and the current master branch. Users will need to recompile their applications against the patched libraries to be protected from this vulnerability. There are no known workarounds for this vulnerability.
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@@ -16,9 +16,9 @@ OE_XSAVE_INITIAL_STATE[OE_MINIMAL_XSTATE_AREA_SIZE/sizeof(uint32_t)] = {
* clear Status, Tag, OpCode, FIP words */
0x037F, 0, 0, 0,
/* Clear FDP bits, set MXCSR to ABI init value of 0x1F80
/* Clear FDP bits, set MXCSR to ABI init value of 0x1FBF
* and MXCSR_MASK to all bits (0XFFFF) */
0, 0, 0x1F80, 0xFFFF,
0, 0, 0x1FBF, 0xFFFF,
/* Clear ST/MM0-7 */
0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
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@@ -35,10 +35,12 @@ OE_XSAVE_INITIAL_STATE[OE_MINIMAL_XSTATE_AREA_SIZE/sizeof(uint32_t)] = {
0, 0, 0, 0, 0, 0, 0, 0,
/* XSAVE Header */
/* Clear XSTATE_BV. Note that this means we don’t support
* compaction mode (XCOMP_BV[63]) to accommodate running
* the same code in simulation mode on older CPUs. */
0, 0, 0, 0,
/* Set XSTATE_BV[1] to 1 (SSE state) */
2, 0,
/* Set XCOMP_BV[1] to 1 (SSE state), allowing non-default
* MXCSR value to be restored.
* Also, set XCOMP_BV[63] to 1 for compaction mode */
2, 0x80000000,
/* Reserved XSAVE header bits */
0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
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