The following commit has been merged into the x86/tdx branch of tip: Commit-ID: 8de62af018cc9262649d7688f7eb1409b2d8f594 Gitweb: https://git.kernel.org/tip/8de62af018cc9262649d7688f7eb1409b2d8f594 Author: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx> AuthorDate: Fri, 27 Jan 2023 01:11:59 +03:00 Committer: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx> CommitterDate: Fri, 27 Jan 2023 09:46:05 -08:00 x86/tdx: Disable NOTIFY_ENABLES == Background == There is a class of side-channel attacks against SGX enclaves called "SGX Step"[1]. These attacks create lots of exceptions inside of enclaves. Basically, run an in-enclave instruction, cause an exception. Over and over. There is a concern that a VMM could attack a TDX guest in the same way by causing lots of #VE's. The TDX architecture includes new countermeasures for these attacks. It basically counts the number of exceptions and can send another *special* exception once the number of VMM-induced #VE's hits a critical threshold[2]. == Problem == But, these special exceptions are independent of any action that the guest takes. They can occur anywhere that the guest executes. This includes sensitive areas like the entry code. The (non-paranoid) #VE handler is incapable of handling exceptions in these areas. == Solution == Fortunately, the special exceptions can be disabled by the guest via write to NOTIFY_ENABLES TDCS field. NOTIFY_ENABLES is disabled by default, but might be enabled by a bootloader, firmware or an earlier kernel before the current kernel runs. Disable NOTIFY_ENABLES feature explicitly and unconditionally. Any NOTIFY_ENABLES-based #VE's that occur before this point will end up in the early #VE exception handler and die due to unexpected exit reason. [1] https://github.com/jovanbulck/sgx-step [2] https://intel.github.io/ccc-linux-guest-hardening-docs/security-spec.html#safety-against-ve-in-kernel-code Signed-off-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx> Signed-off-by: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx> Reviewed-by: Dave Hansen <dave.hansen@xxxxxxxxx> Link: https://lore.kernel.org/all/20230126221159.8635-8-kirill.shutemov%40linux.intel.com --- arch/x86/coco/tdx/tdx.c | 7 +++++++ 1 file changed, 7 insertions(+) diff --git a/arch/x86/coco/tdx/tdx.c b/arch/x86/coco/tdx/tdx.c index c4a1299..b593009 100644 --- a/arch/x86/coco/tdx/tdx.c +++ b/arch/x86/coco/tdx/tdx.c @@ -19,6 +19,10 @@ #define TDX_GET_VEINFO 3 #define TDX_GET_REPORT 4 #define TDX_ACCEPT_PAGE 6 +#define TDX_WR 8 + +/* TDCS fields. To be used by TDG.VM.WR and TDG.VM.RD module calls */ +#define TDCS_NOTIFY_ENABLES 0x9100000000000010 /* TDX hypercall Leaf IDs */ #define TDVMCALL_MAP_GPA 0x10001 @@ -863,6 +867,9 @@ void __init tdx_early_init(void) tdx_parse_tdinfo(&cc_mask); cc_set_mask(cc_mask); + /* Kernel does not use NOTIFY_ENABLES and does not need random #VEs */ + tdx_module_call(TDX_WR, 0, TDCS_NOTIFY_ENABLES, 0, -1ULL, NULL); + /* * All bits above GPA width are reserved and kernel treats shared bit * as flag, not as part of physical address.
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