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Date:   Fri, 4 Jun 2021 12:09:31 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Balbir Singh <bsingharora@...il.com>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        "mingo@...hat.com" <mingo@...hat.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "peterz@...radead.org" <peterz@...radead.org>,
        "torvalds@...ux-foundation.org" <torvalds@...ux-foundation.org>,
        "jpoimboe@...hat.com" <jpoimboe@...hat.com>,
        "x86@...nel.org" <x86@...nel.org>,
        "tony.luck@...el.com" <tony.luck@...el.com>,
        "dave.hansen@...el.com" <dave.hansen@...el.com>,
        "thomas.lendacky@....com" <thomas.lendacky@....com>,
        "benh@...nel.crashing.org" <benh@...nel.crashing.org>,
        linux-hardening@...r.kernel.org
Subject: Re: [PATCH v4 0/5] Next revision of the L1D flush patches

On Fri, Jun 04, 2021 at 08:06:14PM +1000, Balbir Singh wrote:
> On Wed, Apr 28, 2021 at 01:08:05PM -0700, Kees Cook wrote:
> > On Tue, Apr 27, 2021 at 12:24:16AM +0200, Thomas Gleixner wrote:
> > > On Mon, Apr 26 2021 at 10:31, Thomas Gleixner wrote:
> > > > On Thu, Apr 08 2021 at 13:23, Kees Cook wrote:
> > > >>
> > > >> I'd still really like to see this -- it's a big hammer, but that's the
> > > >> point for cases where some new flaw appears and we can point to the
> > > >> toolbox and say "you can mitigate it with this while you wait for new
> > > >> kernel/CPU."
> > > >>
> > > >> Any further thoughts from x86 maintainers? This seems like it addressed
> > > >> all of tglx's review comments.
> > > >
> > > > Sorry for dropping the ball on this. It's in my list of things to deal
> > > > with. Starting to look at it now.
> > > 
> > > So I went through the pile and for remorse I sat down and made the
> > > tweaks I think are necessary myself.
> > > 
> > > I've pushed out the result to
> > > 
> > >   git://git.kernel.org/pub/scm/linux/kernel/git/tglx/devel.git x86/l1dflush
> > 
> > Oh excellent; thank you for doing this!
> >
> 
> Thanks again Thomas!
> 
> I no longer have access to the bare metal hardware, but I was able to test
> this under qemu with some emulation changes. The changes worked as expected.
> 
> Folks on the list/cc, appreciate any tested-by or additional reviewed-by
> tags if you do happen to review/test.

I can't test the behavior (no access to CPU), but I wrote a simple prctl
tester. Perhaps this can be expanded on?


diff --git a/tools/testing/selftests/x86/Makefile b/tools/testing/selftests/x86/Makefile
index 333980375bc7..50c150d35962 100644
--- a/tools/testing/selftests/x86/Makefile
+++ b/tools/testing/selftests/x86/Makefile
@@ -13,7 +13,7 @@ CAN_BUILD_WITH_NOPIE := $(shell ./check_cc.sh $(CC) trivial_program.c -no-pie)
 TARGETS_C_BOTHBITS := single_step_syscall sysret_ss_attrs syscall_nt test_mremap_vdso \
 			check_initial_reg_state sigreturn iopl ioperm \
 			test_vsyscall mov_ss_trap \
-			syscall_arg_fault fsgsbase_restore
+			syscall_arg_fault fsgsbase_restore l1d_flush
 TARGETS_C_32BIT_ONLY := entry_from_vm86 test_syscall_vdso unwind_vdso \
 			test_FCMOV test_FCOMI test_FISTTP \
 			vdso_restorer
diff --git a/tools/testing/selftests/x86/l1d_flush.c b/tools/testing/selftests/x86/l1d_flush.c
new file mode 100644
index 000000000000..ef4e73679d58
--- /dev/null
+++ b/tools/testing/selftests/x86/l1d_flush.c
@@ -0,0 +1,66 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+/*
+ * l1d_flush.c: Exercise the L1D flushing behaviors
+ */
+#define _GNU_SOURCE
+
+#include <stdlib.h>
+#include <stddef.h>
+#include <stdio.h>
+#include <string.h>
+#include <errno.h>
+#include <sys/prctl.h>
+
+#ifndef PR_SET_SPECULATION_CTRL
+# define PR_GET_SPECULATION_CTRL	52
+# define PR_SET_SPECULATION_CTRL	53
+#  define PR_SPEC_ENABLE		(1UL << 1)
+#  define PR_SPEC_DISABLE		(1UL << 2)
+#endif
+
+#ifndef PR_SPEC_L1D_FLUSH
+# define PR_SPEC_L1D_FLUSH		2
+#endif
+
+#include "../kselftest_harness.h"
+
+TEST(toggle)
+{
+	int ret;
+
+	ret = prctl(PR_GET_SPECULATION_CTRL, PR_SPEC_L1D_FLUSH, 0, 0, 0);
+	ASSERT_GE(ret, 0) {
+		TH_LOG("PR_GET_SPECULATION_CTRL, PR_SPEC_L1D_FLUSH failed: %d (%s)", errno, strerror(errno));
+	}
+	EXPECT_EQ((ret & (PR_SPEC_DISABLE | PR_SPEC_ENABLE)), 0) {
+		TH_LOG("PR_SPEC_L1D_FLUSH is already enabled!?");
+	}
+
+	/* Enable */
+	ret = prctl(PR_SET_SPECULATION_CTRL, PR_SPEC_L1D_FLUSH, PR_SPEC_ENABLE, 0, 0);
+	EXPECT_EQ(ret, 0) {
+		if (errno == EPERM)
+			SKIP(return, "Kernel does not support PR_SPEC_L1D_FLUSH (boot with l1d_flush=on with a supported CPU)");
+		TH_LOG("PR_SET_SPECULATION_CTRL, PR_SPEC_L1D_FLUSH, PR_SPEC_ENABLE failed: %d (%s)", errno, strerror(errno));
+	}
+
+	/* Check Enable */
+	ret = prctl(PR_GET_SPECULATION_CTRL, PR_SPEC_L1D_FLUSH, 0, 0, 0);
+	EXPECT_EQ((ret & (PR_SPEC_DISABLE | PR_SPEC_ENABLE)), PR_SPEC_ENABLE) {
+		TH_LOG("PR_SPEC_L1D_FLUSH did not stay enabled");
+	}
+
+	/* Disable */
+	ret = prctl(PR_SET_SPECULATION_CTRL, PR_SPEC_L1D_FLUSH, PR_SPEC_DISABLE, 0, 0);
+	EXPECT_EQ(ret, 0) {
+		TH_LOG("PR_SET_SPECULATION_CTRL, PR_SPEC_L1D_FLUSH, PR_SPEC_DISABLE failed: %d (%s)", errno, strerror(errno));
+	}
+
+	/* Check Disable */
+	ret = prctl(PR_GET_SPECULATION_CTRL, PR_SPEC_L1D_FLUSH, 0, 0, 0);
+	EXPECT_EQ((ret & (PR_SPEC_DISABLE | PR_SPEC_ENABLE)), PR_SPEC_DISABLE) {
+		TH_LOG("PR_SPEC_L1D_FLUSH did not stay disabled");
+	}
+}
+
+TEST_HARNESS_MAIN

-- 
Kees Cook

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