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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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