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Date: Thu, 11 Oct 2018 13:55:51 -0700
From: Kees Cook <keescook@...omium.org>
To: Andy Lutomirski <luto@...nel.org>
Cc: Kristen Carlson Accardi <kristen@...ux.intel.com>,
Kernel Hardening <kernel-hardening@...ts.openwall.com>, Thomas Gleixner <tglx@...utronix.de>,
Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>, "H. Peter Anvin" <hpa@...or.com>,
X86 ML <x86@...nel.org>, LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] x86: entry: flush the cache if syscall error
On Thu, Oct 11, 2018 at 1:48 PM, Andy Lutomirski <luto@...nel.org> wrote:
> On Thu, Oct 11, 2018 at 11:55 AM Kristen Carlson Accardi
> <kristen@...ux.intel.com> wrote:
>>
>> This patch aims to make it harder to perform cache timing attacks on data
>> left behind by system calls. If we have an error returned from a syscall,
>> flush the L1 cache.
>>
>> It's important to note that this patch is not addressing any specific
>> exploit, nor is it intended to be a complete defense against anything.
>> It is intended to be a low cost way of eliminating some of side effects
>> of a failed system call.
>>
>> A performance test using sysbench on one hyperthread and a script which
>> attempts to repeatedly access files it does not have permission to access
>> on the other hyperthread found no significant performance impact.
>>
>> Suggested-by: Alan Cox <alan@...ux.intel.com>
>> Signed-off-by: Kristen Carlson Accardi <kristen@...ux.intel.com>
>> ---
>> arch/x86/Kconfig | 9 +++++++++
>> arch/x86/entry/common.c | 18 ++++++++++++++++++
>> 2 files changed, 27 insertions(+)
>>
>> diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
>> index 1a0be022f91d..bde978eb3b4e 100644
>> --- a/arch/x86/Kconfig
>> +++ b/arch/x86/Kconfig
>> @@ -445,6 +445,15 @@ config RETPOLINE
>> code are eliminated. Since this includes the syscall entry path,
>> it is not entirely pointless.
>>
>> +config SYSCALL_FLUSH
>> + bool "Clear L1 Cache on syscall errors"
>> + default n
>> + help
>> + Selecting 'y' allows the L1 cache to be cleared upon return of
>> + an error code from a syscall if the CPU supports "flush_l1d".
>> + This may reduce the likelyhood of speculative execution style
>> + attacks on syscalls.
>> +
>> config INTEL_RDT
>> bool "Intel Resource Director Technology support"
>> default n
>> diff --git a/arch/x86/entry/common.c b/arch/x86/entry/common.c
>> index 3b2490b81918..26de8ea71293 100644
>> --- a/arch/x86/entry/common.c
>> +++ b/arch/x86/entry/common.c
>> @@ -268,6 +268,20 @@ __visible inline void syscall_return_slowpath(struct pt_regs *regs)
>> prepare_exit_to_usermode(regs);
>> }
>>
>> +__visible inline void l1_cache_flush(struct pt_regs *regs)
>> +{
>> + if (IS_ENABLED(CONFIG_SYSCALL_FLUSH) &&
>> + static_cpu_has(X86_FEATURE_FLUSH_L1D)) {
>> + if (regs->ax == 0 || regs->ax == -EAGAIN ||
>> + regs->ax == -EEXIST || regs->ax == -ENOENT ||
>> + regs->ax == -EXDEV || regs->ax == -ETIMEDOUT ||
>> + regs->ax == -ENOTCONN || regs->ax == -EINPROGRESS)
>
> What about ax > 0? (Or more generally, any ax outside the range of -1
> .. -4095 or whatever the error range is.) As it stands, it looks like
> you'll flush on successful read(), write(), recv(), etc, and that
> could seriously hurt performance on real workloads.
Seems like just changing this with "ax == 0" into "ax >= 0" would solve that?
I think this looks like a good idea. It might be worth adding a
comment about the checks to explain why those errors are whitelisted.
It's a cheap and effective mitigation for "unknown future problems"
that doesn't degrade normal workloads.
>> + return;
>> +
>> + wrmsrl(MSR_IA32_FLUSH_CMD, L1D_FLUSH);
What about CPUs without FLUSH_L1D? Could it be done manually with a
memcpy or something?
-Kees
>> + }
>> +}
>> +
>> #ifdef CONFIG_X86_64
>> __visible void do_syscall_64(unsigned long nr, struct pt_regs *regs)
>> {
>> @@ -290,6 +304,8 @@ __visible void do_syscall_64(unsigned long nr, struct pt_regs *regs)
>> regs->ax = sys_call_table[nr](regs);
>> }
>>
>> + l1_cache_flush(regs);
>> +
>> syscall_return_slowpath(regs);
>> }
>> #endif
>> @@ -338,6 +354,8 @@ static __always_inline void do_syscall_32_irqs_on(struct pt_regs *regs)
>> #endif /* CONFIG_IA32_EMULATION */
>> }
>>
>> + l1_cache_flush(regs);
>> +
>> syscall_return_slowpath(regs);
>> }
>>
>> --
>> 2.14.4
>>
--
Kees Cook
Pixel Security
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