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Subject: Re: [RFC 04/10] x86/mm: Only flush indirect branches when switching
 into non dumpable process
To:     "Woodhouse, David" <dwmw@amazon.co.uk>,
        KarimAllah Ahmed <karahmed@amazon.de>,
        linux-kernel@vger.kernel.org
Cc:     Andi Kleen <ak@linux.intel.com>,
        Andrea Arcangeli <aarcange@redhat.com>,
        Andy Lutomirski <luto@kernel.org>,
        Arjan van de Ven <arjan@linux.intel.com>,
        Ashok Raj <ashok.raj@intel.com>,
        Asit Mallick <asit.k.mallick@intel.com>,
        Borislav Petkov <bp@suse.de>,
        Dan Williams <dan.j.williams@intel.com>,
        Dave Hansen <dave.hansen@intel.com>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        "H . Peter Anvin" <hpa@zytor.com>, Ingo Molnar <mingo@redhat.com>,
        Janakarajan Natarajan <Janakarajan.Natarajan@amd.com>,
        Joerg Roedel <joro@8bytes.org>,
        Jun Nakajima <jun.nakajima@intel.com>,
        Laura Abbott <labbott@redhat.com>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Masami Hiramatsu <mhiramat@kernel.org>,
        Paolo Bonzini <pbonzini@redhat.com>,
        Peter Zijlstra <peterz@infradead.org>,
        =?UTF-8?B?UmFkaW0gS3LEjW3DocWZ?= <rkrcmar@redhat.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Tom Lendacky <thomas.lendacky@amd.com>, kvm@vger.kernel.org,
        x86@kernel.org
References: <1516476182-5153-1-git-send-email-karahmed@amazon.de>
 <1516476182-5153-5-git-send-email-karahmed@amazon.de>
 <1516482364.9814.18.camel@amazon.co.uk>
From:   Tim Chen <tim.c.chen@linux.intel.com>
Message-ID: <bd006c0e-2641-b4ec-9c65-86c5f8c755c2@linux.intel.com>
Date:   Mon, 22 Jan 2018 10:29:23 -0800
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On 01/20/2018 01:06 PM, Woodhouse, David wrote:
> On Sat, 2018-01-20 at 20:22 +0100, KarimAllah Ahmed wrote:
>> From: Tim Chen <tim.c.chen@linux.intel.com>
> 
> I think this is probably From: Andi now rather than From: Tim?

This change is from Andi.


>>  1 file changed, 12 insertions(+), 1 deletion(-)
>>
>> diff --git a/arch/x86/mm/tlb.c b/arch/x86/mm/tlb.c
>> index 304de7d..f64e80c 100644
>> --- a/arch/x86/mm/tlb.c
>> +++ b/arch/x86/mm/tlb.c
>> @@ -225,8 +225,19 @@ void switch_mm_irqs_off(struct mm_struct *prev, struct mm_struct *next,
>>  		 * Avoid user/user BTB poisoning by flushing the branch predictor
>>  		 * when switching between processes. This stops one process from
>>  		 * doing Spectre-v2 attacks on another.
>> +		 *
>> +		 * As an optimization: Flush indirect branches only when
>> +		 * switching into processes that disable dumping.
>> +		 *
>> +		 * This will not flush when switching into kernel threads.
>> +		 * But it would flush when switching into idle and back
>> +		 *
>> +		 * It might be useful to have a one-off cache here
>> +		 * to also not flush the idle case, but we would need some
>> +		 * kind of stable sequence number to remember the previous mm.
>>  		 */
>> -		indirect_branch_prediction_barrier();
>> +		if (tsk && tsk->mm && get_dumpable(tsk->mm) != SUID_DUMP_USER)
>> +			indirect_branch_prediction_barrier();

We could move this close to the cr3 write. The cr3 write provides
barrier against unwanted speculation in the above if check.

Tim