linux-kernel - Re: [PATCH v3 03/20] x86/mm: Save DRs when loading a temporary mm

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Date:   Fri, 22 Feb 2019 00:17:24 +0000
From:   Nadav Amit <namit@...are.com>
To:     Sean Christopherson <sean.j.christopherson@...el.com>
CC:     Rick Edgecombe <rick.p.edgecombe@...el.com>,
        Andy Lutomirski <luto@...nel.org>,
        Ingo Molnar <mingo@...hat.com>,
        LKML <linux-kernel@...r.kernel.org>, X86 ML <x86@...nel.org>,
        "H. Peter Anvin" <hpa@...or.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Damian Tometzki <linux_dti@...oud.com>,
        linux-integrity <linux-integrity@...r.kernel.org>,
        LSM List <linux-security-module@...r.kernel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Kernel Hardening <kernel-hardening@...ts.openwall.com>,
        Linux-MM <linux-mm@...ck.org>, Will Deacon <will.deacon@....com>,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        Kristen Carlson Accardi <kristen@...ux.intel.com>,
        "deneen.t.dock@...el.com" <deneen.t.dock@...el.com>
Subject: Re: [PATCH v3 03/20] x86/mm: Save DRs when loading a temporary mm

> On Feb 21, 2019, at 4:07 PM, Sean Christopherson <sean.j.christopherson@...el.com> wrote:
> 
> On Thu, Feb 21, 2019 at 03:44:34PM -0800, Rick Edgecombe wrote:
>> From: Nadav Amit <namit@...are.com>
>> 
>> Prevent user watchpoints from mistakenly firing while the temporary mm
>> is being used. As the addresses that of the temporary mm might overlap
>> those of the user-process, this is necessary to prevent wrong signals
>> or worse things from happening.
>> 
>> Cc: Andy Lutomirski <luto@...nel.org>
>> Signed-off-by: Nadav Amit <namit@...are.com>
>> ---
>> arch/x86/include/asm/mmu_context.h | 25 +++++++++++++++++++++++++
>> 1 file changed, 25 insertions(+)
>> 
>> diff --git a/arch/x86/include/asm/mmu_context.h b/arch/x86/include/asm/mmu_context.h
>> index d684b954f3c0..0d6c72ece750 100644
>> --- a/arch/x86/include/asm/mmu_context.h
>> +++ b/arch/x86/include/asm/mmu_context.h
>> @@ -13,6 +13,7 @@
>> #include <asm/tlbflush.h>
>> #include <asm/paravirt.h>
>> #include <asm/mpx.h>
>> +#include <asm/debugreg.h>
>> 
>> extern atomic64_t last_mm_ctx_id;
>> 
>> @@ -358,6 +359,7 @@ static inline unsigned long __get_current_cr3_fast(void)
>> 
>> typedef struct {
>> 	struct mm_struct *prev;
>> +	unsigned short bp_enabled : 1;
>> } temp_mm_state_t;
>> 
>> /*
>> @@ -380,6 +382,22 @@ static inline temp_mm_state_t use_temporary_mm(struct mm_struct *mm)
>> 	lockdep_assert_irqs_disabled();
>> 	state.prev = this_cpu_read(cpu_tlbstate.loaded_mm);
>> 	switch_mm_irqs_off(NULL, mm, current);
>> +
>> +	/*
>> +	 * If breakpoints are enabled, disable them while the temporary mm is
>> +	 * used. Userspace might set up watchpoints on addresses that are used
>> +	 * in the temporary mm, which would lead to wrong signals being sent or
>> +	 * crashes.
>> +	 *
>> +	 * Note that breakpoints are not disabled selectively, which also causes
>> +	 * kernel breakpoints (e.g., perf's) to be disabled. This might be
>> +	 * undesirable, but still seems reasonable as the code that runs in the
>> +	 * temporary mm should be short.
>> +	 */
>> +	state.bp_enabled = hw_breakpoint_active();
> 
> Pretty sure caching hw_breakpoint_active() is unnecessary.  It queries a
> per-cpu value, not hardware's DR7 register, and that same value is
> consumed by hw_breakpoint_restore().  No idea if breakpoints can be
> disabled while using a temp mm, but even if that can happen, there's no
> need to restore breakpoints if they've all been disabled, i.e. if
> hw_breakpoint_active() returns false in unuse_temporary_mm().

Good point. I will fix it for next version.

Thanks,
Nadav