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

[Nadav Amit]

> On Feb 21, 2019, at 4:07 PM, Sean Christopherson <sean.j.christopherson@xxxxxxxxx> wrote:
> 
> On Thu, Feb 21, 2019 at 03:44:34PM -0800, Rick Edgecombe wrote:
>> From: Nadav Amit <namit@xxxxxxxxxx>
>> 
>> 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@xxxxxxxxxx>
>> Signed-off-by: Nadav Amit <namit@xxxxxxxxxx>
>> ---
>> 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