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SubjectRe: [PATCH v3 03/20] x86/mm: Save DRs when loading a temporary mm
Date
> On Feb 21, 2019, at 4:07 PM, Sean Christopherson <sean.j.christopherson@intel.com> wrote:
>
> On Thu, Feb 21, 2019 at 03:44:34PM -0800, Rick Edgecombe wrote:
>> From: Nadav Amit <namit@vmware.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@kernel.org>
>> Signed-off-by: Nadav Amit <namit@vmware.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

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