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

[Sean Christopherson <sean.j.christopherson@intel.com>]

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().

> +	if (state.bp_enabled)
> +		hw_breakpoint_disable();
> +
>  	return state;
>  }
>  
> @@ -387,6 +405,13 @@ static inline void unuse_temporary_mm(temp_mm_state_t prev)
>  {
>  	lockdep_assert_irqs_disabled();
>  	switch_mm_irqs_off(NULL, prev.prev, current);
> +
> +	/*
> +	 * Restore the breakpoints if they were disabled before the temporary mm
> +	 * was loaded.
> +	 */
> +	if (prev.bp_enabled)
> +		hw_breakpoint_restore();
>  }
>  
>  #endif /* _ASM_X86_MMU_CONTEXT_H */
> -- 
> 2.17.1
>