Re: [PATCH v10 14/14] powerpc: Use mm_context vas_windows counter to issue CP_ABORT

[Michael Ellerman <mpe@ellerman.id.au>]

Haren Myneni <haren@linux.ibm.com> writes:
> set_thread_uses_vas() sets used_vas flag for a process that opened VAS
> window and issue CP_ABORT during context switch for only that process.
> In multi-thread application, windows can be shared. For example Thread A
> can open a window and Thread B can run COPY/PASTE instructions to send
> NX request which may cause corruption or snooping or a covert channel.
> Also once this flag is set, continue to run CP_ABORT even the VAS window
> is closed.
>
> So define vas-windows counter in process mm_context, increment this
> counter for each window open and decrement it for window close. If
> vas-windows is set, issue CP_ABORT during context switch. It means
> clear the foreign real address mapping only if the process / thread uses
> COPY/PASTE. Then disable it for that process if windows are not open.
>
> Signed-off-by: Haren Myneni <haren@linux.ibm.com>
> Reported-by: Nicholas Piggin <npiggin@gmail.com>
> Suggested-by: Milton Miller <miltonm@us.ibm.com>
> Suggested-by: Nicholas Piggin <npiggin@gmail.com>
> ---
>  arch/powerpc/include/asm/book3s/64/mmu.h    |  3 +++
>  arch/powerpc/include/asm/mmu_context.h      | 22 ++++++++++++++++++++++
>  arch/powerpc/include/asm/processor.h        |  1 -
>  arch/powerpc/kernel/process.c               |  8 ++++++--
>  arch/powerpc/platforms/powernv/vas-window.c |  1 +
>  5 files changed, 32 insertions(+), 3 deletions(-)

This should presumably be tagged:

Fixes: 9d2a4d71332c ("powerpc: Define set_thread_uses_vas()")

I _think_ we don't need to backport it because currently there's no code
in the kernel that will actually trigger the used_vas case, but please
spell that out for me in the change log.

> diff --git a/arch/powerpc/include/asm/book3s/64/mmu.h b/arch/powerpc/include/asm/book3s/64/mmu.h
> index bb3deb7..f0a9ff6 100644
> --- a/arch/powerpc/include/asm/book3s/64/mmu.h
> +++ b/arch/powerpc/include/asm/book3s/64/mmu.h
> @@ -116,6 +116,9 @@ struct patb_entry {
>  	/* Number of users of the external (Nest) MMU */
>  	atomic_t copros;
>  
> +	/* Number of user space windows opened in process mm_context */
> +	atomic_t vas_windows;

This should probably be a refcount_t.

Which is an atomic that has wrappers that catch overflow/underflow cases
for you, like you've open-coded below.

> diff --git a/arch/powerpc/include/asm/mmu_context.h b/arch/powerpc/include/asm/mmu_context.h
> index 360367c..7fd249498 100644
> --- a/arch/powerpc/include/asm/mmu_context.h
> +++ b/arch/powerpc/include/asm/mmu_context.h
> @@ -185,11 +185,33 @@ static inline void mm_context_remove_copro(struct mm_struct *mm)
>  			dec_mm_active_cpus(mm);
>  	}
>  }
> +
> +/*
> + * vas_windows counter shows number of open windows in the mm
> + * context. During context switch, use this counter to clear the
> + * foreign real address mapping (CP_ABORT) for the thread / process
> + * that intend to use COPY/PASTE. When a process closes all windows,
> + * disable CP_ABORT which is expensive to run.
> + */
> +static inline void mm_context_add_vas_windows(struct mm_struct *mm)

I think this would read better if it wasn't plural. "windows" implies
you can add more than one at a time.

So:

static inline void mm_context_add_vas_window(struct mm_struct *mm)

> +{
> +	atomic_inc(&mm->context.vas_windows);
> +}
> +
> +static inline void mm_context_remove_vas_windows(struct mm_struct *mm)
> +{
> +	int c = atomic_dec_if_positive(&mm->context.vas_windows);
> +
> +	/* Detect imbalance between add and remove */
> +	WARN_ON(c < 0);

ie. here.

> +}
>  #else
>  static inline void inc_mm_active_cpus(struct mm_struct *mm) { }
>  static inline void dec_mm_active_cpus(struct mm_struct *mm) { }
>  static inline void mm_context_add_copro(struct mm_struct *mm) { }
>  static inline void mm_context_remove_copro(struct mm_struct *mm) { }
> +static inline void mm_context_add_vas_windows(struct mm_struct *mm) { }
> +static inline void mm_context_remove_vas_windows(struct mm_struct *mm) { }
>  #endif

  
> diff --git a/arch/powerpc/include/asm/processor.h b/arch/powerpc/include/asm/processor.h
> index eedcbfb..bfa336f 100644
> --- a/arch/powerpc/include/asm/processor.h
> +++ b/arch/powerpc/include/asm/processor.h
> @@ -272,7 +272,6 @@ struct thread_struct {
>  	unsigned 	mmcr0;
>  
>  	unsigned 	used_ebb;
> -	unsigned int	used_vas;
>  #endif
>  };
>  
> diff --git a/arch/powerpc/kernel/process.c b/arch/powerpc/kernel/process.c
> index fad50db..a3ecaf9 100644
> --- a/arch/powerpc/kernel/process.c
> +++ b/arch/powerpc/kernel/process.c
> @@ -1221,7 +1221,8 @@ struct task_struct *__switch_to(struct task_struct *prev,
>  		 * mappings, we must issue a cp_abort to clear any state and
>  		 * prevent snooping, corruption or a covert channel.
>  		 */
> -		if (current->thread.used_vas)
> +		if (current->mm &&
> +			atomic_read(&current->mm->context.vas_windows))
>  			asm volatile(PPC_CP_ABORT);
>  	}
>  #endif /* CONFIG_PPC_BOOK3S_64 */
> @@ -1466,7 +1467,10 @@ int set_thread_uses_vas(void)

set_thread_uses_vas() should probably just be moved into vas-window.c
which is its only caller.

>  	if (!cpu_has_feature(CPU_FTR_ARCH_300))
>  		return -EINVAL;
>  
> -	current->thread.used_vas = 1;
> +	if (!current->mm)
> +		return -EINVAL;
> +
> +	mm_context_add_vas_windows(current->mm);
  
I needed to dig a bit to confirm this was the right place to call this.

The call trace is:
  mm_context_add_vas_window()
  set_thread_uses_vas()
  vas_tx_win_open()
  coproc_ioc_tx_win_open()
  coproc_ioctl()

> diff --git a/arch/powerpc/platforms/powernv/vas-window.c b/arch/powerpc/platforms/powernv/vas-window.c
> index 3ffad5a..33dfbbf 100644
> --- a/arch/powerpc/platforms/powernv/vas-window.c
> +++ b/arch/powerpc/platforms/powernv/vas-window.c
> @@ -1333,6 +1333,7 @@ int vas_win_close(struct vas_window *window)
>  			put_pid(window->pid);
>  			if (window->mm) {
>  				mm_context_remove_copro(window->mm);
> +				mm_context_remove_vas_windows(window->mm);
>  				mmdrop(window->mm);
>  			}
>  		}

And similarly here, this is:
  vas_win_close()
  coproc_release()


cheers