Messages in this thread |  | | From | Michael Ellerman <> | Subject | Re: [PATCH v10 14/14] powerpc: Use mm_context vas_windows counter to issue CP_ABORT | Date | Sat, 11 Apr 2020 20:28:50 +1000 |
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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(¤t->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
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