Re: [PATCH 1/5] powerpc/mm: Introduce temporary mm

["Christopher M. Riedl" <cmr@informatik.wtf>]

On Wed Jun 3, 2020 at 8:58 AM, Christophe Leroy wrote:
>
> 
>
> 
> Le 03/06/2020 à 07:19, Christopher M. Riedl a écrit :
> > x86 supports the notion of a temporary mm which restricts access to
> > temporary PTEs to a single CPU. A temporary mm is useful for situations
> > where a CPU needs to perform sensitive operations (such as patching a
> > STRICT_KERNEL_RWX kernel) requiring temporary mappings without exposing
> > said mappings to other CPUs. A side benefit is that other CPU TLBs do
> > not need to be flushed when the temporary mm is torn down.
> > 
> > Mappings in the temporary mm can be set in the userspace portion of the
> > address-space.
> > 
> > Interrupts must be disabled while the temporary mm is in use. HW
> > breakpoints, which may have been set by userspace as watchpoints on
> > addresses now within the temporary mm, are saved and disabled when
> > loading the temporary mm. The HW breakpoints are restored when unloading
> > the temporary mm. All HW breakpoints are indiscriminately disabled while
> > the temporary mm is in use.
> > 
> > Based on x86 implementation:
> > 
> > commit cefa929c034e
> > ("x86/mm: Introduce temporary mm structs")
> > 
> > Signed-off-by: Christopher M. Riedl <cmr@informatik.wtf>
> > ---
> >   arch/powerpc/include/asm/debug.h       |  1 +
> >   arch/powerpc/include/asm/mmu_context.h | 64 ++++++++++++++++++++++++++
> >   arch/powerpc/kernel/process.c          |  5 ++
> >   3 files changed, 70 insertions(+)
> > 
> > diff --git a/arch/powerpc/include/asm/debug.h b/arch/powerpc/include/asm/debug.h
> > index ec57daf87f40..827350c9bcf3 100644
> > --- a/arch/powerpc/include/asm/debug.h
> > +++ b/arch/powerpc/include/asm/debug.h
> > @@ -46,6 +46,7 @@ static inline int debugger_fault_handler(struct pt_regs *regs) { return 0; }
> >   #endif
> >   
> >   void __set_breakpoint(int nr, struct arch_hw_breakpoint *brk);
> > +void __get_breakpoint(int nr, struct arch_hw_breakpoint *brk);
> >   bool ppc_breakpoint_available(void);
> >   #ifdef CONFIG_PPC_ADV_DEBUG_REGS
> >   extern void do_send_trap(struct pt_regs *regs, unsigned long address,
> > diff --git a/arch/powerpc/include/asm/mmu_context.h b/arch/powerpc/include/asm/mmu_context.h
> > index 1a474f6b1992..9269c7c7b04e 100644
> > --- a/arch/powerpc/include/asm/mmu_context.h
> > +++ b/arch/powerpc/include/asm/mmu_context.h
> > @@ -10,6 +10,7 @@
> >   #include <asm/mmu.h>	
> >   #include <asm/cputable.h>
> >   #include <asm/cputhreads.h>
> > +#include <asm/debug.h>
> >   
> >   /*
> >    * Most if the context management is out of line
> > @@ -300,5 +301,68 @@ static inline int arch_dup_mmap(struct mm_struct *oldmm,
> >   	return 0;
> >   }
> >   
> > +struct temp_mm {
> > +	struct mm_struct *temp;
> > +	struct mm_struct *prev;
> > +	bool is_kernel_thread;
> > +	struct arch_hw_breakpoint brk[HBP_NUM_MAX];
> > +};
> > +
> > +static inline void init_temp_mm(struct temp_mm *temp_mm, struct mm_struct *mm)
> > +{
> > +	temp_mm->temp = mm;
> > +	temp_mm->prev = NULL;
> > +	temp_mm->is_kernel_thread = false;
> > +	memset(&temp_mm->brk, 0, sizeof(temp_mm->brk));
> > +}
> > +
> > +static inline void use_temporary_mm(struct temp_mm *temp_mm)
> > +{
> > +	lockdep_assert_irqs_disabled();
> > +
> > +	temp_mm->is_kernel_thread = current->mm == NULL;
> > +	if (temp_mm->is_kernel_thread)
> > +		temp_mm->prev = current->active_mm;
> > +	else
> > +		temp_mm->prev = current->mm;
>
> 
> Is that necessary to make different for kernel threads ? When I look at
> x86 implementation, they don't do such a thing.
>

Yup, in do_slb_fault we error out if the current->mm is NULL resulting
in spectacular fails during patching w/ hash mmu.

> 
> > +
> > +	/*
> > +	 * Hash requires a non-NULL current->mm to allocate a userspace address
> > +	 * when handling a page fault. Does not appear to hurt in Radix either.
> > +	 */
> > +	current->mm = temp_mm->temp;
> > +	switch_mm_irqs_off(NULL, temp_mm->temp, current);
> > +
> > +	if (ppc_breakpoint_available()) {
> > +		struct arch_hw_breakpoint null_brk = {0};
> > +		int i = 0;
> > +
> > +		for (; i < nr_wp_slots(); ++i) {
> > +			__get_breakpoint(i, &temp_mm->brk[i]);
> > +			if (temp_mm->brk[i].type != 0)
> > +				__set_breakpoint(i, &null_brk);
> > +		}
> > +	}
> > +}
> > +
> > +static inline void unuse_temporary_mm(struct temp_mm *temp_mm)
> > +{
> > +	lockdep_assert_irqs_disabled();
> > +
> > +	if (temp_mm->is_kernel_thread)
> > +		current->mm = NULL;
> > +	else
> > +		current->mm = temp_mm->prev;
> > +	switch_mm_irqs_off(NULL, temp_mm->prev, current);
> > +
> > +	if (ppc_breakpoint_available()) {
> > +		int i = 0;
> > +
> > +		for (; i < nr_wp_slots(); ++i)
> > +			if (temp_mm->brk[i].type != 0)
> > +				__set_breakpoint(i, &temp_mm->brk[i]);
> > +	}
> > +}
> > +
> >   #endif /* __KERNEL__ */
> >   #endif /* __ASM_POWERPC_MMU_CONTEXT_H */
> > diff --git a/arch/powerpc/kernel/process.c b/arch/powerpc/kernel/process.c
> > index 048d64c4e115..3973144f6980 100644
> > --- a/arch/powerpc/kernel/process.c
> > +++ b/arch/powerpc/kernel/process.c
> > @@ -825,6 +825,11 @@ static inline int set_breakpoint_8xx(struct arch_hw_breakpoint *brk)
> >   	return 0;
> >   }
> >   
> > +void __get_breakpoint(int nr, struct arch_hw_breakpoint *brk)
> > +{
> > +	memcpy(brk, this_cpu_ptr(&current_brk[nr]), sizeof(*brk));
> > +}
> > +
> >   void __set_breakpoint(int nr, struct arch_hw_breakpoint *brk)
> >   {
> >   	memcpy(this_cpu_ptr(&current_brk[nr]), brk, sizeof(*brk));
> > 
>
> 
> Christophe
>
> 
>
>