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Date: Fri, 01 May 2020 15:30:34 -0500
From: "Christopher M. Riedl" <cmr@...ormatik.wtf>
To: "Christophe Leroy" <christophe.leroy@....fr>,
<linuxppc-dev@...ts.ozlabs.org>, <kernel-hardening@...ts.openwall.com>
Subject: Re: [RFC PATCH v2 1/5] powerpc/mm: Introduce temporary mm
On Wed Apr 29, 2020 at 7:39 AM, Christophe Leroy wrote:
>
>
>
>
> Le 29/04/2020 à 04:05, 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@...ormatik.wtf>
> > ---
> > arch/powerpc/include/asm/debug.h | 1 +
> > arch/powerpc/include/asm/mmu_context.h | 54 ++++++++++++++++++++++++++
> > arch/powerpc/kernel/process.c | 5 +++
> > 3 files changed, 60 insertions(+)
> >
> > diff --git a/arch/powerpc/include/asm/debug.h b/arch/powerpc/include/asm/debug.h
> > index 7756026b95ca..b945bc16c932 100644
> > --- a/arch/powerpc/include/asm/debug.h
> > +++ b/arch/powerpc/include/asm/debug.h
> > @@ -45,6 +45,7 @@ static inline int debugger_break_match(struct pt_regs *regs) { return 0; }
> > static inline int debugger_fault_handler(struct pt_regs *regs) { return 0; }
> > #endif
> >
> > +void __get_breakpoint(struct arch_hw_breakpoint *brk);
> > void __set_breakpoint(struct arch_hw_breakpoint *brk);
> > bool ppc_breakpoint_available(void);
> > #ifdef CONFIG_PPC_ADV_DEBUG_REGS
> > diff --git a/arch/powerpc/include/asm/mmu_context.h b/arch/powerpc/include/asm/mmu_context.h
> > index 360367c579de..57a8695fe63f 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
> > @@ -270,5 +271,58 @@ 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;
> > +};
> > +
> > +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;
> > +
> > + /*
> > + * 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()) {
> > + __get_breakpoint(&temp_mm->brk);
> > + if (temp_mm->brk.type != 0)
> > + hw_breakpoint_disable();
> > + }
> > +}
> > +
> > +static inline void unuse_temporary_mm(struct temp_mm *temp_mm)
>
>
> Not sure "unuse" is a best naming, allthought I don't have a better
> suggestion a the moment. If not using temporary_mm anymore, what are we
> using now ?
>
>
I'm not too fond of 'unuse' either, but it's what x86 uses and I
couldn't come up with anything better on the spot. Maybe 'undo' is
better since we're switching back to whatever mm was in use before?
> > +{
> > + 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() && temp_mm->brk.type != 0)
> > + __set_breakpoint(&temp_mm->brk);
> > +}
> > +
> > #endif /* __KERNEL__ */
> > #endif /* __ASM_POWERPC_MMU_CONTEXT_H */
> > diff --git a/arch/powerpc/kernel/process.c b/arch/powerpc/kernel/process.c
> > index 9c21288f8645..ec4cf890d92c 100644
> > --- a/arch/powerpc/kernel/process.c
> > +++ b/arch/powerpc/kernel/process.c
> > @@ -800,6 +800,11 @@ static inline int set_breakpoint_8xx(struct arch_hw_breakpoint *brk)
> > return 0;
> > }
> >
> > +void __get_breakpoint(struct arch_hw_breakpoint *brk)
> > +{
> > + memcpy(brk, this_cpu_ptr(¤t_brk), sizeof(*brk));
> > +}
> > +
> > void __set_breakpoint(struct arch_hw_breakpoint *brk)
> > {
> > memcpy(this_cpu_ptr(¤t_brk), brk, sizeof(*brk));
> >
>
>
> Christophe
>
>
>
>
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