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Date:   Wed, 11 Aug 2021 13:02:49 -0500
From:   "Christopher M. Riedl" <cmr@...ux.ibm.com>
To:     "Christophe Leroy" <christophe.leroy@...roup.eu>,
        <linuxppc-dev@...ts.ozlabs.org>
Cc:     <keescook@...omium.org>, <peterz@...radead.org>, <x86@...nel.org>,
        <npiggin@...il.com>, <linux-hardening@...r.kernel.org>,
        <tglx@...utronix.de>, <dja@...ens.net>
Subject: Re: [PATCH v5 5/8] powerpc/64s: Introduce temporary mm for Radix
 MMU

On Thu Aug 5, 2021 at 4:27 AM CDT, Christophe Leroy wrote:
>
>
> Le 13/07/2021 à 07:31, 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. Another 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.
>
> Can you explain more about that breakpoint stuff ? Why is it a special
> case here at all ? Isn't it
> the same when you switch from one user task to another one ? x86 commit
> doesn't say anythink about
> breakpoints.
>

We do not check if the breakpoint is on a kernel address (perf can do
this IIUC) and just disable all of them. I had to dig, but x86 has a
comment with their implementation at arch/x86/kernel/alternative.c:743.

I can reword that part of the commit message if it's unclear.

> > 
> > Based on x86 implementation:
> > 
> > commit cefa929c034e
> > ("x86/mm: Introduce temporary mm structs")
> > 
> > Signed-off-by: Christopher M. Riedl <cmr@...ux.ibm.com>
> > 
> > ---
> > 
> > v5:  * Drop support for using a temporary mm on Book3s64 Hash MMU.
> > 
> > v4:  * Pass the prev mm instead of NULL to switch_mm_irqs_off() when
> >         using/unusing the temp mm as suggested by Jann Horn to keep
> >         the context.active counter in-sync on mm/nohash.
> >       * Disable SLB preload in the temporary mm when initializing the
> >         temp_mm struct.
> >       * Include asm/debug.h header to fix build issue with
> >         ppc44x_defconfig.
> > ---
> >   arch/powerpc/include/asm/debug.h |  1 +
> >   arch/powerpc/kernel/process.c    |  5 +++
> >   arch/powerpc/lib/code-patching.c | 56 ++++++++++++++++++++++++++++++++
> >   3 files changed, 62 insertions(+)
> > 
> > diff --git a/arch/powerpc/include/asm/debug.h b/arch/powerpc/include/asm/debug.h
> > index 86a14736c76c3..dfd82635ea8b3 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/kernel/process.c b/arch/powerpc/kernel/process.c
> > index 185beb2905801..a0776200772e8 100644
> > --- a/arch/powerpc/kernel/process.c
> > +++ b/arch/powerpc/kernel/process.c
> > @@ -865,6 +865,11 @@ 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));
> > diff --git a/arch/powerpc/lib/code-patching.c b/arch/powerpc/lib/code-patching.c
> > index 54b6157d44e95..3122d8e4cc013 100644
> > --- a/arch/powerpc/lib/code-patching.c
> > +++ b/arch/powerpc/lib/code-patching.c
> > @@ -17,6 +17,9 @@
> >   #include <asm/code-patching.h>
> >   #include <asm/setup.h>
> >   #include <asm/inst.h>
> > +#include <asm/mmu_context.h>
> > +#include <asm/debug.h>
> > +#include <asm/tlb.h>
> >   
> >   static int __patch_instruction(u32 *exec_addr, struct ppc_inst instr, u32 *patch_addr)
> >   {
> > @@ -45,6 +48,59 @@ int raw_patch_instruction(u32 *addr, struct ppc_inst instr)
> >   }
> >   
> >   #ifdef CONFIG_STRICT_KERNEL_RWX
> > +
> > +struct temp_mm {
> > +	struct mm_struct *temp;
> > +	struct mm_struct *prev;
> > +	struct arch_hw_breakpoint brk[HBP_NUM_MAX];
> > +};
> > +
> > +static inline void init_temp_mm(struct temp_mm *temp_mm, struct mm_struct *mm)
> > +{
> > +	/* We currently only support temporary mm on the Book3s64 Radix MMU */
> > +	WARN_ON(!radix_enabled());
> > +
> > +	temp_mm->temp = mm;
> > +	temp_mm->prev = NULL;
> > +	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->prev = current->active_mm;
> > +	switch_mm_irqs_off(temp_mm->prev, temp_mm->temp, current);
> > +
> > +	WARN_ON(!mm_is_thread_local(temp_mm->temp));
> > +
> > +	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)
>
> not sure about the naming.
>
> Maybe start_using_temp_mm() and stop_using_temp_mm() would be more
> explicit.
>

Hehe I think we've discussed this before - naming things is hard :) I'll
take your suggestions for the next spin.

>
> > +{
> > +	lockdep_assert_irqs_disabled();
> > +
> > +	switch_mm_irqs_off(temp_mm->temp, 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]);
> > +	}
> > +}
> > +
> >   static DEFINE_PER_CPU(struct vm_struct *, text_poke_area);
> >   
> >   #if IS_BUILTIN(CONFIG_LKDTM)
> > 
>
> You'll probably get a bisecting hasard with those unused 'static inline'
> functions in a .c file
> because that patch alone probably fails build.

I just built the patch without any issue. The compiler only complains
for unused 'static' (non-inline) functions right?

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