[PATCH v5 5/8] powerpc/64s: Introduce temporary mm for Radix MMU

[Christophe Leroy]

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.

> 
> Based on x86 implementation:
> 
> commit cefa929c034e
> ("x86/mm: Introduce temporary mm structs")
> 
> Signed-off-by: Christopher M. Riedl <cmr at linux.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.


> +{
> +	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.