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Message-ID: <df3d65fe-0c13-10dc-8508-b59b6daa3fdc@c-s.fr>
Date: Wed, 29 Apr 2020 07:39:06 +0200
From: Christophe Leroy <christophe.leroy@....fr>
To: "Christopher M. Riedl" <cmr@...ormatik.wtf>,
linuxppc-dev@...ts.ozlabs.org, kernel-hardening@...ts.openwall.com
Subject: Re: [RFC PATCH v2 1/5] powerpc/mm: Introduce temporary mm
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 ?
> +{
> + 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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