Re: [PATCH v6 2/5] powerpc/kprobes: Mark newly allocated probes as RO

[Christophe Leroy <christophe.leroy@c-s.fr>]

Le 24/12/2019 à 06:55, Russell Currey a écrit :
> With CONFIG_STRICT_KERNEL_RWX=y and CONFIG_KPROBES=y, there will be one
> W+X page at boot by default.  This can be tested with
> CONFIG_PPC_PTDUMP=y and CONFIG_PPC_DEBUG_WX=y set, and checking the
> kernel log during boot.
> 
> powerpc doesn't implement its own alloc() for kprobes like other
> architectures do, but we couldn't immediately mark RO anyway since we do
> a memcpy to the page we allocate later.  After that, nothing should be
> allowed to modify the page, and write permissions are removed well
> before the kprobe is armed.
> 
> The memcpy() would fail if >1 probes were allocated, so use
> patch_instruction() instead which is safe for RO.
> 
> Reviewed-by: Daniel Axtens <dja@axtens.net>
> Signed-off-by: Russell Currey <ruscur@russell.cc>
> ---
>   arch/powerpc/kernel/kprobes.c | 6 ++++--
>   1 file changed, 4 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/powerpc/kernel/kprobes.c b/arch/powerpc/kernel/kprobes.c
> index 2d27ec4feee4..b72761f0c9e3 100644
> --- a/arch/powerpc/kernel/kprobes.c
> +++ b/arch/powerpc/kernel/kprobes.c
> @@ -24,6 +24,7 @@
>   #include <asm/sstep.h>
>   #include <asm/sections.h>
>   #include <linux/uaccess.h>
> +#include <linux/set_memory.h>
>   
>   DEFINE_PER_CPU(struct kprobe *, current_kprobe) = NULL;
>   DEFINE_PER_CPU(struct kprobe_ctlblk, kprobe_ctlblk);
> @@ -124,13 +125,14 @@ int arch_prepare_kprobe(struct kprobe *p)
>   	}
>   
>   	if (!ret) {
> -		memcpy(p->ainsn.insn, p->addr,
> -				MAX_INSN_SIZE * sizeof(kprobe_opcode_t));
> +		patch_instruction(p->ainsn.insn, *p->addr);
>   		p->opcode = *p->addr;
>   		flush_icache_range((unsigned long)p->ainsn.insn,
>   			(unsigned long)p->ainsn.insn + sizeof(kprobe_opcode_t));

patch_instruction() already does the flush, no need to flush again with 
flush_icache_range()

>   	}
>   
> +	set_memory_ro((unsigned long)p->ainsn.insn, 1);
> +

I don't really understand, why do you need to set this ro ? Or why do 
you need to change the memcpy() to patch_instruction() if the area is 
not already ro ?

If I understand correctly, p->ainsn.insn is within a special executable 
page allocated via module_alloc(). Wouldn't it be more correct to modify 
kprobe get_insn_slot() logic so that allocated page is ROX instead of RWX ?

>   	p->ainsn.boostable = 0;
>   	return ret;
>   }
> 

Christophe