Subject: Re: [PATCH v3 09/13] powerpc: extend execmem_params for kprobes allocations

From: Mike Rapoport <[email-masked]>
Date: Sat, 23 Sep 2023 19:27:42 +0300

Hi Christophe,

On Fri, Sep 22, 2023 at 10:32:46AM +0000, Christophe Leroy wrote:
> Hi Mike,
> 
> Le 18/09/2023 à 09:29, Mike Rapoport a écrit :
> > From: "Mike Rapoport (IBM)" <[email-masked]>
> > 
> > powerpc overrides kprobes::alloc_insn_page() to remove writable
> > permissions when STRICT_MODULE_RWX is on.
> > 
> > Add definition of EXECMEM_KRPOBES to execmem_params to allow using the
> > generic kprobes::alloc_insn_page() with the desired permissions.
> > 
> > As powerpc uses breakpoint instructions to inject kprobes, it does not
> > need to constrain kprobe allocations to the modules area and can use the
> > entire vmalloc address space.
> 
> I don't understand what you mean here. Does it mean kprobe allocation 
> doesn't need to be executable ? I don't think so based on the pgprot you 
> set.
> 
> On powerpc book3s/32, vmalloc space is not executable. Only modules 
> space is executable. X/NX cannot be set on a per page basis, it can only 
> be set on a 256 Mbytes segment basis.
> 
> See commit c49643319715 ("powerpc/32s: Only leave NX unset on segments 
> used for modules") and 6ca055322da8 ("powerpc/32s: Use dedicated segment 
> for modules with STRICT_KERNEL_RWX") and 7bee31ad8e2f ("powerpc/32s: Fix 
> is_module_segment() when MODULES_VADDR is defined").
> 
> So if your intention is still to have an executable kprobes, then you 
> can't use vmalloc address space.

Right, and I've fixed the KPROBES range to uses the same range as MODULES.
The commit message is stale and I need to update it.
 
> Christophe
> 
> > 
> > Signed-off-by: Mike Rapoport (IBM) <[email-masked]>
> > ---
> >   arch/powerpc/kernel/kprobes.c | 14 --------------
> >   arch/powerpc/kernel/module.c  | 11 +++++++++++
> >   2 files changed, 11 insertions(+), 14 deletions(-)
> > 
> > diff --git a/arch/powerpc/kernel/kprobes.c b/arch/powerpc/kernel/kprobes.c
> > index 62228c7072a2..14c5ddec3056 100644
> > --- a/arch/powerpc/kernel/kprobes.c
> > +++ b/arch/powerpc/kernel/kprobes.c
> > @@ -126,20 +126,6 @@ kprobe_opcode_t *arch_adjust_kprobe_addr(unsigned long addr, unsigned long offse
> >   	return (kprobe_opcode_t *)(addr + offset);
> >   }
> >   
> > -void *alloc_insn_page(void)
> > -{
> > -	void *page;
> > -
> > -	page = execmem_text_alloc(EXECMEM_KPROBES, PAGE_SIZE);
> > -	if (!page)
> > -		return NULL;
> > -
> > -	if (strict_module_rwx_enabled())
> > -		set_memory_rox((unsigned long)page, 1);
> > -
> > -	return page;
> > -}
> > -
> >   int arch_prepare_kprobe(struct kprobe *p)
> >   {
> >   	int ret = 0;
> > diff --git a/arch/powerpc/kernel/module.c b/arch/powerpc/kernel/module.c
> > index 824d9541a310..bf2c62aef628 100644
> > --- a/arch/powerpc/kernel/module.c
> > +++ b/arch/powerpc/kernel/module.c
> > @@ -95,6 +95,9 @@ static struct execmem_params execmem_params __ro_after_init = {
> >   		[EXECMEM_DEFAULT] = {
> >   			.alignment = 1,
> >   		},
> > +		[EXECMEM_KPROBES] = {
> > +			.alignment = 1,
> > +		},
> >   		[EXECMEM_MODULE_DATA] = {
> >   			.alignment = 1,
> >   		},
> > @@ -135,5 +138,13 @@ struct execmem_params __init *execmem_arch_params(void)
> >   
> >   	range->pgprot = prot;
> >   
> > +	execmem_params.ranges[EXECMEM_KPROBES].start = VMALLOC_START;
> > +	execmem_params.ranges[EXECMEM_KPROBES].start = VMALLOC_END;
> > +
> > +	if (strict_module_rwx_enabled())
> > +		execmem_params.ranges[EXECMEM_KPROBES].pgprot = PAGE_KERNEL_ROX;
> > +	else
> > +		execmem_params.ranges[EXECMEM_KPROBES].pgprot = PAGE_KERNEL_EXEC;
> > +
> >   	return &execmem_params;
> >   }

-- 
Sincerely yours,
Mike.