Re: [PATCH v2 1/4] arm64: patching: always use fixmap

[Jarkko Sakkinen]

On Wed Apr 3, 2024 at 7:51 PM EEST, Mark Rutland wrote:
> On Wed, Apr 03, 2024 at 07:20:31PM +0300, Jarkko Sakkinen wrote:
> > On Wed Apr 3, 2024 at 6:01 PM EEST, Mark Rutland wrote:
> > > For historical reasons, patch_map() won't bother to fixmap non-image
> > > addresses when CONFIG_STRICT_MODULE_RWX=n, matching the behaviour prior
> > > to the introduction of CONFIG_STRICT_MODULE_RWX. However, as arm64
> > > doesn't select CONFIG_ARCH_OPTIONAL_KERNEL_RWX, CONFIG_MODULES implies
> > > CONFIG_STRICT_MODULE_RWX, so any kernel built with module support will
> > > use the fixmap for any non-image address.
> > 
> > Not familiar with the config flag but I'd guess it is essentially
> > w^x enforcement right for the sections?
>
> Essentially, yes.
>
> > > Historically we only used patch_map() for the kernel image and modules,
> > > but these days its also used by BPF and KPROBES to write to read-only
> > > pages of executable text. Currently these both depend on CONFIG_MODULES,
> > > but we'd like to change that in subsequent patches, which will require
> > > using the fixmap regardless of CONFIG_STRICT_MODULE_RWX.
> > >
> > > This patch changes patch_map() to always use the fixmap, and simplifies
> > > the logic:
> > >
> > > * Use is_image_text() directly in the if-else, rather than using a
> > >   temporary boolean variable.
> > >
> > > * Use offset_in_page() to get the offset within the mapping.
> > >
> > > * Remove uintaddr and cast the address directly when using
> > >   is_image_text().
> > >
> > > For kernels built with CONFIG_MODULES=y, there should be no functional
> > > change as a result of this patch.
> > >
> > > For kernels built with CONFIG_MODULES=n, patch_map() will use the fixmap
> > > for non-image addresses, but there are no extant users with non-image
> > > addresses when CONFIG_MODULES=n, and hence there should be no functional
> > > change as a result of this patch alone.
> > >
> > > Signed-off-by: Mark Rutland <mark.rutland@xxxxxxx>
> > > Cc: Catalin Marinas <catalin.marinas@xxxxxxx>
> > > Cc: Will Deacon <will@xxxxxxxxxx>
> > > ---
> > >  arch/arm64/kernel/patching.c | 10 +++-------
> > >  1 file changed, 3 insertions(+), 7 deletions(-)
> > >
> > > Catalin, Will, this is a prerequisite for the final two patches in the
> > > series. Are you happy for this go via the tracing tree?
> > >
> > > Mark.
> > >
> > > diff --git a/arch/arm64/kernel/patching.c b/arch/arm64/kernel/patching.c
> > > index 2555349303684..f0f3a2a82ca5a 100644
> > > --- a/arch/arm64/kernel/patching.c
> > > +++ b/arch/arm64/kernel/patching.c
> > > @@ -30,20 +30,16 @@ static bool is_image_text(unsigned long addr)
> > >  
> > >  static void __kprobes *patch_map(void *addr, int fixmap)
> > >  {
> > > -	unsigned long uintaddr = (uintptr_t) addr;
> > > -	bool image = is_image_text(uintaddr);
> > >  	struct page *page;
> > >  
> > > -	if (image)
> > > +	if (is_image_text((unsigned long)addr))
> > >  		page = phys_to_page(__pa_symbol(addr));
> > > -	else if (IS_ENABLED(CONFIG_STRICT_MODULE_RWX))
> > > -		page = vmalloc_to_page(addr);
> > >  	else
> > > -		return addr;
> > > +		page = vmalloc_to_page(addr);
> > >  
> > >  	BUG_ON(!page);
> > >  	return (void *)set_fixmap_offset(fixmap, page_to_phys(page) +
> > > -			(uintaddr & ~PAGE_MASK));
> > > +					 offset_in_page(addr));
> > 
> > nit: could be a single line but i guess it is up to the taste (and
> > subsystem maintainer). I.e. checkpatch will allow it at least.
> > 
> > I don't mind it too much just mentioning for completeness.
>
> At that point it goes to 93 chars long, and I stuck with the existing line
> wrapping at 80 chars. I'd rather have a temporary 'phys_addr_t phys' variable
> and do:
>
> 	phys = page_to_phys(page) + offset_in_page(addr);
> 	return (void *)set_fixmap(fixmap, phys);
>
> ... but I'll leave this as-is for now.
>
> > >  }
> > >  
> > >  static void __kprobes patch_unmap(int fixmap)
> > 
> > If my assumption about the config flag holds this makes sense:
> > 
> > Reviewed-by: Jarkko Sakkinen <jarkko@xxxxxxxxx>
>
> Thanks! I assume that should be "kernel.org", with an 'o' ;)

Yes, that's correct, not from Gibraltar :-)

BR, Jarkko