Re: [PATCH] powerpc/signal32: Use struct_group() to zero spe regs

[Christophe Leroy]

Le 19/11/2021 à 17:28, Kees Cook a écrit :
> On Fri, Nov 19, 2021 at 08:46:27AM +0000, LEROY Christophe wrote:
> > Le 18/11/2021 à 21:36, Kees Cook a écrit :
> > > In preparation for FORTIFY_SOURCE performing compile-time and run-time
> > > field bounds checking for memset(), avoid intentionally writing across
> > > neighboring fields.
> > >
> > > Add a struct_group() for the spe registers so that memset() can correctly reason
> > > about the size:
> > >
> > >      In function 'fortify_memset_chk',
> > >          inlined from 'restore_user_regs.part.0' at arch/powerpc/kernel/signal_32.c:539:3:
> > >      >> include/linux/fortify-string.h:195:4: error: call to '__write_overflow_field' declared with attribute warning: detected write beyond size of field (1st parameter); maybe use struct_group()? [-Werror=attribute-warning]
> > >        195 |    __write_overflow_field();
> > >            |    ^~~~~~~~~~~~~~~~~~~~~~~~
> > >
> > > Reported-by: kernel test robot <lkp@xxxxxxxxx>
> > > Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
> >
> > Reviewed-by: Christophe Leroy <christophe.leroy@xxxxxxxxxx>
> >
> > However, is it really worth adding that grouping ? Wouldn't it be
> > cleaner to handle evr[] and acc separately ? Now that we are using
> > unsafe variants of get/put user performance wouldn't be impacted.
>
> I'm fine with whatever is desired here. I reworked an earlier version of
> this patch based on mpe's feedback, so I can certain rework it again. :)

Well, with oddities like the below, it may not be straight forward. If 
the objective is to enable FORTIFY_SOURCE, maybe that's good enough.

Let see if Michael has any opinion.


> > I have some doubts about things like:
> >
> > 	unsafe_copy_to_user(&frame->mc_vregs, current->thread.evr,
> > 				    ELF_NEVRREG * sizeof(u32), failed);
> >
> > Because as far as I can see, ELF_NEVRREG is 34 but mc_vregs is a table
> > of 33 u32 and is at the end of the structure:
> >
> > 	struct mcontext {
> > 		elf_gregset_t	mc_gregs;
> > 		elf_fpregset_t	mc_fregs;
> > 		unsigned long	mc_pad[2];
> > 		elf_vrregset_t	mc_vregs __attribute__((__aligned__(16)));
> > 	};
> >
> > 	typedef elf_vrreg_t elf_vrregset_t[ELF_NVRREG];
> >
> > 	# define ELF_NEVRREG	34	/* includes acc (as 2) */
> > 	# define ELF_NVRREG	33	/* includes vscr */
>
> I don't know these internals very well -- do you want me to change this
> specifically somehow? With the BUILD_BUG_ON()s added, there's no binary
> change here -- I wanted to make sure nothing was different in the
> output.

Neither do I. I was just scared by what I saw while reviewing your 
patch. A cleanup is probably required but it can be another patch.

Christophe