Re: [PATCH RFC] s390: Fix nospec table alignments

[Josh Poimboeuf <jpoimboe@kernel.org>]

On Sat, Aug 27, 2022 at 08:23:17PM +0200, Heiko Carstens wrote:
> On Fri, Aug 26, 2022 at 04:55:44PM -0700, Josh Poimboeuf wrote:
> > Add proper alignment for .nospec_call_table and .nospec_return_table in
> > vmlinux.
> > 
> > Fixes: f19fbd5ed642 ("s390: introduce execute-trampolines for branches")
> > Signed-off-by: Josh Poimboeuf <jpoimboe@kernel.org>
> > ---
> > This is RFC because I don't know anything about s390 behavior for
> > unaligned data accesses, but this seemed to fix an issue for me.
> > 
> > While working on another s390 issue, I was getting intermittent boot
> > failures in __nospec_revert() when it tried to access 'instr[0]'.  I
> > noticed the __nospec_call_start address ended in 'ff'.  This patch
> > seemed to fix it.  I have no idea why it was (only sometimes) failing in
> > the first place.
> > 
> > The intermittent part of it is probably at least partially explained by
> > CONFIG_RANDOMIZE_BASE.  Except now I can no longer recreate it :-/
> > 
> > Regardless, this patch seems correct.  I just can't explain what I saw.
> > Any ideas?
> > 
> >  arch/s390/kernel/vmlinux.lds.S | 1 +
> >  1 file changed, 1 insertion(+)
> > 
> > diff --git a/arch/s390/kernel/vmlinux.lds.S b/arch/s390/kernel/vmlinux.lds.S
> > index 2e526f11b91e..5ea3830af0cc 100644
> > --- a/arch/s390/kernel/vmlinux.lds.S
> > +++ b/arch/s390/kernel/vmlinux.lds.S
> > @@ -131,6 +131,7 @@ SECTIONS
> >  	/*
> >  	 * Table with the patch locations to undo expolines
> >  	*/
> > +	. = ALIGN(4);
> >  	.nospec_call_table : {
> >  		__nospec_call_start = . ;
> >  		*(.s390_indirect*)
> 
> On s390 labels must be at an even address due to instructions like
> "larl" (load address relative long), which generate a pc-relative
> address adding the number of words encoded into the instruction to the
> current IP.
> 
> With commit e6ed91fd0768 ("s390/alternatives: remove padding
> generation code") I managed to reduce the size of struct alt_instr by
> one byte, so it is now only 11 bytes in size. So depending on the size
> / number of members of the __alt_instructions array nospec_call_table
> starts at an uneven address, which would explain this.
> 
> Unfortunately I was unable to let any compiler generate code, that
> would use the larl instruction. Instead the address of
> nospec_call_table was loaded indirectly via the GOT, which again works
> always, regardless if the table starts at an even or uneven address.
> 
> This needs to be fixed anyway, and your patch certainly is correct.
> 
> Could you maybe share your kernel config + compiler version, if you
> are still able to reproduce this?

I think the trick is to disable CONFIG_RELOCATABLE.  When I compile with
CONFIG_RELOCATABLE=n and "gcc version 11.3.1 20220421 (Red Hat 11.3.1-2)
(GCC)", I get the following in nospec_init_branches():

 2a8:   c0 20 00 00 00 00       larl    %r2,2a8 <nospec_init_branches+0x30>     2aa: R_390_PC32DBL      __nospec_call_start+0x2

That said, I still haven't been able to figure out how to recreate the
program check in __nospec_revert(), even when the nospec_call_table
starts at an odd offset.

BTW, I only discovered this when testing with my pending patches which
propose getting rid of -fPIE for CONFIG_RELOCATABLE, so that more than
64k sections can be supported.  But that's a separate topic :-)

-- 
Josh