Re: [RFC PATCH] powerpc/lib: Fixing use a temporary mm for code patching

[Christophe Leroy]

Le 15/04/2020 Ã 18:22, Christopher M Riedl a ÃcritÂ:
> > On April 15, 2020 4:12 AM Christophe Leroy <christophe.leroy@xxxxxx> wrote:
> >
> >   
> > Le 15/04/2020 Ã 07:16, Christopher M Riedl a ÃcritÂ:
> > > > On March 26, 2020 9:42 AM Christophe Leroy <christophe.leroy@xxxxxx> wrote:
> > > >
> > > >    
> > > > This patch fixes the RFC series identified below.
> > > > It fixes three points:
> > > > - Failure with CONFIG_PPC_KUAP
> > > > - Failure to write do to lack of DIRTY bit set on the 8xx
> > > > - Inadequaly complex WARN post verification
> > > >
> > > > However, it has an impact on the CPU load. Here is the time
> > > > needed on an 8xx to run the ftrace selftests without and
> > > > with this series:
> > > > - Without CONFIG_STRICT_KERNEL_RWX		==> 38 seconds
> > > > - With CONFIG_STRICT_KERNEL_RWX			==> 40 seconds
> > > > - With CONFIG_STRICT_KERNEL_RWX + this series	==> 43 seconds
> > > >
> > > > Link: https://patchwork.ozlabs.org/project/linuxppc-dev/list/?series=166003
> > > > Signed-off-by: Christophe Leroy <christophe.leroy@xxxxxx>
> > > > ---
> > > >    arch/powerpc/lib/code-patching.c | 5 ++++-
> > > >    1 file changed, 4 insertions(+), 1 deletion(-)
> > > >
> > > > diff --git a/arch/powerpc/lib/code-patching.c b/arch/powerpc/lib/code-patching.c
> > > > index f156132e8975..4ccff427592e 100644
> > > > --- a/arch/powerpc/lib/code-patching.c
> > > > +++ b/arch/powerpc/lib/code-patching.c
> > > > @@ -97,6 +97,7 @@ static int map_patch(const void *addr, struct patch_mapping *patch_mapping)
> > > >    	}
> > > >    
> > > >    	pte = mk_pte(page, pgprot);
> > > > +	pte = pte_mkdirty(pte);
> > > >    	set_pte_at(patching_mm, patching_addr, ptep, pte);
> > > >    
> > > >    	init_temp_mm(&patch_mapping->temp_mm, patching_mm);
> > > > @@ -168,7 +169,9 @@ static int do_patch_instruction(unsigned int *addr, unsigned int instr)
> > > >    			(offset_in_page((unsigned long)addr) /
> > > >    				sizeof(unsigned int));
> > > >    
> > > > +	allow_write_to_user(patch_addr, sizeof(instr));
> > > >    	__patch_instruction(addr, instr, patch_addr);
> > > > +	prevent_write_to_user(patch_addr, sizeof(instr));
> > >
> > > On radix we can map the page with PAGE_KERNEL protection which ends up
> > > setting EAA[0] in the radix PTE. This means the KUAP (AMR) protection is
> > > ignored (ISA v3.0b Fig. 35) since we are accessing the page from MSR[PR]=0.
> > >
> > > Can we employ a similar approach on the 8xx? I would prefer *not* to wrap
> > > the __patch_instruction() with the allow_/prevent_write_to_user() KUAP things
> > > because this is a temporary kernel mapping which really isn't userspace in
> > > the usual sense.
> >
> > On the 8xx, that's pretty different.
> >
> > The PTE doesn't control whether a page is user page or a kernel page.
> > The only thing that is set in the PTE is whether a page is linked to a
> > given PID or not.
> > PAGE_KERNEL tells that the page can be addressed with any PID.
> >
> > The user access right is given by a kind of zone, which is in the PGD
> > entry. Every pages above PAGE_OFFSET are defined as belonging to zone 0.
> > Every pages below PAGE_OFFSET are defined as belonging to zone 1.
> >
> > By default, zone 0 can only be accessed by kernel, and zone 1 can only
> > be accessed by user. When kernel wants to access zone 1, it temporarily
> > changes properties of zone 1 to allow both kernel and user accesses.
> >
> > So, if your mapping is below PAGE_OFFSET, it is in zone 1 and kernel
> > must unlock it to access it.
> >
> >
> > And this is more or less the same on hash/32. This is managed by segment
> > registers. One segment register corresponds to a 256Mbytes area. Every
> > pages below PAGE_OFFSET can only be read by default by kernel. Only user
> > can write if the PTE allows it. When the kernel needs to write at an
> > address below PAGE_OFFSET, it must change the segment properties in the
> > corresponding segment register.
> >
> > So, for both cases, if we want to have it local to a task while still
> > allowing kernel access, it means we have to define a new special area
> > between TASK_SIZE and PAGE_OFFSET which belongs to kernel zone.
> >
> > That looks complex to me for a small benefit, especially as 8xx is not
> > SMP and neither are most of the hash/32 targets.
>
> Agreed. So I guess the solution is to differentiate between radix/non-radix
> and use PAGE_SHARED for non-radix along with the KUAP functions when KUAP
> is enabled. Hmm, I need to think about this some more, especially if it's
> acceptable to temporarily map kernel text as PAGE_SHARED for patching. Do
> you see any obvious problems on 8xx and hash/32 w/ using PAGE_SHARED?

No it shouldn't be a problem AFAICS, except maybe the CPU overhead it 
brings as I mentioned previously (ftrace selftests going from 40 to 43 
seconds ie 8% overhead.

> I don't necessarily want to drop the local mm patching idea for non-radix
> platforms since that means we would have to maintain two implementations.

What's the problem with RADIX, why can't PAGE_SHARED be used on radix ?

Christophe