Re: [PATCH v6 1/7] powerpc/mm: Implement set_memory() routines

[Russell Currey <ruscur@russell.cc>]

On Wed, 2020-03-11 at 17:03 +1100, Daniel Axtens wrote:
> Russell Currey <ruscur@russell.cc> writes:
> 
> > The set_memory_{ro/rw/nx/x}() functions are required for
> > STRICT_MODULE_RWX,
> > and are generally useful primitives to have.  This implementation
> > is
> > designed to be completely generic across powerpc's many MMUs.
> > 
> > It's possible that this could be optimised to be faster for
> > specific
> > MMUs, but the focus is on having a generic and safe implementation
> > for
> > now.
> > 
> > This implementation does not handle cases where the caller is
> > attempting
> > to change the mapping of the page it is executing from, or if
> > another
> > CPU is concurrently using the page being altered.  These cases
> > likely
> > shouldn't happen, but a more complex implementation with MMU-
> > specific code
> > could safely handle them, so that is left as a TODO for now.
> > 
> > These functions do nothing if STRICT_KERNEL_RWX is not enabled.
> > 
> > Signed-off-by: Russell Currey <ruscur@russell.cc>
> > Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
> > ---
> > v6: Merge patch 8/8 from v5, handling RWX not being enabled.
> >     Add note to change_page_attr() in case it's ever made non-
> > static
> > ---
> >  arch/powerpc/Kconfig                  |  1 +
> >  arch/powerpc/include/asm/set_memory.h | 32 +++++++++++
> >  arch/powerpc/mm/Makefile              |  2 +-
> >  arch/powerpc/mm/pageattr.c            | 79
> > +++++++++++++++++++++++++++
> >  4 files changed, 113 insertions(+), 1 deletion(-)
> >  create mode 100644 arch/powerpc/include/asm/set_memory.h
> >  create mode 100644 arch/powerpc/mm/pageattr.c
> > 
> > diff --git a/arch/powerpc/Kconfig b/arch/powerpc/Kconfig
> > index 497b7d0b2d7e..bd074246e34e 100644
> > --- a/arch/powerpc/Kconfig
> > +++ b/arch/powerpc/Kconfig
> > @@ -129,6 +129,7 @@ config PPC
> >  	select ARCH_HAS_PTE_SPECIAL
> >  	select ARCH_HAS_MEMBARRIER_CALLBACKS
> >  	select ARCH_HAS_SCALED_CPUTIME		if
> > VIRT_CPU_ACCOUNTING_NATIVE && PPC_BOOK3S_64
> > +	select ARCH_HAS_SET_MEMORY
> >  	select ARCH_HAS_STRICT_KERNEL_RWX	if ((PPC_BOOK3S_64 ||
> > PPC32) && !HIBERNATION)
> >  	select ARCH_HAS_TICK_BROADCAST		if
> > GENERIC_CLOCKEVENTS_BROADCAST
> >  	select ARCH_HAS_UACCESS_FLUSHCACHE
> > diff --git a/arch/powerpc/include/asm/set_memory.h
> > b/arch/powerpc/include/asm/set_memory.h
> > new file mode 100644
> > index 000000000000..64011ea444b4
> > --- /dev/null
> > +++ b/arch/powerpc/include/asm/set_memory.h
> > @@ -0,0 +1,32 @@
> > +/* SPDX-License-Identifier: GPL-2.0 */
> > +#ifndef _ASM_POWERPC_SET_MEMORY_H
> > +#define _ASM_POWERPC_SET_MEMORY_H
> > +
> > +#define SET_MEMORY_RO	0
> > +#define SET_MEMORY_RW	1
> > +#define SET_MEMORY_NX	2
> > +#define SET_MEMORY_X	3
> > +
> > +int change_memory_attr(unsigned long addr, int numpages, long
> > action);
> > +
> > +static inline int set_memory_ro(unsigned long addr, int numpages)
> > +{
> > +	return change_memory_attr(addr, numpages, SET_MEMORY_RO);
> > +}
> > +
> > +static inline int set_memory_rw(unsigned long addr, int numpages)
> > +{
> > +	return change_memory_attr(addr, numpages, SET_MEMORY_RW);
> > +}
> > +
> > +static inline int set_memory_nx(unsigned long addr, int numpages)
> > +{
> > +	return change_memory_attr(addr, numpages, SET_MEMORY_NX);
> > +}
> > +
> > +static inline int set_memory_x(unsigned long addr, int numpages)
> > +{
> > +	return change_memory_attr(addr, numpages, SET_MEMORY_X);
> > +}
> > +
> > +#endif
> > diff --git a/arch/powerpc/mm/Makefile b/arch/powerpc/mm/Makefile
> > index 5e147986400d..a998fdac52f9 100644
> > --- a/arch/powerpc/mm/Makefile
> > +++ b/arch/powerpc/mm/Makefile
> > @@ -5,7 +5,7 @@
> >  
> >  ccflags-$(CONFIG_PPC64)	:= $(NO_MINIMAL_TOC)
> >  
> > -obj-y				:= fault.o mem.o pgtable.o
> > mmap.o \
> > +obj-y				:= fault.o mem.o pgtable.o
> > mmap.o pageattr.o \
> >  				   init_$(BITS).o pgtable_$(BITS).o \
> >  				   pgtable-frag.o ioremap.o
> > ioremap_$(BITS).o \
> >  				   init-common.o mmu_context.o drmem.o
> > diff --git a/arch/powerpc/mm/pageattr.c
> > b/arch/powerpc/mm/pageattr.c
> > new file mode 100644
> > index 000000000000..748fa56d9db0
> > --- /dev/null
> > +++ b/arch/powerpc/mm/pageattr.c
> > @@ -0,0 +1,79 @@
> > +// SPDX-License-Identifier: GPL-2.0
> > +
> > +/*
> > + * MMU-generic set_memory implementation for powerpc
> > + *
> > + * Copyright 2019, IBM Corporation.
> > + */
> > +
> > +#include <linux/mm.h>
> > +#include <linux/set_memory.h>
> > +
> > +#include <asm/mmu.h>
> > +#include <asm/page.h>
> > +#include <asm/pgtable.h>
> > +
> > +
> > +/*
> > + * Updates the attributes of a page in three steps:
> > + *
> > + * 1. invalidate the page table entry
> > + * 2. flush the TLB
> > + * 3. install the new entry with the updated attributes
> > + *
> > + * This is unsafe if the caller is attempting to change the
> > mapping of the
> > + * page it is executing from, or if another CPU is concurrently
> > using the
> > + * page being altered.
> > + *
> > + * TODO make the implementation resistant to this.
> > + *
> > + * NOTE: can be dangerous to call without STRICT_KERNEL_RWX
> > + */
> > +static int change_page_attr(pte_t *ptep, unsigned long addr, void
> > *data)
> > +{
> > +	long action = (long)data;
> > +	pte_t pte;
> > +
> > +	spin_lock(&init_mm.page_table_lock);
> > +
> > +	/* invalidate the PTE so it's safe to modify */
> > +	pte = ptep_get_and_clear(&init_mm, addr, ptep);
> > +	flush_tlb_kernel_range(addr, addr + PAGE_SIZE);
> > +
> > +	/* modify the PTE bits as desired, then apply */
> > +	switch (action) {
> > +	case SET_MEMORY_RO:
> > +		pte = pte_wrprotect(pte);
> > +		break;
> > +	case SET_MEMORY_RW:
> > +		pte = pte_mkwrite(pte);
> > +		break;
> > +	case SET_MEMORY_NX:
> > +		pte = pte_exprotect(pte);
> > +		break;
> > +	case SET_MEMORY_X:
> > +		pte = pte_mkexec(pte);
> > +		break;
> > +	default:
> > +		break;
> 
> Should this have a WARN_ON_ONCE to let you know you're doing
> something
> that doesn't work? I know it's only ever called by things in this
> file,
> but still... Anyway it's very minor and I'm not fussed either way.

True, might as well.

> 
> > +	}
> > +
> > +	set_pte_at(&init_mm, addr, ptep, pte);
> > +	spin_unlock(&init_mm.page_table_lock);
> 
> Initially I thought: shouldn't you put the PTL lock/unlock in the
> outer
> function? Then I remembered that apply_to_page_range can potentially
> allocate new page table entries which would deadlock if you held the
> lock.
> 
> Speaking of which - apply_to_page_range will create new pte entries
> if
> you apply it over an address range that isn't filled in. That doesn't
> really make sense here - should you use apply_to_existing_page_range
> instead?
> 
> You _might_ be able to move the PTL lock if you use
> apply_to_existing_page_range but I'm not completely sure if that's
> safe
> or if the speed boost is worth it. You could check mm/memory.c if you
> wanted.

Seems like I should definitely be using apply_to_existing_page_range()
but I'm not too keen on moving the lock in case it's unsafe - and these
only get called on module load so it's not a particularly hot path.

> > +
> > +	return 0;
> > +}
> > +
> > +int change_memory_attr(unsigned long addr, int numpages, long
> > action)
> > +{
> > +	unsigned long start = ALIGN_DOWN(addr, PAGE_SIZE);
> > +	unsigned long sz = numpages * PAGE_SIZE;
> > +
> > +	if (!IS_ENABLED(CONFIG_STRICT_KERNEL_RWX))
> > +		return 0;
> > +
> > +	if (!numpages)
> > +		return 0;
> 
> What happens if numpages is negative? Doesn't the guard need to check
> for that rather than just for zero?

I don't know why numpages isn't unsigned in the set_memory API, that
sounds like another potential patch.

Anyway, yes.

> With those caveats, and noting that I've been focused only on:
>  - lock/unlock paths
>  - integer arithmetic
>  - stuff about apply_page_range semantics
> this patch is:
> 
> Reviewed-by: Daniel Axtens <dja@axtens.net>

Thanks for the review, I wasn't aware apply_to_existing_page_range()
existed.

> 
> Regards,
> Daniel
> 
> > +
> > +	return apply_to_page_range(&init_mm, start, sz,
> > change_page_attr, (void *)action);
> > +}
> > -- 
> > 2.25.1