Re: [PATCH] powerpc/mm: Fix KUAP warning by providing copy_from_kernel_nofault_allowed()

[Christophe Leroy]

Le 05/12/2020 à 09:48, Christoph Hellwig a écrit :
> On Sat, Dec 05, 2020 at 08:43:06AM +0000, Christophe Leroy wrote:
> > Since commit c33165253492 ("powerpc: use non-set_fs based maccess
> > routines"), userspace access is not granted anymore when using
> > copy_from_kernel_nofault()
> >
> > However, kthread_probe_data() uses copy_from_kernel_nofault()
> > to check validity of pointers. When the pointer is NULL,
> > it points to userspace, leading to a KUAP fault and triggering
> > the following big hammer warning many times when you request
> > a sysrq "show task":
>
>
>
> > To avoid that, copy_from_kernel_nofault_allowed() is used to check
> > whether the address is a valid kernel address. But the default
> > version of it returns true for any address.
> >
> > Provide a powerpc version of copy_from_kernel_nofault_allowed()
> > that returns false when the address is below TASK_USER_MAX,
> > so that copy_from_kernel_nofault() will return -ERANGE.
>
> Looks good.  I wonder if we should just default to the TASK_SIZE_MAX
> check in  copy_from_kernel_nofault_allowed for architectures that select
> CONFIG_ARCH_HAS_NON_OVERLAPPING_ADDRESS_SPACE?

Yes maybe that would be better.

Can you cook a patch an get it into 5.10 ?

Christophe

> > Reported-by: Qian Cai <qcai@xxxxxxxxxx>
> > Fixes: c33165253492 ("powerpc: use non-set_fs based maccess routines")
> > Cc: Christoph Hellwig <hch@xxxxxx>
> > Cc: Al Viro <viro@xxxxxxxxxxxxxxxxxx>
> > Signed-off-by: Christophe Leroy <christophe.leroy@xxxxxxxxxx>
> > ---
> > This issue was introduced in 5.10. I didn't mark it for stable, hopping it will go into 5.10-rc7
> > ---
> >   arch/powerpc/mm/Makefile  | 2 +-
> >   arch/powerpc/mm/maccess.c | 9 +++++++++
> >   2 files changed, 10 insertions(+), 1 deletion(-)
> >   create mode 100644 arch/powerpc/mm/maccess.c
> >
> > diff --git a/arch/powerpc/mm/Makefile b/arch/powerpc/mm/Makefile
> > index 5e147986400d..55b4a8bd408a 100644
> > --- a/arch/powerpc/mm/Makefile
> > +++ b/arch/powerpc/mm/Makefile
> > @@ -5,7 +5,7 @@
> >   
> >   ccflags-$(CONFIG_PPC64)	:= $(NO_MINIMAL_TOC)
> >   
> > -obj-y				:= fault.o mem.o pgtable.o mmap.o \
> > +obj-y				:= fault.o mem.o pgtable.o mmap.o maccess.o \
> >   				   init_$(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/maccess.c b/arch/powerpc/mm/maccess.c
> > new file mode 100644
> > index 000000000000..56e97c0fb233
> > --- /dev/null
> > +++ b/arch/powerpc/mm/maccess.c
> > @@ -0,0 +1,9 @@
> > +// SPDX-License-Identifier: GPL-2.0-only
> > +
> > +#include <linux/uaccess.h>
> > +#include <linux/kernel.h>
> > +
> > +bool copy_from_kernel_nofault_allowed(const void *unsafe_src, size_t size)
> > +{
> > +	return (unsigned long)unsafe_src >= TASK_SIZE_MAX;
> > +}
> > --
> > 2.25.0
> ---end quoted text---