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

Christophe Leroy
2020-12-05T10:04:27Z

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@redhat.com>
>> Fixes: c33165253492 ("powerpc: use non-set_fs based maccess routines")
>> Cc: Christoph Hellwig <hch@lst.de>
>> Cc: Al Viro <viro@zeniv.linux.org.uk>
>> Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
>> ---
>> 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---
>