Re: [PATCH -next, v2] powerpc: add support for syscall stack randomization

[Michael Ellerman <mpe@ellerman.id.au>]

xiujianfeng <xiujianfeng@huawei.com> writes:
> friendly ping....

I will consider this for v5.20 once the merge window has closed (after
v5.19-rc1 is release).

cheers

> 在 2022/5/16 15:32, Xiu Jianfeng 写道:
>> Add support for adding a random offset to the stack while handling
>> syscalls. This patch uses mftb() instead of get_random_int() for better
>> performance.
>>
>> In order to avoid unconditional stack canaries on syscall entry (due to
>> the use of alloca()), also disable stack protector to avoid triggering
>> needless checks and slowing down the entry path. As there is no general
>> way to control stack protector coverage with a function attribute, this
>> must be disabled at the compilation unit level.
>>
>> Signed-off-by: Xiu Jianfeng <xiujianfeng@huawei.com>
>>
>> ---
>> Changes in v2:
>>    -move choose choose_random_kstack_offset() to the end of system_call_exception
>>    -allow full 6 (10) bits of entropy
>>    -disable stack-protector for interrupt.c
>> ---
>>   arch/powerpc/Kconfig            |  1 +
>>   arch/powerpc/kernel/Makefile    |  7 +++++++
>>   arch/powerpc/kernel/interrupt.c | 19 ++++++++++++++++++-
>>   3 files changed, 26 insertions(+), 1 deletion(-)
>>
>> diff --git a/arch/powerpc/Kconfig b/arch/powerpc/Kconfig
>> index 98309eeae09c..2f0019a0054e 100644
>> --- a/arch/powerpc/Kconfig
>> +++ b/arch/powerpc/Kconfig
>> @@ -192,6 +192,7 @@ config PPC
>>   	select HAVE_ARCH_KASAN			if PPC32 && PPC_PAGE_SHIFT <= 14
>>   	select HAVE_ARCH_KASAN_VMALLOC		if PPC32 && PPC_PAGE_SHIFT <= 14
>>   	select HAVE_ARCH_KFENCE			if PPC_BOOK3S_32 || PPC_8xx || 40x
>> +	select HAVE_ARCH_RANDOMIZE_KSTACK_OFFSET
>>   	select HAVE_ARCH_KGDB
>>   	select HAVE_ARCH_MMAP_RND_BITS
>>   	select HAVE_ARCH_MMAP_RND_COMPAT_BITS	if COMPAT
>> diff --git a/arch/powerpc/kernel/Makefile b/arch/powerpc/kernel/Makefile
>> index 4ddd161aef32..5c5e85b8229b 100644
>> --- a/arch/powerpc/kernel/Makefile
>> +++ b/arch/powerpc/kernel/Makefile
>> @@ -40,6 +40,13 @@ CFLAGS_cputable.o += -DDISABLE_BRANCH_PROFILING
>>   CFLAGS_btext.o += -DDISABLE_BRANCH_PROFILING
>>   endif
>>   
>> +#ifdef CONFIG_RANDOMIZE_KSTACK_OFFSET
>> +# Remove stack protector to avoid triggering unneeded stack canary
>> +# checks due to randomize_kstack_offset.
>> +CFLAGS_REMOVE_interrupt.o = -fstack-protector -fstack-protector-strong
>> +CFLAGS_interrupt.o += -fno-stack-protector
>> +#endif
>> +
>>   obj-y				:= cputable.o syscalls.o \
>>   				   irq.o align.o signal_$(BITS).o pmc.o vdso.o \
>>   				   process.o systbl.o idle.o \
>> diff --git a/arch/powerpc/kernel/interrupt.c b/arch/powerpc/kernel/interrupt.c
>> index 784ea3289c84..d7cdcb6fc336 100644
>> --- a/arch/powerpc/kernel/interrupt.c
>> +++ b/arch/powerpc/kernel/interrupt.c
>> @@ -4,6 +4,7 @@
>>   #include <linux/err.h>
>>   #include <linux/compat.h>
>>   #include <linux/sched/debug.h> /* for show_regs */
>> +#include <linux/randomize_kstack.h>
>>   
>>   #include <asm/kup.h>
>>   #include <asm/cputime.h>
>> @@ -78,10 +79,12 @@ notrace long system_call_exception(long r3, long r4, long r5,
>>   				   long r6, long r7, long r8,
>>   				   unsigned long r0, struct pt_regs *regs)
>>   {
>> +	long ret;
>>   	syscall_fn f;
>>   
>>   	kuap_lock();
>>   
>> +	add_random_kstack_offset();
>>   	regs->orig_gpr3 = r3;
>>   
>>   	if (IS_ENABLED(CONFIG_PPC_IRQ_SOFT_MASK_DEBUG))
>> @@ -229,7 +232,21 @@ notrace long system_call_exception(long r3, long r4, long r5,
>>   		f = (void *)sys_call_table[r0];
>>   	}
>>   
>> -	return f(r3, r4, r5, r6, r7, r8);
>> +	ret = f(r3, r4, r5, r6, r7, r8);
>> +	/*
>> +	 * Ultimately, this value will get limited by KSTACK_OFFSET_MAX(),
>> +	 * so the maximum stack offset is 1k bytes(10 bits).
>> +	 *
>> +	 * The actual entropy will be further reduced by the compiler when
>> +	 * applying stack alignment constraints: the powerpc architecture
>> +	 * may have two kinds of stack alignment(16-bytes and 8-bytes).
>> +	 *
>> +	 * So the resulting 6 or 7 bits of entropy is seen in SP[9:4] or SP[9:3].
>> +	 *
>> +	 */
>> +	choose_random_kstack_offset(mftb());
>> +
>> +	return ret;
>>   }
>>   
>>   static notrace void booke_load_dbcr0(void)