Subject: Re: [PATCH] arm: fix page faults in do_alignment

From: Jing Xiangfeng <[email-masked]>
Date: Sat, 31 Aug 2019 09:49:45 +0800

On 2019/8/30 21:35, Russell King - ARM Linux admin wrote:
> On Fri, Aug 30, 2019 at 09:31:17PM +0800, Jing Xiangfeng wrote:
>> The function do_alignment can handle misaligned address for user and
>> kernel space. If it is a userspace access, do_alignment may fail on
>> a low-memory situation, because page faults are disabled in
>> probe_kernel_address.
>>
>> Fix this by using __copy_from_user stead of probe_kernel_address.
>>
>> Fixes: b255188 ("ARM: fix scheduling while atomic warning in alignment handling code")
>> Signed-off-by: Jing Xiangfeng <[email-masked]>
> 
> NAK.
> 
> The "scheduling while atomic warning in alignment handling code" is
> caused by fixing up the page fault while trying to handle the
> mis-alignment fault generated from an instruction in atomic context.

__might_sleep is called in the function  __get_user which lead to that bug.
And that bug is triggered in a kernel space. Page fault can not be generated.
Right?

> Your patch re-introduces that bug.
> 
>> ---
>>  arch/arm/mm/alignment.c | 16 +++++++++++++---
>>  1 file changed, 13 insertions(+), 3 deletions(-)
>>
>> diff --git a/arch/arm/mm/alignment.c b/arch/arm/mm/alignment.c
>> index 04b3643..2ccabd3 100644
>> --- a/arch/arm/mm/alignment.c
>> +++ b/arch/arm/mm/alignment.c
>> @@ -774,6 +774,7 @@ static ssize_t alignment_proc_write(struct file *file, const char __user *buffer
>>  	unsigned long instr = 0, instrptr;
>>  	int (*handler)(unsigned long addr, unsigned long instr, struct pt_regs *regs);
>>  	unsigned int type;
>> +	mm_segment_t fs;
>>  	unsigned int fault;
>>  	u16 tinstr = 0;
>>  	int isize = 4;
>> @@ -784,16 +785,22 @@ static ssize_t alignment_proc_write(struct file *file, const char __user *buffer
>>  
>>  	instrptr = instruction_pointer(regs);
>>  
>> +	fs = get_fs();
>> +	set_fs(KERNEL_DS);
>>  	if (thumb_mode(regs)) {
>>  		u16 *ptr = (u16 *)(instrptr & ~1);
>> -		fault = probe_kernel_address(ptr, tinstr);
>> +		fault = __copy_from_user(tinstr,
>> +				(__force const void __user *)ptr,
>> +				sizeof(tinstr));
>>  		tinstr = __mem_to_opcode_thumb16(tinstr);
>>  		if (!fault) {
>>  			if (cpu_architecture() >= CPU_ARCH_ARMv7 &&
>>  			    IS_T32(tinstr)) {
>>  				/* Thumb-2 32-bit */
>>  				u16 tinst2 = 0;
>> -				fault = probe_kernel_address(ptr + 1, tinst2);
>> +				fault = __copy_from_user(tinst2,
>> +						(__force const void __user *)(ptr+1),
>> +						sizeof(tinst2));
>>  				tinst2 = __mem_to_opcode_thumb16(tinst2);
>>  				instr = __opcode_thumb32_compose(tinstr, tinst2);
>>  				thumb2_32b = 1;
>> @@ -803,10 +810,13 @@ static ssize_t alignment_proc_write(struct file *file, const char __user *buffer
>>  			}
>>  		}
>>  	} else {
>> -		fault = probe_kernel_address((void *)instrptr, instr);
>> +		fault = __copy_from_user(instr,
>> +				(__force const void __user *)instrptr,
>> +				sizeof(instr));
>>  		instr = __mem_to_opcode_arm(instr);
>>  	}
>>  
>> +	set_fs(fs);
>>  	if (fault) {
>>  		type = TYPE_FAULT;
>>  		goto bad_or_fault;
>> -- 
>> 1.8.3.1
>>
>>
> 

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