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

From: Russell King - ARM Linux admin <[email-masked]>
Date: Fri, 30 Aug 2019 14:35:22 +0100

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.

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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