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

From: Russell King - ARM Linux admin <[email-masked]>
Date: Fri, 30 Aug 2019 14:48:29 +0100

Please fix your email.

  [email-masked]
      host mx7.huawei.com [168.195.93.46]
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SPF is *not* required for email.

If you wish to impose such restrictions on email, then I reserve the
right to ignore your patches until this issue is resolved! ;)

On Fri, Aug 30, 2019 at 02:35:22PM +0100, 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.
> 
> 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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> 

-- 
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