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

From: [email-masked] (Eric W. Biederman)
Date: Mon, 16 Sep 2019 09:31:20 -0500

Russell King - ARM Linux admin <[email-masked]> writes:

> On Fri, Sep 06, 2019 at 04:17:59PM +0100, Russell King - ARM Linux admin wrote:
>> On Mon, Sep 02, 2019 at 12:36:56PM -0500, Eric W. Biederman wrote:
>> > Russell King - ARM Linux admin <[email-masked]> writes:
>> > 
>> > > On Fri, Aug 30, 2019 at 04:02:48PM -0500, Eric W. Biederman wrote:
>> > >> Russell King - ARM Linux admin <[email-masked]> writes:
>> > >> 
>> > >> > On Fri, Aug 30, 2019 at 02:45:36PM -0500, Eric W. Biederman wrote:
>> > >> >> Russell King - ARM Linux admin <[email-masked]> writes:
>> > >> >> 
>> > >> >> > 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.
>> > >> >> 
>> > >> >> And the patch that fixed scheduling while atomic apparently introduced a
>> > >> >> regression.  Admittedly a regression that took 6 years to track down but
>> > >> >> still.
>> > >> >
>> > >> > Right, and given the number of years, we are trading one regression for
>> > >> > a different regression.  If we revert to the original code where we
>> > >> > fix up, we will end up with people complaining about a "new" regression
>> > >> > caused by reverting the previous fix.  Follow this policy and we just
>> > >> > end up constantly reverting the previous revert.
>> > >> >
>> > >> > The window is very small - the page in question will have had to have
>> > >> > instructions read from it immediately prior to the handler being entered,
>> > >> > and would have had to be made "old" before subsequently being unmapped.
>> > >> 
>> > >> > Rather than excessively complicating the code and making it even more
>> > >> > inefficient (as in your patch), we could instead retry executing the
>> > >> > instruction when we discover that the page is unavailable, which should
>> > >> > cause the page to be paged back in.
>> > >> 
>> > >> My patch does not introduce any inefficiencies.  It onlys moves the
>> > >> check for user_mode up a bit.  My patch did duplicate the code.
>> > >> 
>> > >> > If the page really is unavailable, the prefetch abort should cause a
>> > >> > SEGV to be raised, otherwise the re-execution should replace the page.
>> > >> >
>> > >> > The danger to that approach is we page it back in, and it gets paged
>> > >> > back out before we're able to read the instruction indefinitely.
>> > >> 
>> > >> I would think either a little code duplication or a function that looks
>> > >> at user_mode(regs) and picks the appropriate kind of copy to do would be
>> > >> the best way to go.  Because what needs to happen in the two cases for
>> > >> reading the instruction are almost completely different.
>> > >
>> > > That is what I mean.  I'd prefer to avoid that with the large chunk of
>> > > code.  How about instead adding a local replacement for
>> > > probe_kernel_address() that just sorts out the reading, rather than
>> > > duplicating all the code to deal with thumb fixup.
>> > 
>> > So something like this should be fine?
>> > 
>> > Jing Xiangfeng can you test this please?  I think this fixes your issue
>> > but I don't currently have an arm development box where I could test this.
>> 
>> Sorry, only just got around to this again.  What I came up with is this:
>
> I've heard nothing, so I've done nothing...

Sorry it wasn't clear you were looking for feedback.

This looks functionally equivalent to the last test version I posted and
that Jing Xiangfeng confirms solves his issue.

So I say please merge whichever version you like.

Eric

>> 8<===
>> From: Russell King <[email-masked]>
>> Subject: [PATCH] ARM: mm: fix alignment
>> 
>> Signed-off-by: Russell King <[email-masked]>
>> ---
>>  arch/arm/mm/alignment.c | 44 ++++++++++++++++++++++++++++++++++++--------
>>  1 file changed, 36 insertions(+), 8 deletions(-)
>> 
>> diff --git a/arch/arm/mm/alignment.c b/arch/arm/mm/alignment.c
>> index 6067fa4de22b..529f54d94709 100644
>> --- a/arch/arm/mm/alignment.c
>> +++ b/arch/arm/mm/alignment.c
>> @@ -765,6 +765,36 @@ do_alignment_t32_to_handler(unsigned long *pinstr, struct pt_regs *regs,
>>  	return NULL;
>>  }
>>  
>> +static int alignment_get_arm(struct pt_regs *regs, u32 *ip, unsigned long *inst)
>> +{
>> +	u32 instr = 0;
>> +	int fault;
>> +
>> +	if (user_mode(regs))
>> +		fault = get_user(instr, ip);
>> +	else
>> +		fault = probe_kernel_address(ip, instr);
>> +
>> +	*inst = __mem_to_opcode_arm(instr);
>> +
>> +	return fault;
>> +}
>> +
>> +static int alignment_get_thumb(struct pt_regs *regs, u16 *ip, u16 *inst)
>> +{
>> +	u16 instr = 0;
>> +	int fault;
>> +
>> +	if (user_mode(regs))
>> +		fault = get_user(instr, ip);
>> +	else
>> +		fault = probe_kernel_address(ip, instr);
>> +
>> +	*inst = __mem_to_opcode_thumb16(instr);
>> +
>> +	return fault;
>> +}
>> +
>>  static int
>>  do_alignment(unsigned long addr, unsigned int fsr, struct pt_regs *regs)
>>  {
>> @@ -772,10 +802,10 @@ do_alignment(unsigned long addr, unsigned int fsr, struct pt_regs *regs)
>>  	unsigned long instr = 0, instrptr;
>>  	int (*handler)(unsigned long addr, unsigned long instr, struct pt_regs *regs);
>>  	unsigned int type;
>> -	unsigned int fault;
>>  	u16 tinstr = 0;
>>  	int isize = 4;
>>  	int thumb2_32b = 0;
>> +	int fault;
>>  
>>  	if (interrupts_enabled(regs))
>>  		local_irq_enable();
>> @@ -784,15 +814,14 @@ do_alignment(unsigned long addr, unsigned int fsr, struct pt_regs *regs)
>>  
>>  	if (thumb_mode(regs)) {
>>  		u16 *ptr = (u16 *)(instrptr & ~1);
>> -		fault = probe_kernel_address(ptr, tinstr);
>> -		tinstr = __mem_to_opcode_thumb16(tinstr);
>> +
>> +		fault = alignment_get_thumb(regs, ptr, &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);
>> -				tinst2 = __mem_to_opcode_thumb16(tinst2);
>> +				u16 tinst2;
>> +				fault = alignment_get_thumb(regs, ptr + 1, &tinst2);
>>  				instr = __opcode_thumb32_compose(tinstr, tinst2);
>>  				thumb2_32b = 1;
>>  			} else {
>> @@ -801,8 +830,7 @@ do_alignment(unsigned long addr, unsigned int fsr, struct pt_regs *regs)
>>  			}
>>  		}
>>  	} else {
>> -		fault = probe_kernel_address((void *)instrptr, instr);
>> -		instr = __mem_to_opcode_arm(instr);
>> +		fault = alignment_get_arm(regs, (void *)instrptr, &instr);
>>  	}
>>  
>>  	if (fault) {
>> -- 
>> 2.7.4
>> 
>> -- 
>> RMK's Patch system: https://www.armlinux.org.uk/developer/patches/
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