Subject: Re: cxl: fix setting of _PAGE_USER bit when handling page faults

From: "Aneesh Kumar K.V" <[email-masked]>
Date: Mon, 28 Mar 2016 19:12:20 +0530

Michael Ellerman <[email-masked]> writes:

> [ text/plain ]
> On Fri, 2016-18-03 at 04:01:21 UTC, Andrew Donnellan wrote:
>> When handling page faults, cxl_handle_page_fault() checks whether the page
>> should be accessible by userspace and have its _PAGE_USER access bit set.
>> _PAGE_USER should be set if the context's kernel flag isn't set, or if the
>> page falls outside of kernel memory.
>> 
>> However, the check currently uses the wrong operator, causing it to always
>> evalute to true. As such, we always set the _PAGE_USER bit, even when it
>> should be restricted to the kernel.
>> 
>> Fix the check so that the _PAGE_USER bit is set only as intended.
>> 
> ..
>> diff --git a/drivers/misc/cxl/fault.c b/drivers/misc/cxl/fault.c
>> index 9a8650b..a76cb8a 100644
>> --- a/drivers/misc/cxl/fault.c
>> +++ b/drivers/misc/cxl/fault.c
>> @@ -152,7 +152,7 @@ static void cxl_handle_page_fault(struct cxl_context *ctx,
>>  	access = _PAGE_PRESENT;
>>  	if (dsisr & CXL_PSL_DSISR_An_S)
>>  		access |= _PAGE_RW;
>> -	if ((!ctx->kernel) || ~(dar & (1ULL << 63)))
>> +	if ((!ctx->kernel) || !(dar & (1ULL << 63)))
>>  		access |= _PAGE_USER;
>
> I think you can (should) use is_kernel_addr() for the DAR check.
>
> I'm also slightly worried by that logic in the case of a non-kernel context.
>
> ie. if ctx->kernel is false, we get:
>
> 	if (true || !is_kernel_addr(dar))
>  		access |= _PAGE_USER;
>
> Which means we just add _PAGE_USER for any address. What am I missing here?

I noticed this when doing radix support and have a variant posted at

https://lists.ozlabs.org/pipermail/linuxppc-dev/2016-March/141036.html

-aneesh