On 12.05.21 10:04, David Laight wrote:
> From: Joerg
>> Sent: 12 May 2021 08:55
>>
>> From: Joerg Roedel <jroedel@suse.de>
>>
>> The put_user() and get_user() functions do checks on the address which is
>> passed to them. They check whether the address is actually a user-space
>> address and whether its fine to access it. They also call might_fault()
>> to indicate that they could fault and possibly sleep.
>>
>> All of these checks are neither wanted nor required in the #VC exception
>> handler, which can be invoked from almost any context and also for MMIO
>> instructions from kernel space on kernel memory. All the #VC handler
>> wants to know is whether a fault happened when the access was tried.
>>
>> This is provided by __put_user()/__get_user(), which just do the access
>> no matter what.
> 
> That can't be right at all.
> __put/get_user() are only valid on user addresses and will try to
> fault in a missing page - so can sleep.
> 
> At best this is abused the calls.

You want something like xen_safe_[read|write]_ulong().


Juergen