On 12.05.21 10:04, David Laight wrote:

From: Joerg

quoted

Sent: 12 May 2021 08:55

From: Joerg Roedel <redacted>

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