Re: [Xen-devel] [PATCH v2 3/3] x86/mm-locks: apply a bias to lock levels for control domain

[George Dunlap <George.Dunlap@xxxxxxxxxx>]

> On Dec 21, 2018, at 9:41 AM, Roger Pau Monne <roger.pau@xxxxxxxxxx> wrote:
> 
> paging_log_dirty_op function takes mm locks from a subject domain and
> then attempts to perform copy to operations against the caller domain
> in order to copy the result of the hypercall into the caller provided
> buffer.
> 
> This works fine when the caller is a non-paging domain, but triggers a
> lock order panic when the caller is a paging domain due to the fact
> that at the point where the copy to operation is performed the subject
> domain paging lock is locked, and the copy operation requires
> locking the caller p2m lock which has a lower level.
> 
> Fix this limitation by adding a bias to the level of control domain mm
> locks, so that the lower control domain mm lock always has a level
> greater than the higher unprivileged domain lock level. This allows
> locking the subject domain mm locks and then locking the control
> domain mm locks, while keeping the same lock ordering and the changes
> mostly confined to mm-locks.h.
> 
> Note that so far only this flow (locking a subject domain locks and
> then the control domain ones) has been identified, but not all
> possible code paths have been inspected. Hence this solution attempts
> to be a non-intrusive fix for the problem at hand, without discarding
> further changes in the future if other valid code paths are found that
> require more complex lock level ordering.
> 
> Signed-off-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>

Reviewed-by: George Dunlap <george.dunlap@xxxxxxxxxx>

…but given the nature of the change, I’d like to see it pass at least one 
ad-hoc osstest run before it gets checked in.  (That probably means you’ll have 
to chase someone else to do that.)

 -George

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