Re: [PATCH v2] xen/privcmd: fix error exit of privcmd_ioctl_dm_op()

[Juergen Gross]

On 25.08.22 11:50, Jan Beulich wrote:
> On 25.08.2022 11:26, Juergen Gross wrote:
> > The error exit of privcmd_ioctl_dm_op() is calling unlock_pages()
> > potentially with pages being NULL, leading to a NULL dereference.
> >
> > Additionally lock_pages() doesn't check for pin_user_pages_fast()
> > having been completely successful, resulting in potentially not
> > locking all pages into memory. This could result in sporadic failures
> > when using the related memory in user mode.
> >
> > Fix all of that by calling unlock_pages() always with the real number
> > of pinned pages, which will be zero in case pages being NULL, and by
> > checking the number of patches pinned by pin_user_pages_fast()
>
> Nit: s/patches/pages/
>
> > matching the expected number of pages.
> >
> > Cc: <stable@xxxxxxxxxxxxxxx>
> > Fixes: ab520be8cd5d ("xen/privcmd: Add IOCTL_PRIVCMD_DM_OP")
> > Reported-by: Rustam Subkhankulov <subkhankulov@xxxxxxxxx>
> > Signed-off-by: Juergen Gross <jgross@xxxxxxxx>
>
> Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>
>
> I have a question / suggestion, though:
>
> > --- a/drivers/xen/privcmd.c
> > +++ b/drivers/xen/privcmd.c
> > @@ -602,6 +602,10 @@ static int lock_pages(
> >   		*pinned += page_count;
> >   		nr_pages -= page_count;
> >   		pages += page_count;
> > +
> > +		/* Exact reason isn't known, EFAULT is one possibility. */
> > +		if (page_count < requested)
> > +			return -EFAULT;
> >   	}
>
> I don't really know the inner workings of pin_user_pages_fast()
> nor what future plans there are with it. To be as independent of
> its behavior as possible, how about bailing here only when
> page_count actually is zero (i.e. no forward progress)?

This would require to rework the loop in lock_pages() to be able to
handle only a partial buffer.

This would add some complexity, but OTOH I'd get an exact error code
back in case of failure.

I'll have a try and see how the result would look like.


Juergen

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