Re: kern/54994: Critical bug in uarea_poolpage_alloc() for archs with __HAVE_CPU_UAREA_ROUTINES

[Jason Thorpe <thorpej%me.com@localhost>]

> On Feb 21, 2020, at 3:15 AM, rokuyama.rk%gmail.com@localhost wrote:
> 
> Since low-level routines for these archs assume physically contignous
> u-area, this results in a catastrophe; pages happens to follow the 1st
> page of u-area will be randomly overwritten.

I don't have an Alpha handy to run your test case, but I don't think this is catastrophic on all platforms.  As far as I can recall, the Alpha does not actually require a physically contiguous u-area.  MIPS is the same in this regard (although it probably need to make sure it keeps UPAGES wired TLB entries for the u-area when the direct-mapping isn't used ... it certainly used to do that).  The comment in the Alpha's cpu_uarea_alloc() is merely there to call out the requirement in order to be able to use the direct-mapped region.

Oh, turns out MIPS already has much more complex rules for when the direct-mapped region is used, so I suspect it's fine as well.  I haven't audited the rest.

In any case, the short answer is: Only an issue if the PA of the u-area is provided to the CPU, not an issue if it does everything with the VA of the u-area.

-- thorpej