Re: [PATCH] mm: migrate: add missing flush_dcache_page for non-mapped page migrate

[Lars Persson]

On 2/25/19 4:07 PM, Vlastimil Babka wrote:
> On 2/19/19 1:32 PM, Lars Persson wrote:
> > Our MIPS 1004Kc SoCs were seeing random userspace crashes with SIGILL
> > and SIGSEGV that could not be traced back to a userspace code
> > bug. They had all the magic signs of an I/D cache coherency issue.
> >
> > Now recently we noticed that the /proc/sys/vm/compact_memory interface
> > was quite efficient at provoking this class of userspace crashes.
> >
> > Studying the code in mm/migrate.c there is a distinction made between
> > migrating a page that is mapped at the instant of migration and one
> > that is not mapped. Our problem turned out to be the non-mapped pages.
> >
> > For the non-mapped page the code performs a copy of the page content
> > and all relevant meta-data of the page without doing the required
> > D-cache maintenance. This leaves dirty data in the D-cache of the CPU
> > and on the 1004K cores this data is not visible to the I-cache. A
> > subsequent page-fault that triggers a mapping of the page will happily
> > serve the process with potentially stale code.
> >
> > What about ARM then, this bug should have seen greater exposure? Well
> > ARM became immune to this flaw back in 2010, see commit c01778001a4f
> > ("ARM: 6379/1: Assume new page cache pages have dirty D-cache").
> >
> > My proposed fix moves the D-cache maintenance inside move_to_new_page
> > to make it common for both cases.
> >
> > Signed-off-by: Lars Persson <larper@xxxxxxxx>
>
> What about CC stable and a Fixes tag, would it be applicable here?

Yes this is candidate for stable so let's add:
Cc: <stable@xxxxxxxxxxxxxxx>

I do not find a good candidate for a Fixes tag.