Hi, Andrew,
On Mon, Apr 17, 2023 at 04:48:22PM -0700, Andrew Morton wrote:
On Mon, 17 Apr 2023 15:53:13 -0400 Peter Xu peterx@redhat.com wrote:
When we try to unshare a pinned page for a private hugetlb, uffd-wp bit can get lost during unsharing. Fix it by carrying it over.
This should be very rare, only if an unsharing happened on a private hugetlb page with uffd-wp protected (e.g. in a child which shares the same page with parent with UFFD_FEATURE_EVENT_FORK enabled).
What are the user-visible consequences of the bug?
When above condition met, one can lose uffd-wp bit on the privately mapped hugetlb page. It allows the page to be writable even if it should still be wr-protected. I assume it can mean data loss.
However it's very hard to trigger. When I wrote the reproducer (provided in the last patch) I needed to use the newest gup_test cmd introduced by David to trigger it because I don't even know another way to do a proper RO longerm pin.
Besides that, it needs a bunch of other conditions all met:
(1) hugetlb being mapped privately, (2) userfaultfd registered with WP and EVENT_FORK, (3) the user app fork()s, then, (4) RO longterm pin onto a wr-protected anonymous page.
If it's not impossible to hit in production I'd say extremely rare.
Cc: linux-stable stable@vger.kernel.org
When proposing a backport, it's better to present the patch as a standalone thing, against current -linus. I'll then queue it in mm-hotfixes and shall send it upstream during this -rc cycle.
As presented, this patch won't go upstream until after 6.3 is released, and as it comes later in time, more backporting effort might be needed.
I can rework things if this fix is reasonably urgent (the "user-visible consequences" info is the guide). If not urgent, we can leave things as they are.
IMHO it's not urgent so suitable for mm-unstable (current base of this set; sorry if I forgot to mention it explicitly). I'll post (and remember to post) patches on top of mm-stable if they're urgent, or e.g. bugs introduced in current release.
I copied stable for the pure logic of fixing a bug in old kernels. The consequence of hitting the bug is very bad but chance to hit is very low.
Thanks,