mm/hugetlb.c: fix a bug within a BUG(): inconsistent pte comparison

The following crash happens for me when running the -mm selftests (below).
Specifically, it happens while running the uffd-stress subtests:

kernel BUG at mm/hugetlb.c:7249!
invalid opcode: 0000 [#1] PREEMPT SMP NOPTI
CPU: 0 PID: 3238 Comm: uffd-stress Not tainted 6.4.0-hubbard-github+ #109
Hardware name: ASUS X299-A/PRIME X299-A, BIOS 1503 08/03/2018
RIP: 0010:huge_pte_alloc+0x12c/0x1a0
...
Call Trace:
 <TASK>
 ? __die_body+0x63/0xb0
 ? die+0x9f/0xc0
 ? do_trap+0xab/0x180
 ? huge_pte_alloc+0x12c/0x1a0
 ? do_error_trap+0xc6/0x110
 ? huge_pte_alloc+0x12c/0x1a0
 ? handle_invalid_op+0x2c/0x40
 ? huge_pte_alloc+0x12c/0x1a0
 ? exc_invalid_op+0x33/0x50
 ? asm_exc_invalid_op+0x16/0x20
 ? __pfx_put_prev_task_idle+0x10/0x10
 ? huge_pte_alloc+0x12c/0x1a0
 hugetlb_fault+0x1a3/0x1120
 ? finish_task_switch+0xb3/0x2a0
 ? lock_is_held_type+0xdb/0x150
 handle_mm_fault+0xb8a/0xd40
 ? find_vma+0x5d/0xa0
 do_user_addr_fault+0x257/0x5d0
 exc_page_fault+0x7b/0x1f0
 asm_exc_page_fault+0x22/0x30

That happens because a BUG() statement in huge_pte_alloc() attempts to
check that a pte, if present, is a hugetlb pte, but it does so in a
non-lockless-safe manner that leads to a false BUG() report.

We got here due to a couple of bugs, each of which by itself was not quite
enough to cause a problem:

First of all, before commit c33c794("mm: ptep_get() conversion"), the
BUG() statement in huge_pte_alloc() was itself fragile: it relied upon
compiler behavior to only read the pte once, despite using it twice in the
same conditional.

Next, commit c33c794 ("mm: ptep_get() conversion") broke that
delicate situation, by causing all direct pte reads to be done via
READ_ONCE().  And so READ_ONCE() got called twice within the same BUG()
conditional, leading to comparing (potentially, occasionally) different
versions of the pte, and thus to false BUG() reports.

Fix this by taking a single snapshot of the pte before using it in the
BUG conditional.

Now, that commit is only partially to blame here but, people doing
bisections will invariably land there, so this will help them find a fix
for a real crash.  And also, the previous behavior was unlikely to ever
expose this bug--it was fragile, yet not actually broken.

So that's why I chose this commit for the Fixes tag, rather than the
commit that created the original BUG() statement.

Link: https://lkml.kernel.org/r/20230701010442.2041858-1-jhubbard@nvidia.com
Fixes: c33c794 ("mm: ptep_get() conversion")
Signed-off-by: John Hubbard <jhubbard@nvidia.com>
Acked-by: James Houghton <jthoughton@google.com>
Acked-by: Muchun Song <songmuchun@bytedance.com>
Reviewed-by: Ryan Roberts <ryan.roberts@arm.com>
Acked-by: Mike Kravetz <mike.kravetz@oracle.com>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Alex Williamson <alex.williamson@redhat.com>
Cc: Alexander Potapenko <glider@google.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Andrey Konovalov <andreyknvl@gmail.com>
Cc: Andrey Ryabinin <ryabinin.a.a@gmail.com>
Cc: Christian Brauner <brauner@kernel.org>
Cc: Christoph Hellwig <hch@infradead.org>
Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: Dave Airlie <airlied@gmail.com>
Cc: Dimitri Sivanich <dimitri.sivanich@hpe.com>
Cc: Dmitry Vyukov <dvyukov@google.com>
Cc: Ian Rogers <irogers@google.com>
Cc: Jason Gunthorpe <jgg@ziepe.ca>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Cc: Lorenzo Stoakes <lstoakes@gmail.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Miaohe Lin <linmiaohe@huawei.com>
Cc: Michal Hocko <mhocko@kernel.org>
Cc: Mike Rapoport (IBM) <rppt@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Naoya Horiguchi <naoya.horiguchi@nec.com>
Cc: Oleksandr Tyshchenko <oleksandr_tyshchenko@epam.com>
Cc: Pavel Tatashin <pasha.tatashin@soleen.com>
Cc: Roman Gushchin <roman.gushchin@linux.dev>
Cc: SeongJae Park <sj@kernel.org>
Cc: Shakeel Butt <shakeelb@google.com>
Cc: Uladzislau Rezki (Sony) <urezki@gmail.com>
Cc: Vincenzo Frascino <vincenzo.frascino@arm.com>
Cc: Yu Zhao <yuzhao@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

mm/hugetlb.c

@@ -7246,7 +7246,12 @@ pte_t *huge_pte_alloc(struct mm_struct *mm, struct vm_area_struct *vma,
 				pte = (pte_t *)pmd_alloc(mm, pud, addr);
 		}
 	}
-	BUG_ON(pte && pte_present(ptep_get(pte)) && !pte_huge(ptep_get(pte)));
+
+	if (pte) {
+		pte_t pteval = ptep_get_lockless(pte);
+
+		BUG_ON(pte_present(pteval) && !pte_huge(pteval));
+	}
 
 	return pte;
 }