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 c33c794828f2("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 c33c794828f2 ("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.

Fixes c33c794828f2 ("mm: ptep_get() conversion")

Acked-by James Houghton <[email protected]>

Acked-by Muchun Song <[email protected]>

Cc Adrian Hunter <[email protected]>

Cc Al Viro <[email protected]>

Cc Alex Williamson <[email protected]>

Cc Alexander Potapenko <[email protected]>

Cc Alexander Shishkin <[email protected]>

Cc Andrew Morton <[email protected]>

Cc Andrey Konovalov <[email protected]>

Cc Andrey Ryabinin <[email protected]>

Cc Christian Brauner <[email protected]>

Cc Christoph Hellwig <[email protected]>

Cc Daniel Vetter <[email protected]>

Cc Dave Airlie <[email protected]>

Cc Dimitri Sivanich <[email protected]>

Cc Dmitry Vyukov <[email protected]>

Cc Ian Rogers <[email protected]>

Cc Jason Gunthorpe <[email protected]>

Cc Jiri Olsa <[email protected]>

Cc Johannes Weiner <[email protected]>

Cc Kirill A. Shutemov <[email protected]>

Cc Lorenzo Stoakes <[email protected]>

Cc Mark Rutland <[email protected]>

Cc Matthew Wilcox <[email protected]>

Cc Miaohe Lin <[email protected]>

Cc Michal Hocko <[email protected]>

Cc Mike Kravetz <[email protected]>

Cc Mike Rapoport (IBM) <[email protected]>

Cc Namhyung Kim <[email protected]>

Cc Naoya Horiguchi <[email protected]>

Cc Oleksandr Tyshchenko <[email protected]>

Cc Pavel Tatashin <[email protected]>

Cc Roman Gushchin <[email protected]>

Cc Ryan Roberts <[email protected]>

Cc SeongJae Park <[email protected]>

Cc Shakeel Butt <[email protected]>

Cc Uladzislau Rezki (Sony) <[email protected]>

Cc Vincenzo Frascino <[email protected]>

Cc Yu Zhao <[email protected]>

Signed-off-by John Hubbard <[email protected]>

---

Changes since v1:

Added Acked-by's.

Fixed as per Ryan Roberts (thanks!): changed to ptep_get_lockless().

 mm/hugetlb.c | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

mm/hugetlb.c +6 -1

index bce28cca73a1..64a3239b6407 100644
@@ -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;
 }

base-commit: bf1fa6f15553df04f2bdd06190ccd5f388ab0777