mm-hugetlbc-fix-a-bug-within-a-bug-inconsistent-pte-comparison.patch removed from -mm tree

The quilt patch titled
Subject: mm/hugetlb.c: fix a bug within a BUG(): inconsistent pte comparison
has been removed from the -mm tree. Its filename was
mm-hugetlbc-fix-a-bug-within-a-bug-inconsistent-pte-comparison.patch

This patch was dropped because it was merged into the mm-hotfixes-stable branch of git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm

------------------------------------------------------
From: John Hubbard <[email protected]>
Subject: mm/hugetlb.c: fix a bug within a BUG(): inconsistent pte comparison
Date: Fri, 30 Jun 2023 18:04:42 -0700

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.

---

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

@@ -7246,7 +7246,12 @@ pte_t *huge_pte_alloc(struct mm_struct *
 				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;
 }

_

Patches currently in -mm which might be from [email protected] are