linux-kernel - [PATCH 5.7 004/124] khugepaged: adjust VM_BUG_ON_MM() in __khugepaged

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Message-Id: <20200824082409.605505185@linuxfoundation.org>
Date:   Mon, 24 Aug 2020 10:28:58 +0200
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     linux-kernel@...r.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        stable@...r.kernel.org, syzbot <syzkaller@...glegroups.com>,
        Hugh Dickins <hughd@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Yang Shi <shy828301@...il.com>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Song Liu <songliubraving@...com>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Eric Dumazet <edumazet@...gle.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Sasha Levin <sashal@...nel.org>
Subject: [PATCH 5.7 004/124] khugepaged: adjust VM_BUG_ON_MM() in __khugepaged_enter()

From: Hugh Dickins <hughd@...gle.com>

[ Upstream commit f3f99d63a8156c7a4a6b20aac22b53c5579c7dc1 ]

syzbot crashes on the VM_BUG_ON_MM(khugepaged_test_exit(mm), mm) in
__khugepaged_enter(): yes, when one thread is about to dump core, has set
core_state, and is waiting for others, another might do something calling
__khugepaged_enter(), which now crashes because I lumped the core_state
test (known as "mmget_still_valid") into khugepaged_test_exit().  I still
think it's best to lump them together, so just in this exceptional case,
check mm->mm_users directly instead of khugepaged_test_exit().

Fixes: bbe98f9cadff ("khugepaged: khugepaged_test_exit() check mmget_still_valid()")
Reported-by: syzbot <syzkaller@...glegroups.com>
Signed-off-by: Hugh Dickins <hughd@...gle.com>
Signed-off-by: Andrew Morton <akpm@...ux-foundation.org>
Acked-by: Yang Shi <shy828301@...il.com>
Cc: "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
Cc: Andrea Arcangeli <aarcange@...hat.com>
Cc: Song Liu <songliubraving@...com>
Cc: Mike Kravetz <mike.kravetz@...cle.com>
Cc: Eric Dumazet <edumazet@...gle.com>
Cc: <stable@...r.kernel.org>	[4.8+]
Link: http://lkml.kernel.org/r/alpine.LSU.2.11.2008141503370.18085@eggly.anvils
Signed-off-by: Linus Torvalds <torvalds@...ux-foundation.org>
Signed-off-by: Sasha Levin <sashal@...nel.org>
---
 mm/khugepaged.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/mm/khugepaged.c b/mm/khugepaged.c
index 9e7cec2840927..cb17091d0a202 100644
--- a/mm/khugepaged.c
+++ b/mm/khugepaged.c
@@ -435,7 +435,7 @@ int __khugepaged_enter(struct mm_struct *mm)
 		return -ENOMEM;
 
 	/* __khugepaged_exit() must not run from under us */
-	VM_BUG_ON_MM(khugepaged_test_exit(mm), mm);
+	VM_BUG_ON_MM(atomic_read(&mm->mm_users) == 0, mm);
 	if (unlikely(test_and_set_bit(MMF_VM_HUGEPAGE, &mm->flags))) {
 		free_mm_slot(mm_slot);
 		return 0;
-- 
2.25.1