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t=1787731087; bh=nnoL+gJ+u5T/AkuD/0NsDWUD538fTPQqnzcWVX2qpxM=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=R2dV6cTXD9GdewtiqUfKySuqcT6BtP0s9bpk5E/nuQKxpyfzmQdAeT8kun1/uuhNm kCGNmi/jBZ+YLddStZKk73F6XLoeDG1SPP22G8ydWHNhrToXRbi9CkD5HXxI9Do7Rt IgEjFf/K/szKePsOjZChVM7HHeMcXy4pT0b4+P5sRHTlMZi6PeUGEDmic8uqFMQZVo 41fkV8rV4sKs//ct96ax6jAgzdGCIRCsguG/AtU7QubMSLtUVX3hfrn8WFMXCdubtE XEenT5Z0YTD/YsPX/67csAQeFNMvq+cmKrqB2vHyAV5XeJoN5OtvSU3Y+aMzNZQI1b 5CDvbfo3S8jog== Message-ID: Date: Wed, 26 Aug 2026 09:58:01 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v3 2/3] mm: khugepaged: fix folio is used after pte_unmap_unlock() To: Vernon Yang Cc: akpm@linux-foundation.org, ljs@kernel.org, nico.pache@linux.dev, ryan.roberts@arm.com, dev.jain@arm.com, baohua@kernel.org, lance.yang@linux.dev, usama.arif@linux.dev, zokeefe@google.com, linux-kernel@vger.kernel.org, linux-mm@kvack.org, stable@vger.kernel.org, Vernon Yang References: <20260824092935.73892-1-vernon2gm@gmail.com> <20260824092935.73892-3-vernon2gm@gmail.com> <0bc399df-2555-4378-bee4-eacbff434bdd@kernel.org> From: "David Hildenbrand (Arm)" Content-Language: en-US Autocrypt: addr=david@kernel.org; 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charset=UTF-8 Content-Transfer-Encoding: 7bit X-Rspamd-Server: rspam03 X-Rspamd-Queue-Id: 95615160003 X-Stat-Signature: exaqt7x3anoxnkz1aq4os3zj4aamkag4 X-Rspam-User: X-HE-Tag: 1787731088-28499 X-HE-Meta: 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 iUw7iLGJ 3HdvwS7/Kx9wAC+FyeDhGx9jW4KMra4kbkX4voHnim1SuipXOnhnxZDGwpMdBOSxf3qszxCJtUXQwYVGxFZE0Afn2NTLrFgbr/RBulz2ISQMAGDH2GRLnhW9ET86cXaPb/J/pgfPntcvO6FBLIm+2u5yNVjETpZ+fWetpzLJ3pIDnlRji8rQAE2hMwsYBTnUbw6Jm204QCWzdahQ3gUtXxwoal5ULuX/Hhynk0UiiPye0xJsuyIKSOOrod76L4GDRnKuW33xUqagt2QOdjoA7vXZEYFJTWuW3FAiSOo/6jJpo0AkW+BO3SbYsfBdkzAHRASPKrWvICyePx30= Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 8/26/26 04:46, Vernon Yang wrote: > On Mon, Aug 24, 2026 at 01:57:15PM +0200, David Hildenbrand (Arm) wrote: >> On 8/24/26 11:29, Vernon Yang wrote: >>> From: Vernon Yang >>> >>> After the page table lock has dropped, the folio can be freed >>> concurrently. The trace_mm_khugepaged_scan_pmd() is left with >>> a dangling folio pointer. >>> >>> So using the folio_pfn() before dropping the page table lock, >>> closing use-after-free window. >>> >>> Fixes: 7d2eba0557c1 ("mm: add tracepoint for scanning pages") >>> Cc: stable@vger.kernel.org >>> Signed-off-by: Vernon Yang >>> --- >>> include/trace/events/huge_memory.h | 6 +++--- >>> mm/khugepaged.c | 5 ++++- >>> 2 files changed, 7 insertions(+), 4 deletions(-) >>> >>> diff --git a/include/trace/events/huge_memory.h b/include/trace/events/huge_memory.h >>> index 7b526528f85b..fa828967e1fb 100644 >>> --- a/include/trace/events/huge_memory.h >>> +++ b/include/trace/events/huge_memory.h >>> @@ -55,10 +55,10 @@ SCAN_STATUS >>> >>> TRACE_EVENT(mm_khugepaged_scan_pmd, >>> >>> - TP_PROTO(struct mm_struct *mm, struct folio *folio, >>> + TP_PROTO(struct mm_struct *mm, unsigned long pfn, >>> int referenced, int none_or_zero, int status, int unmapped), >>> >>> - TP_ARGS(mm, folio, referenced, none_or_zero, status, unmapped), >>> + TP_ARGS(mm, pfn, referenced, none_or_zero, status, unmapped), >>> >>> TP_STRUCT__entry( >>> __field(struct mm_struct *, mm) >>> @@ -71,7 +71,7 @@ TRACE_EVENT(mm_khugepaged_scan_pmd, >>> >>> TP_fast_assign( >>> __entry->mm = mm; >>> - __entry->pfn = folio ? folio_pfn(folio) : -1; >>> + __entry->pfn = pfn; >>> __entry->referenced = referenced; >>> __entry->none_or_zero = none_or_zero; >>> __entry->status = status; >>> diff --git a/mm/khugepaged.c b/mm/khugepaged.c >>> index 00337405c0e0..4e0fca5942dd 100644 >>> --- a/mm/khugepaged.c >>> +++ b/mm/khugepaged.c >>> @@ -1618,6 +1618,7 @@ static enum scan_result collapse_scan_pmd(struct mm_struct *mm, >>> enum scan_result result = SCAN_FAIL; >>> struct page *page = NULL; >>> struct folio *folio = NULL; >>> + unsigned long pfn = -1; >>> unsigned long addr; >>> unsigned long enabled_orders; >>> spinlock_t *ptl; >>> @@ -1780,6 +1781,8 @@ static enum scan_result collapse_scan_pmd(struct mm_struct *mm, >>> result = SCAN_SUCCEED; >>> } >>> out_unmap: >>> + if (folio) >>> + pfn = folio_pfn(folio); >> >> Should we reset the folio to NULL at the beginning of the loop? Then we really >> only trace the PFN if it really was problematic. > > Yes, this is a pre-existing bug, and I'll fix it together. Thanks! > > But it is not at the beginning of the loop, it is at the __ending__ of > the loop, for the same reason as PATCH#1. See my reply to patch #1, hth. -- Cheers, David