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b=QTWtYKU2TyoxPXjcHz+SiSddeSqqdvSFOswAYcm578/7B/fTaz0fQyG5Icg3+VHLm v6Dq4OMR7o5VtmB4eXuQDSNPvgtsNPC++8zGFQlNPs1rWtlwyJuFbPXYmdfDhq8DGt 8tU65ngkv7fDdyLxAha7PE8Okts73aKvC88MxdbnLJ/l1E1xohN6E3i8Dsy18loMEx hNojwg8rtowHlQGSIrFMObNhvk7T7n43KE+4tcMLPRXKdv3iemHiUrwcUczRjQippe L77nQ0Oovfubn2l3n1a2/Fb7FOwPkIV+0umIYUTyLvsVJjCqeF0MZIQiRuy7uCOY9l 0P4eTZM0rGbjg== Message-ID: <5ae7a11c-1db4-44cd-bf67-a98cfd5ca08b@kernel.org> Date: Mon, 7 Sep 2026 14:23:33 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v2] mm/migrate_device: avoid out-of-bounds writes for compound folios To: Matthew Brost Cc: Hui Su , akpm@linux-foundation.org, balbirs@nvidia.com, ziy@nvidia.com, joshua.hahnjy@gmail.com, rakie.kim@sk.com, byungchul@sk.com, gourry@gourry.net, ying.huang@linux.alibaba.com, apopple@nvidia.com, linux-mm@kvack.org, linux-kernel@vger.kernel.org, stable@vger.kernel.org References: <20260817120758.669807-3-sh_def@163.com> <4ead5df7-f000-4087-83e4-13aae036e0b7@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: rspam05 X-Rspamd-Queue-Id: CB0C84000D X-Stat-Signature: 83j4pa9ikcikr8hignccqceordjqzfsg X-Rspam-User: X-HE-Tag: 1788783819-711726 X-HE-Meta: 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 FrIiLJ4E zqH+kJpDKi94xHferEbMfkQ5OnJw7f5mHqV+vlOGuKk7Ucm2p7HDzdLQrqu1wGcILxQtdYViuV5O6hql6+JRzTN4Gyk6Rty18Rpb2qpbR/Gt5ITYoEyO9dKQBQCRH/9AscGyT2enfw1UEPjLOtZli9khhpux7mzg7Mnodim3mdrBIx9piPxzw4CBkaoiQotR67HMJ3yKTIOIvt548pnTJG68UCgU7gASTk5yY+ZghxxkVftAUemuasvXTeNW8tchtjW09ADYPfg3lYWMkt7eX4Hr1Oh3/tMKtC1L4Pyfj3CxFor/fum/5w1LlGsb5jlX2vdoeF2IP1ZB5cyREHMkOs7bcoVG+ASUyYaht07QPR4Yv3xIdbCOQ/GPpQ//vBOgb0DznYWKpGI0jLhs2n/QAQemRKmCE6KA7HPgOCmZVkSmOznF9sSNeEeQWQuaULkijBtDAsJqn7G5I/SY= Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 8/28/26 06:44, Matthew Brost wrote: > On Thu, Aug 27, 2026 at 05:14:40PM +0200, David Hildenbrand (Arm) wrote: >> On 8/17/26 14:08, Hui Su wrote: >>> migrate_device_range() and migrate_device_pfns() clear the entries >>> following a compound folio so that the PFN arrays retain their >>> page-granular representation. >>> >>> If a compound folio extends beyond the end of the caller-provided range, >>> the loops clear all following folio entries without limiting them to the >>> number of slots remaining in the npages-sized array, causing an >>> out-of-bounds write. >>> >>> Do not proceed with a compound folio if its page-granular representation >>> does not fit entirely in the remaining PFN array. If this happens, drop >>> any reference and lock acquired for the folio, clear the remaining >>> entries, and stop collecting. >>> >>> Observed with a KASAN x86 QEMU kernel using the HMM >>> migrate_anon_huge_zero selftest. Closing /dev/hmm_dmirror0 after >>> migrating an anonymous huge page to device memory exercises: >>> >>> dmirror_fops_release() >>> -> dmirror_device_evict_chunk() >>> -> migrate_device_range() >>> >>> Fixes: a30b48bf1b24 ("mm/migrate_device: implement THP migration of zone device pages") >>> Cc: stable@vger.kernel.org >>> Signed-off-by: Hui Su >>> --- >>> Changes in v2: >>> - Do not partially represent a compound folio when it does not fit in >>> the remaining PFN array. >>> - Drop any reference and lock acquired for that folio, clear the >>> remaining entries, and stop collecting, as suggested by Balbir Singh. >>> >>> v1: https://lore.kernel.org/lkml/20260817074350.442493-2-sh_def@163.com/ >>> >>> mm/migrate_device.c | 18 ++++++++++++++++++ >>> 1 file changed, 18 insertions(+) >>> >>> diff --git a/mm/migrate_device.c b/mm/migrate_device.c >>> index 908d2d4ec43a..69b8d0660bab 100644 >>> --- a/mm/migrate_device.c >>> +++ b/mm/migrate_device.c >>> @@ -1400,6 +1400,15 @@ int migrate_device_range(unsigned long *src_pfns, unsigned long start, >>> >>> src_pfns[i] = migrate_device_pfn_lock(pfn); >>> nr = folio_nr_pages(folio); >>> + if (nr > npages - i) { >>> + if (src_pfns[i] & MIGRATE_PFN_MIGRATE) { >>> + folio_unlock(folio); >>> + folio_put(folio); >>> + } >>> + memset(&src_pfns[i], 0, >>> + (npages - i) * sizeof(*src_pfns)); >>> + break; >>> + } >>> if (nr > 1) { >>> src_pfns[i] |= MIGRATE_PFN_COMPOUND; >>> for (j = 1; j < nr; j++) >>> @@ -1434,6 +1443,15 @@ int migrate_device_pfns(unsigned long *src_pfns, unsigned long npages) >>> >>> src_pfns[i] = migrate_device_pfn_lock(src_pfns[i]); >>> nr = folio_nr_pages(folio); >>> + if (nr > npages - i) { >>> + if (src_pfns[i] & MIGRATE_PFN_MIGRATE) { >>> + folio_unlock(folio); >>> + folio_put(folio); >>> + } >>> + memset(&src_pfns[i], 0, >>> + (npages - i) * sizeof(*src_pfns)); >>> + break; >>> + } >>> if (nr > 1) { >>> src_pfns[i] |= MIGRATE_PFN_COMPOUND; >>> for (j = 1; j < nr; j++) >> >> The code duplication here makes me angry. :) And using a memset on one branch but >> not on the other is weird. >> >> But is this the right fix or rather what >> >> https://lore.kernel.org/r/20260805231041.3791771-3-matthew.brost@intel.com >> > > I'm not sure my patch is right now that I think about, or if this patch > is right, or your suggestion either :). > >> tried? That fix would also need a cleanup but seems code-wise simpler. A cleanup could look like: >> >> diff --git a/mm/migrate_device.c b/mm/migrate_device.c >> index 762c5cee8fecc..480bf59bc2425 100644 >> --- a/mm/migrate_device.c >> +++ b/mm/migrate_device.c >> @@ -1419,10 +1419,9 @@ int migrate_device_range(unsigned long *src_pfns, unsigned long start, >> for (pfn = start, i = 0; i < npages; pfn++, i++) { >> struct page *page = pfn_to_page(pfn); >> struct folio *folio = page_folio(page); >> - unsigned int nr = 1; >> + unsigned int nr = min(folio_nr_pages(folio), npages - i); >> >> src_pfns[i] = migrate_device_pfn_lock(pfn); >> - nr = folio_nr_pages(folio); >> if (nr > 1) { >> src_pfns[i] |= MIGRATE_PFN_COMPOUND; >> for (j = 1; j < nr; j++) >> @@ -1453,10 +1452,9 @@ int migrate_device_pfns(unsigned long *src_pfns, unsigned long npages) >> for (i = 0; i < npages; i++) { >> struct page *page = pfn_to_page(src_pfns[i]); >> struct folio *folio = page_folio(page); >> - unsigned int nr = 1; >> + unsigned int nr = min(folio_nr_pages(folio), npages - i); > > I think the semantics of both these function should be that if a user > passes in a PFN array and one of the PFNs exceeds the `npages` limit, > that is a misuse of this function and we should unwind and return an > error. The caller is providing the PFNs and should therefore know the > backing page alignment, so `npages` should always end exactly on a page > boundary. The sole caller of migrate_device_pfns function is > `drm_gpusvm/pagemap`, which certainly adheres to this requirement. > > The two callers of migrate_device_range, also seemly adheres to this > requirement. That's what I was thinking: because how should we partially migrate folios? Doesn't make sense :) If this cannot be triggered today, neither Fixes: nor CC: stable is appropriate. Agreed that we should just warn. -- Cheers, David