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[2003:d8:2f19:6500:e1c1:8216:4c25:efe4]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-3a4d1ed8740sm5369659f8f.83.2025.05.26.05.59.54 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Mon, 26 May 2025 05:59:55 -0700 (PDT) Message-ID: <87521d93-cc03-480d-a2ef-3ef8c84481c9@redhat.com> Date: Mon, 26 May 2025 14:59:54 +0200 Precedence: bulk X-Mailing-List: stable@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH] mm/hugetlb: fix kernel NULL pointer dereference when replacing free hugetlb folios To: Ge Yang , akpm@linux-foundation.org Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org, stable@vger.kernel.org, 21cnbao@gmail.com, baolin.wang@linux.alibaba.com, muchun.song@linux.dev, osalvador@suse.de, liuzixing@hygon.cn References: <1747884137-26685-1-git-send-email-yangge1116@126.com> <07b7d4fd-c600-4de1-aea4-037e148da79b@126.com> From: David Hildenbrand Content-Language: en-US Autocrypt: addr=david@redhat.com; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit On 26.05.25 14:57, Ge Yang wrote: > > > 在 2025/5/26 20:41, David Hildenbrand 写道: >> On 22.05.25 05:22, yangge1116@126.com wrote: >>> From: Ge Yang >>> >>> A kernel crash was observed when replacing free hugetlb folios: >>> >>> BUG: kernel NULL pointer dereference, address: 0000000000000028 >>> PGD 0 P4D 0 >>> Oops: Oops: 0000 [#1] SMP NOPTI >>> CPU: 28 UID: 0 PID: 29639 Comm: test_cma.sh Tainted 6.15.0-rc6-zp #41 >>> PREEMPT(voluntary) >>> RIP: 0010:alloc_and_dissolve_hugetlb_folio+0x1d/0x1f0 >>> RSP: 0018:ffffc9000b30fa90 EFLAGS: 00010286 >>> RAX: 0000000000000000 RBX: 0000000000342cca RCX: ffffea0043000000 >>> RDX: ffffc9000b30fb08 RSI: ffffea0043000000 RDI: 0000000000000000 >>> RBP: ffffc9000b30fb20 R08: 0000000000001000 R09: 0000000000000000 >>> R10: ffff88886f92eb00 R11: 0000000000000000 R12: ffffea0043000000 >>> R13: 0000000000000000 R14: 00000000010c0200 R15: 0000000000000004 >>> FS:  00007fcda5f14740(0000) GS:ffff8888ec1d8000(0000) >>> knlGS:0000000000000000 >>> CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033 >>> CR2: 0000000000000028 CR3: 0000000391402000 CR4: 0000000000350ef0 >>> Call Trace: >>> >>>   replace_free_hugepage_folios+0xb6/0x100 >>>   alloc_contig_range_noprof+0x18a/0x590 >>>   ? srso_return_thunk+0x5/0x5f >>>   ? down_read+0x12/0xa0 >>>   ? srso_return_thunk+0x5/0x5f >>>   cma_range_alloc.constprop.0+0x131/0x290 >>>   __cma_alloc+0xcf/0x2c0 >>>   cma_alloc_write+0x43/0xb0 >>>   simple_attr_write_xsigned.constprop.0.isra.0+0xb2/0x110 >>>   debugfs_attr_write+0x46/0x70 >>>   full_proxy_write+0x62/0xa0 >>>   vfs_write+0xf8/0x420 >>>   ? srso_return_thunk+0x5/0x5f >>>   ? filp_flush+0x86/0xa0 >>>   ? srso_return_thunk+0x5/0x5f >>>   ? filp_close+0x1f/0x30 >>>   ? srso_return_thunk+0x5/0x5f >>>   ? do_dup2+0xaf/0x160 >>>   ? srso_return_thunk+0x5/0x5f >>>   ksys_write+0x65/0xe0 >>>   do_syscall_64+0x64/0x170 >>>   entry_SYSCALL_64_after_hwframe+0x76/0x7e >>> >>> There is a potential race between __update_and_free_hugetlb_folio() >>> and replace_free_hugepage_folios(): >>> >>> CPU1                              CPU2 >>> __update_and_free_hugetlb_folio   replace_free_hugepage_folios >>>                                      folio_test_hugetlb(folio) >>>                                      -- It's still hugetlb folio. >>> >>>    __folio_clear_hugetlb(folio) >>>    hugetlb_free_folio(folio) >>>                                      h = folio_hstate(folio) >>>                                      -- Here, h is NULL pointer >>> >>> When the above race condition occurs, folio_hstate(folio) returns >>> NULL, and subsequent access to this NULL pointer will cause the >>> system to crash. To resolve this issue, execute folio_hstate(folio) >>> under the protection of the hugetlb_lock lock, ensuring that >>> folio_hstate(folio) does not return NULL. >>> >>> Fixes: 04f13d241b8b ("mm: replace free hugepage folios after migration") >>> Signed-off-by: Ge Yang >>> Cc: >>> --- >>>   mm/hugetlb.c | 8 ++++++++ >>>   1 file changed, 8 insertions(+) >>> >>> diff --git a/mm/hugetlb.c b/mm/hugetlb.c >>> index 3d3ca6b..6c2e007 100644 >>> --- a/mm/hugetlb.c >>> +++ b/mm/hugetlb.c >>> @@ -2924,12 +2924,20 @@ int replace_free_hugepage_folios(unsigned long >>> start_pfn, unsigned long end_pfn) >>>       while (start_pfn < end_pfn) { >>>           folio = pfn_folio(start_pfn); >>> + >>> +        /* >>> +         * The folio might have been dissolved from under our feet, >>> so make sure >>> +         * to carefully check the state under the lock. >>> +         */ >>> +        spin_lock_irq(&hugetlb_lock); >>>           if (folio_test_hugetlb(folio)) { >>>               h = folio_hstate(folio); >>>           } else { >>> +            spin_unlock_irq(&hugetlb_lock); >>>               start_pfn++; >>>               continue; >>>           } >>> +        spin_unlock_irq(&hugetlb_lock); >> >> As mentioned elsewhere, this will grab the hugetlb_lock for each and >> every pfn in the range if there are no hugetlb folios (common case). >> >> That should certainly *not* be done. >> >> In case we see !folio_test_hugetlb(), we should just move on. >> > > The main reason for acquiring the hugetlb_lock here is to obtain a valid > hstate, as the alloc_and_dissolve_hugetlb_folio() function requires > hstate as a parameter. This approach is indeed not performance-friendly. > However, in the patch available at > https://lore.kernel.org/lkml/1747987559-23082-1-git-send-email-yangge1116@126.com/, > all these operations will be removed. No, you still take locks on anything that is !folio_ref_count(folio). We should really have an early folio_test_hugetlb(folio) check. hugetlb is on most systems out there the absolute *corner case*. -- Cheers, David / dhildenb