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[37.188.135.150]) by smtp.gmail.com with ESMTPSA id t16sm3816015wra.17.2020.03.26.06.55.26 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 26 Mar 2020 06:55:27 -0700 (PDT) Date: Thu, 26 Mar 2020 14:55:26 +0100 From: Michal Hocko To: "Aneesh Kumar K.V" Cc: linux-mm@kvack.org, akpm@linux-foundation.org, linux-kernel@vger.kernel.org, mpe@ellerman.id.au, linuxppc-dev@lists.ozlabs.org, Sachin Sant , Baoquan He , Dan Williams , Pankaj Gupta , David Hildenbrand , Wei Yang , Oscar Salvador , Mike Rapoport , stable@vger.kernel.org Subject: Re: [PATCH v2] mm/sparse: Fix kernel crash with pfn_section_valid check Message-ID: <20200326135526.GR27965@dhcp22.suse.cz> References: <20200326133235.343616-1-aneesh.kumar@linux.ibm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200326133235.343616-1-aneesh.kumar@linux.ibm.com> Sender: stable-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: stable@vger.kernel.org On Thu 26-03-20 19:02:35, Aneesh Kumar K.V wrote: > Fixes the below crash > > BUG: Kernel NULL pointer dereference on read at 0x00000000 > Faulting instruction address: 0xc000000000c3447c > Oops: Kernel access of bad area, sig: 11 [#1] > LE PAGE_SIZE=64K MMU=Hash SMP NR_CPUS=2048 NUMA pSeries > CPU: 11 PID: 7519 Comm: lt-ndctl Not tainted 5.6.0-rc7-autotest #1 > ... > NIP [c000000000c3447c] vmemmap_populated+0x98/0xc0 > LR [c000000000088354] vmemmap_free+0x144/0x320 > Call Trace: > section_deactivate+0x220/0x240 > __remove_pages+0x118/0x170 > arch_remove_memory+0x3c/0x150 > memunmap_pages+0x1cc/0x2f0 > devm_action_release+0x30/0x50 > release_nodes+0x2f8/0x3e0 > device_release_driver_internal+0x168/0x270 > unbind_store+0x130/0x170 > drv_attr_store+0x44/0x60 > sysfs_kf_write+0x68/0x80 > kernfs_fop_write+0x100/0x290 > __vfs_write+0x3c/0x70 > vfs_write+0xcc/0x240 > ksys_write+0x7c/0x140 > system_call+0x5c/0x68 > > The crash is due to NULL dereference at > > test_bit(idx, ms->usage->subsection_map); due to ms->usage = NULL; in pfn_section_valid() > > With commit: d41e2f3bd546 ("mm/hotplug: fix hot remove failure in SPARSEMEM|!VMEMMAP case") > section_mem_map is set to NULL after depopulate_section_mem(). This > was done so that pfn_page() can work correctly with kernel config that disables > SPARSEMEM_VMEMMAP. With that config pfn_to_page does > > __section_mem_map_addr(__sec) + __pfn; > where > > static inline struct page *__section_mem_map_addr(struct mem_section *section) > { > unsigned long map = section->section_mem_map; > map &= SECTION_MAP_MASK; > return (struct page *)map; > } > > Now with SPASEMEM_VMEMAP enabled, mem_section->usage->subsection_map is used to > check the pfn validity (pfn_valid()). Since section_deactivate release > mem_section->usage if a section is fully deactivated, pfn_valid() check after > a subsection_deactivate cause a kernel crash. > > static inline int pfn_valid(unsigned long pfn) > { > ... > return early_section(ms) || pfn_section_valid(ms, pfn); > } > > where > > static inline int pfn_section_valid(struct mem_section *ms, unsigned long pfn) > { > int idx = subsection_map_index(pfn); > > return test_bit(idx, ms->usage->subsection_map); > } > > Avoid this by clearing SECTION_HAS_MEM_MAP when mem_section->usage is freed. > For architectures like ppc64 where large pages are used for vmmemap mapping (16MB), > a specific vmemmap mapping can cover multiple sections. Hence before a vmemmap > mapping page can be freed, the kernel needs to make sure there are no valid sections > within that mapping. Clearing the section valid bit before > depopulate_section_memap enables this. I believe that the necessity of clearing the section before the tear down is worth a comment into the code. Because this is just way to easy to miss or not be aware at all while looking into the code without git balme. > Fixes: d41e2f3bd546 ("mm/hotplug: fix hot remove failure in SPARSEMEM|!VMEMMAP case") > Reported-by: Sachin Sant > Tested-by: Sachin Sant > Cc: Baoquan He > Cc: Michael Ellerman > Cc: Dan Williams > Cc: Pankaj Gupta > Cc: David Hildenbrand > Cc: Michal Hocko > Cc: Wei Yang > Cc: Oscar Salvador > Cc: Mike Rapoport > Cc: > Signed-off-by: Aneesh Kumar K.V Acked-by: Michal Hocko Thanks! > --- > mm/sparse.c | 6 ++++++ > 1 file changed, 6 insertions(+) > > diff --git a/mm/sparse.c b/mm/sparse.c > index aadb7298dcef..65599e8bd636 100644 > --- a/mm/sparse.c > +++ b/mm/sparse.c > @@ -781,6 +781,12 @@ static void section_deactivate(unsigned long pfn, unsigned long nr_pages, > ms->usage = NULL; > } > memmap = sparse_decode_mem_map(ms->section_mem_map, section_nr); > + /* > + * Mark the section invalid so that valid_section() > + * return false. This prevents code from dereferencing > + * ms->usage array. > + */ > + ms->section_mem_map &= ~SECTION_HAS_MEM_MAP; > } > > if (section_is_early && memmap) > -- > 2.25.1 > -- Michal Hocko SUSE Labs