Messages in this thread | | | Subject | Re: [PATCH] mm/sparse: Fix kernel crash with pfn_section_valid check | From | "Aneesh Kumar K.V" <> | Date | Thu, 26 Mar 2020 15:26:22 +0530 |
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On 3/26/20 3:10 PM, Michal Hocko wrote: > On Wed 25-03-20 08:49:14, 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 > > It would be great to match this to the specific source code.
The crash is due to NULL dereference at
test_bit(idx, ms->usage->subsection_map); due to ms->usage = NULL;
that is explained in later part of the commit. > >> __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 >> >> 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. > > I am sorry, I haven't noticed that during the review of the commit > mentioned above. This is all subtle as hell, I have to say. > > Why do we have to free usage before deactivaing section memmap? Now that > we have a late section_mem_map reset shouldn't we tear down the usage in > the same branch? >
We still need to make the section invalid before we call into depopulate_section_memmap(). Because architecture like powerpc can share vmemmap area across sections (16MB mapping of vmemmap area) and we use vmemmap_popluated() to make that decision.
>> Fixes: d41e2f3bd546 ("mm/hotplug: fix hot remove failure in SPARSEMEM|!VMEMMAP case") >> Cc: Baoquan He <bhe@redhat.com> >> Reported-by: Sachin Sant <sachinp@linux.vnet.ibm.com> >> Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.ibm.com> >> --- >> mm/sparse.c | 2 ++ >> 1 file changed, 2 insertions(+) >> >> diff --git a/mm/sparse.c b/mm/sparse.c >> index aadb7298dcef..3012d1f3771a 100644 >> --- a/mm/sparse.c >> +++ b/mm/sparse.c >> @@ -781,6 +781,8 @@ 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 */ >> + ms->section_mem_map &= ~SECTION_HAS_MEM_MAP; > > Btw. this comment is not really helping at all.
That is marking the section invalid so that
static inline int valid_section(struct mem_section *section) { return (section && (section->section_mem_map & SECTION_HAS_MEM_MAP)); }
returns false.
> /* > * section->usage is gone and VMEMMAP's pfn_valid depens > * on it (see pfn_section_valid) > */ >> } >> >> if (section_is_early && memmap) >> -- >> 2.25.1 >> >
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