On 15.04.2020 04:54, Andrew Morton wrote: > > The patch titled > Subject: mm/ksm: fix NULL pointer dereference when KSM zero page is enabled > has been added to the -mm tree. Its filename is > mm-ksm-fix-null-pointer-dereference-when-ksm-zero-page-is-enabled.patch > > This patch should soon appear at > http://ozlabs.org/~akpm/mmots/broken-out/mm-ksm-fix-null-pointer-dereference-when-ksm-zero-page-is-enabled.patch > and later at > http://ozlabs.org/~akpm/mmotm/broken-out/mm-ksm-fix-null-pointer-dereference-when-ksm-zero-page-is-enabled.patch > > Before you just go and hit "reply", please: > a) Consider who else should be cc'ed > b) Prefer to cc a suitable mailing list as well > c) Ideally: find the original patch on the mailing list and do a > reply-to-all to that, adding suitable additional cc's > > *** Remember to use Documentation/process/submit-checklist.rst when testing your code *** > > The -mm tree is included into linux-next and is updated > there every 3-4 working days > > ------------------------------------------------------ > From: Muchun Song <songmuchun@xxxxxxxxxxxxx> > Subject: mm/ksm: fix NULL pointer dereference when KSM zero page is enabled > > find_mergeable_vma can return NULL. In this case, it leads to crash when > we access vma->vm_mm(its offset is 0x40) later in write_protect_page. And > this case did happen on our server. The following calltrace is captured > in kernel 4.19 with the following patch applied and KSM zero page enabled > on our server. > > commit e86c59b1b12d ("mm/ksm: improve deduplication of zero pages with colouring") > > So add a vma check to fix it. > > -------------------------------------------------------------------------- > BUG: unable to handle kernel NULL pointer dereference at 0000000000000040 > Oops: 0000 [#1] SMP NOPTI > CPU: 9 PID: 510 Comm: ksmd Kdump: loaded Tainted: G OE 4.19.36.bsk.9-amd64 #4.19.36.bsk.9 > Hardware name: FOXCONN R-5111/GROOT, BIOS IC1B111F 08/17/2019 > RIP: 0010:try_to_merge_one_page+0xc7/0x760 > Code: 24 58 65 48 33 34 25 28 00 00 00 89 e8 0f 85 a3 06 00 00 48 83 c4 > 60 5b 5d 41 5c 41 5d 41 5e 41 5f c3 48 8b 46 08 a8 01 75 b8 <49> > 8b 44 24 40 4c 8d 7c 24 20 b9 07 00 00 00 4c 89 e6 4c 89 ff 48 > RSP: 0018:ffffadbdd9fffdb0 EFLAGS: 00010246 > RAX: ffffda83ffd4be08 RBX: ffffda83ffd4be40 RCX: 0000002c6e800000 > RDX: 0000000000000000 RSI: ffffda83ffd4be40 RDI: 0000000000000000 > RBP: ffffa11939f02ec0 R08: 0000000094e1a447 R09: 00000000abe76577 > R10: 0000000000000962 R11: 0000000000004e6a R12: 0000000000000000 > R13: ffffda83b1e06380 R14: ffffa18f31f072c0 R15: ffffda83ffd4be40 > FS: 0000000000000000(0000) GS:ffffa0da43b80000(0000) knlGS:0000000000000000 > CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033 > CR2: 0000000000000040 CR3: 0000002c77c0a003 CR4: 00000000007626e0 > DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000 > DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400 > PKRU: 55555554 > Call Trace: > ? follow_page_pte+0x36d/0x5e0 > ksm_scan_thread+0x115e/0x1960 > ? remove_wait_queue+0x60/0x60 > kthread+0xf5/0x130 > ? try_to_merge_with_ksm_page+0x90/0x90 > ? kthread_create_worker_on_cpu+0x70/0x70 > ret_from_fork+0x1f/0x30 > -------------------------------------------------------------------------- > > Link: http://lkml.kernel.org/r/20200414132905.83819-1-songmuchun@xxxxxxxxxxxxx > Fixes: e86c59b1b12d ("mm/ksm: improve deduplication of zero pages with colouring") > Signed-off-by: Muchun Song <songmuchun@xxxxxxxxxxxxx> > Co-developed-by: Xiongchun Duan <duanxiongchun@xxxxxxxxxxxxx> > Reviewed-by: David Hildenbrand <david@xxxxxxxxxx> Reviewed-by: Kirill Tkhai <ktkhai@xxxxxxxxxxxxx> > Cc: Hugh Dickins <hughd@xxxxxxxxxx> > Cc: Yang Shi <yang.shi@xxxxxxxxxxxxxxxxx> > Cc: Claudio Imbrenda <imbrenda@xxxxxxxxxxxxxxxxxx> > Cc: Markus Elfring <Markus.Elfring@xxxxxx> > Cc: <stable@xxxxxxxxxxxxxxx> > Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> > --- > > mm/ksm.c | 7 +++++-- > 1 file changed, 5 insertions(+), 2 deletions(-) > > --- a/mm/ksm.c~mm-ksm-fix-null-pointer-dereference-when-ksm-zero-page-is-enabled > +++ a/mm/ksm.c > @@ -2112,8 +2112,11 @@ static void cmp_and_merge_page(struct pa > > down_read(&mm->mmap_sem); > vma = find_mergeable_vma(mm, rmap_item->address); > - err = try_to_merge_one_page(vma, page, > - ZERO_PAGE(rmap_item->address)); > + if (vma) > + err = try_to_merge_one_page(vma, page, > + ZERO_PAGE(rmap_item->address)); > + else > + err = -EFAULT; > up_read(&mm->mmap_sem); In case of vma is out of date, we also may consider to exit this function without calling unstable_tree_search_insert(). > /* > * In case of failure, the page was not really empty, so we > _ > > Patches currently in -mm which might be from songmuchun@xxxxxxxxxxxxx are > > mm-ksm-fix-null-pointer-dereference-when-ksm-zero-page-is-enabled.patch >
