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dkim=pass header.d=oracle.com header.s=corp-2021-07-09 header.b=sdMGF+r7; dkim=pass header.d=oracle.onmicrosoft.com header.s=selector2-oracle-onmicrosoft-com header.b=voDZv6s6; spf=none (imf04.hostedemail.com: domain of maciej.szmigiero@oracle.com has no SPF policy when checking 205.220.177.32) smtp.mailfrom=maciej.szmigiero@oracle.com; dmarc=pass (policy=none) header.from=oracle.com X-Rspam-User: X-HE-Tag: 1647870168-850533 Content-Transfer-Encoding: quoted-printable X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On 21.03.2022 12:01, Paolo Bonzini wrote: > On 3/21/22 11:25, syzbot wrote: >> syz repro:=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 https://urldefense.com/v3/__h= ttps://syzkaller.appspot.com/x/repro.syz?x=3D12a2d0a9700000__;!!ACWV5N9M2= RV99hQ!bJGc10O9acwj6GeDIyIdP0zHAuWUpAyb7E4gom6naJO0VKxLGw2oijJnPqByG7ye0U= q2ZA$ C reproducer:=C2=A0=C2=A0 https://urldefense.com/v3/__https://syzka= ller.appspot.com/x/repro.c?x=3D13d34fd9700000__;!!ACWV5N9M2RV99hQ!bJGc10O= 9acwj6GeDIyIdP0zHAuWUpAyb7E4gom6naJO0VKxLGw2oijJnPqByG7xoEv26SQ$ >> The issue was bisected to: >> >> commit ed922739c9199bf515a3e7fec3e319ce1edeef2a >> Author: Maciej S. Szmigiero >> Date:=C2=A0=C2=A0 Mon Dec 6 19:54:28 2021 +0000 >> >> =C2=A0=C2=A0=C2=A0=C2=A0 KVM: Use interval tree to do fast hva lookup = in memslots >> >> bisection log:=C2=A0 https://urldefense.com/v3/__https://syzkaller.app= spot.com/x/bisect.txt?x=3D142aa59d700000__;!!ACWV5N9M2RV99hQ!bJGc10O9acwj= 6GeDIyIdP0zHAuWUpAyb7E4gom6naJO0VKxLGw2oijJnPqByG7xEhtZ-FQ$ final oops:=C2= =A0=C2=A0=C2=A0=C2=A0 https://urldefense.com/v3/__https://syzkaller.appsp= ot.com/x/report.txt?x=3D162aa59d700000__;!!ACWV5N9M2RV99hQ!bJGc10O9acwj6G= eDIyIdP0zHAuWUpAyb7E4gom6naJO0VKxLGw2oijJnPqByG7zcn2K3LQ$ console output:= https://urldefense.com/v3/__https://syzkaller.appspot.com/x/log.txt?x=3D= 122aa59d700000__;!!ACWV5N9M2RV99hQ!bJGc10O9acwj6GeDIyIdP0zHAuWUpAyb7E4gom= 6naJO0VKxLGw2oijJnPqByG7wzducgVQ$=20 >=20 > It bisects here just because the patch introduces the warning; the issu= e is a mmu_notifier_invalidate_range_start with an empty range.=C2=A0 The= offending system call >=20 > mremap(&(0x7f000000d000/0x2000)=3Dnil, 0xfffffffffffffe74, 0x1000, 0x3,= &(0x7f0000007000/0x1000)=3Dnil) >=20 > really means old_len =3D=3D 0 (it's page-aligned at the beginning of sy= s_mremap), and flags includes MREMAP_FIXED so it goes down to mremap_to a= nd from there to move_page_tables.=C2=A0 No function on this path attempt= s to special case old_len =3D=3D 0, the immediate fix would be >=20 > diff --git a/mm/mremap.c b/mm/mremap.c > index 002eec83e91e..0e175aef536e 100644 > --- a/mm/mremap.c > +++ b/mm/mremap.c > @@ -486,6 +486,9 @@ unsigned long move_page_tables(struct vm_area_struc= t > =C2=A0=C2=A0=C2=A0=C2=A0 pmd_t *old_pmd, *new_pmd; > =C2=A0=C2=A0=C2=A0=C2=A0 pud_t *old_pud, *new_pud; >=20 > +=C2=A0=C2=A0=C2=A0 if (!len) > +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 return 0; > + > =C2=A0=C2=A0=C2=A0=C2=A0 old_end =3D old_addr + len; > =C2=A0=C2=A0=C2=A0=C2=A0 flush_cache_range(vma, old_addr, old_end); >=20 > but there are several other ways to fix this elsewhere in the call chai= n: >=20 > - check for old_len =3D=3D 0 somewhere in mremap_to >=20 > - skip the call in __mmu_notifier_invalidate_range_start and __mmu_noti= fier_invalidate_range_end, if people agree not to play whack-a-mole with = the callers of mmu_notifier_invalidate_range_*. >=20 > - remove the warning in KVM This probably depends whether it is actually legal to call MMU notifiers with a zero range, the first time this warning triggered it was the calle= r that was fixed [1]. By the way, the warning-on-zero-range was added during memslots patch set review process [2], but I think it ultimately does make sense. > Thanks, >=20 > Paolo >=20 Thanks, Maciej [1]: https://lore.kernel.org/kvm/20211228234257.1926057-1-seanjc@google.c= om/ [2]: https://lore.kernel.org/kvm/YKWaFwgMNSaQQuQP@google.com/