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Fri, 19 Aug 2022 05:00:25 -0700 (PDT) X-Google-Smtp-Source: AA6agR5caDrp322/e96KsmCuJMmXZw/WOMTkleWULgzj2Ub4gzaN6wjZDkazZ60FrxIjYHnsYZlkovLyQVTpeHxJSHU= X-Received: by 2002:ab0:a99:0:b0:39d:de9a:3d58 with SMTP id d25-20020ab00a99000000b0039dde9a3d58mr845050uak.36.1660910425241; Fri, 19 Aug 2022 05:00:25 -0700 (PDT) MIME-Version: 1.0 References: <20220802151550.159076-1-wangkefeng.wang@huawei.com> <20220811160020.1e6823094217e8d6d3aaebdf@linux-foundation.org> In-Reply-To: <20220811160020.1e6823094217e8d6d3aaebdf@linux-foundation.org> Reply-To: abhishek.shah@columbia.edu From: Abhishek Shah Date: Fri, 19 Aug 2022 08:00:00 -0400 Message-ID: Subject: Re: [PATCH] mm: ksm: fix data-race in __ksm_enter / run_store To: Andrew Morton Cc: Kefeng Wang , linux-mm@kvack.org, linux-kernel@vger.kernel.org, Gabriel Ryan Content-Type: multipart/alternative; boundary="000000000000e6903605e696d729" X-Proofpoint-ORIG-GUID: o70TP4tW92TBrBUNgVh_xewS9o8flg2l X-Proofpoint-GUID: o70TP4tW92TBrBUNgVh_xewS9o8flg2l X-CU-OB: Yes X-Proofpoint-Virus-Version: vendor=baseguard engine=ICAP:2.0.205,Aquarius:18.0.895,Hydra:6.0.517,FMLib:17.11.122.1 definitions=2022-08-19_06,2022-08-18_01,2022-06-22_01 X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 clxscore=1011 lowpriorityscore=10 priorityscore=1501 phishscore=0 spamscore=0 impostorscore=10 adultscore=0 malwarescore=0 mlxscore=0 mlxlogscore=999 bulkscore=10 suspectscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.12.0-2207270000 definitions=main-2208190044 ARC-Seal: i=1; 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dkim=pass header.d=columbia.edu header.s=pps01 header.b=Tcfwf6If; dmarc=none; spf=pass (imf26.hostedemail.com: domain of as5258@columbia.edu designates 148.163.139.74 as permitted sender) smtp.mailfrom=as5258@columbia.edu X-Rspamd-Queue-Id: C1A4614000F X-HE-Tag: 1660910427-401120 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: --000000000000e6903605e696d729 Content-Type: text/plain; charset="UTF-8" Hi all, I looked through the vulnerability some more and came up with this so far. Please let me know what you think Consider the following interleaving. 1. In *__ksm_enter*, thread 1 inserts the new mm into the list of mms at a position determined by *ksm_run* being set to KSM_RUN_MERGE (see here ) 2. In *run_store*, thread 2 changes *ksm_run* to KSM_RUN_UNMERGE here and executes unmerge_and_remove_all_rmap_items, where it can free the newly added mm via *mmdrop* here . 3. In *__ksm_enter*, thread 2 continues execution and updates the fields of the new mm (see here ), although it was freed, resulting in a use-after-free vulnerability. Thanks! On Thu, Aug 11, 2022 at 7:00 PM Andrew Morton wrote: > On Tue, 2 Aug 2022 23:15:50 +0800 Kefeng Wang > wrote: > > > Abhishek reported a data-race issue, > > OK, but it would be better to perform an analysis of the alleged bug, > describe the potential effects if the race is hit, etc. > > > --- a/mm/ksm.c > > +++ b/mm/ksm.c > > @@ -2507,6 +2507,7 @@ int __ksm_enter(struct mm_struct *mm) > > { > > struct mm_slot *mm_slot; > > int needs_wakeup; > > + bool ksm_run_unmerge; > > > > mm_slot = alloc_mm_slot(); > > if (!mm_slot) > > @@ -2515,6 +2516,10 @@ int __ksm_enter(struct mm_struct *mm) > > /* Check ksm_run too? Would need tighter locking */ > > needs_wakeup = list_empty(&ksm_mm_head.mm_list); > > > > + mutex_lock(&ksm_thread_mutex); > > + ksm_run_unmerge = !!(ksm_run & KSM_RUN_UNMERGE); > > + mutex_unlock(&ksm_thread_mutex); > > > > spin_lock(&ksm_mmlist_lock); > > insert_to_mm_slots_hash(mm, mm_slot); > > /* > > @@ -2527,7 +2532,7 @@ int __ksm_enter(struct mm_struct *mm) > > * scanning cursor, otherwise KSM pages in newly forked mms will be > > * missed: then we might as well insert at the end of the list. > > */ > > - if (ksm_run & KSM_RUN_UNMERGE) > > + if (ksm_run_unmerge) > > run_store() can alter ksm_run right here, so __ksm_enter() is still > acting on the old setting? > > > list_add_tail(&mm_slot->mm_list, &ksm_mm_head.mm_list); > > else > > list_add_tail(&mm_slot->mm_list, > &ksm_scan.mm_slot->mm_list); > > --000000000000e6903605e696d729 Content-Type: text/html; charset="UTF-8" Content-Transfer-Encoding: quoted-printable
Hi all,=C2=A0

=C2=A0I looked through=C2=A0the vulnerability=C2= =A0some more and came up with this so far. Please let me know what you thin= k

=
Consider the = following interleaving.=C2=A0
  1. In=C2=A0__ksm_enter, thread 1 inserts the new mm into= the list of mms at a position determined by=C2=A0ksm_run=C2=A0being= set to KSM_RUN_MERGE (see=C2=A0here)
    2. In=C2=A0run_store, thread 2 cha= nges=C2=A0ksm_run=C2=A0to KSM_RUN_UNMERGE=C2=A0h= ere=C2=A0and executes unmerge_and_remove_all_rmap_items, where it can f= ree the newly added mm via=C2=A0mmdrop=C2=A0here= .=C2=A0
  2. In=C2=A0__= ksm_enter, thread 2 continues execution and updates the fields of the n= ew mm (see=C2=A0here), although it was freed, resul= ting in a use-after-free vulnerability.=C2=A0
Thanks!
=

= On Thu, Aug 11, 2022 at 7:00 PM Andrew Morton <akpm@linux-foundation.org> wrote:
On Tue, 2 Aug 2022 23:15:50 +0= 800 Kefeng Wang <wangkefeng.wang@huawei.com> wrote:

> Abhishek reported a data-race issue,

OK, but it would be better to perform an analysis of the alleged bug,
describe the potential effects if the race is hit, etc.

> --- a/mm/ksm.c
> +++ b/mm/ksm.c
> @@ -2507,6 +2507,7 @@ int __ksm_enter(struct mm_struct *mm)
>=C2=A0 {
>=C2=A0 =C2=A0 =C2=A0 =C2=A0struct mm_slot *mm_slot;
>=C2=A0 =C2=A0 =C2=A0 =C2=A0int needs_wakeup;
> +=C2=A0 =C2=A0 =C2=A0bool ksm_run_unmerge;
>=C2=A0
>=C2=A0 =C2=A0 =C2=A0 =C2=A0mm_slot =3D alloc_mm_slot();
>=C2=A0 =C2=A0 =C2=A0 =C2=A0if (!mm_slot)
> @@ -2515,6 +2516,10 @@ int __ksm_enter(struct mm_struct *mm)
>=C2=A0 =C2=A0 =C2=A0 =C2=A0/* Check ksm_run too?=C2=A0 Would need tight= er locking */
>=C2=A0 =C2=A0 =C2=A0 =C2=A0needs_wakeup =3D list_empty(&ksm_mm_head= .mm_list);
>=C2=A0
> +=C2=A0 =C2=A0 =C2=A0mutex_lock(&ksm_thread_mutex);
> +=C2=A0 =C2=A0 =C2=A0ksm_run_unmerge =3D !!(ksm_run & KSM_RUN_UNME= RGE);
> +=C2=A0 =C2=A0 =C2=A0mutex_unlock(&ksm_thread_mutex);
>
>=C2=A0 =C2=A0 =C2=A0 =C2=A0spin_lock(&ksm_mmlist_lock);
>=C2=A0 =C2=A0 =C2=A0 =C2=A0insert_to_mm_slots_hash(mm, mm_slot);
>=C2=A0 =C2=A0 =C2=A0 =C2=A0/*
> @@ -2527,7 +2532,7 @@ int __ksm_enter(struct mm_struct *mm)
>=C2=A0 =C2=A0 =C2=A0 =C2=A0 * scanning cursor, otherwise KSM pages in n= ewly forked mms will be
>=C2=A0 =C2=A0 =C2=A0 =C2=A0 * missed: then we might as well insert at t= he end of the list.
>=C2=A0 =C2=A0 =C2=A0 =C2=A0 */
> -=C2=A0 =C2=A0 =C2=A0if (ksm_run & KSM_RUN_UNMERGE)
> +=C2=A0 =C2=A0 =C2=A0if (ksm_run_unmerge)

run_store() can alter ksm_run right here, so __ksm_enter() is still
acting on the old setting?

>=C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0list_add_tail(&a= mp;mm_slot->mm_list, &ksm_mm_head.mm_list);
>=C2=A0 =C2=A0 =C2=A0 =C2=A0else
>=C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0list_add_tail(&a= mp;mm_slot->mm_list, &ksm_scan.mm_slot->mm_list);

--000000000000e6903605e696d729--