On Tue, 27 Jun 2023 17:43:25 +0530
Shyam Prasad N <nspmangalore@xxxxxxxxx> wrote:
> On Tue, Jun 27, 2023 at 1:04 PM Winston Wen <wentao@xxxxxxxxxxxxx>
> wrote:
> >
> > On Tue, 27 Jun 2023 12:24:04 +0530
> > Shyam Prasad N <nspmangalore@xxxxxxxxx> wrote:
> >
> > > On Mon, Jun 26, 2023 at 2:09 PM Winston Wen <wentao@xxxxxxxxxxxxx>
> > > wrote:
> > > >
> > > > On Mon, 26 Jun 2023 12:24:35 +0530
> > > > Shyam Prasad N <nspmangalore@xxxxxxxxx> wrote:
> > > >
> > > > > On Mon, Jun 26, 2023 at 10:54 AM Steve French
> > > > > <smfrench@xxxxxxxxx> wrote:
> > > > > >
> > > > > > Added Cc: stable and Shyam's RB and merged into cifs-2.6.git
> > > > > > for-next
> > > > > >
> > > > > > On Mon, Jun 26, 2023 at 12:15 AM Shyam Prasad N
> > > > > > <nspmangalore@xxxxxxxxx> wrote:
> > > > > > >
> > > > > > > On Mon, Jun 26, 2023 at 9:25 AM Winston Wen
> > > > > > > <wentao@xxxxxxxxxxxxx> wrote:
> > > > > > > >
> > > > > > > > We switch session state to SES_EXITING without
> > > > > > > > cifs_tcp_ses_lock now, it may lead to potential
> > > > > > > > use-after-free issue.
> > > > > > > >
> > > > > > > > Consider the following execution processes:
> > > > > > > >
> > > > > > > > Thread 1:
> > > > > > > > __cifs_put_smb_ses()
> > > > > > > > spin_lock(&cifs_tcp_ses_lock)
> > > > > > > > if (--ses->ses_count > 0)
> > > > > > > > spin_unlock(&cifs_tcp_ses_lock)
> > > > > > > > return
> > > > > > > > spin_unlock(&cifs_tcp_ses_lock)
> > > > > > > > ---> **GAP**
> > > > > > > > spin_lock(&ses->ses_lock)
> > > > > > > > if (ses->ses_status == SES_GOOD)
> > > > > > > > ses->ses_status = SES_EXITING
> > > > > > > > spin_unlock(&ses->ses_lock)
> > > > > > > >
> > > > > > > > Thread 2:
> > > > > > > > cifs_find_smb_ses()
> > > > > > > > spin_lock(&cifs_tcp_ses_lock)
> > > > > > > > list_for_each_entry(ses, ...)
> > > > > > > > spin_lock(&ses->ses_lock)
> > > > > > > > if (ses->ses_status == SES_EXITING)
> > > > > > > > spin_unlock(&ses->ses_lock)
> > > > > > > > continue
> > > > > > > > ...
> > > > > > > > spin_unlock(&ses->ses_lock)
> > > > > > > > if (ret)
> > > > > > > > cifs_smb_ses_inc_refcount(ret)
> > > > > > > > spin_unlock(&cifs_tcp_ses_lock)
> > > > > > > >
> > > > > > > > If thread 1 is preempted in the gap and thread 2 start
> > > > > > > > executing, thread 2 will get the session, and soon
> > > > > > > > thread 1 will switch the session state to SES_EXITING
> > > > > > > > and start releasing it, even though thread 1 had
> > > > > > > > increased the session's refcount and still uses it.
> > > > > > > >
> > > > > > > > So switch session state under cifs_tcp_ses_lock to
> > > > > > > > eliminate this gap.
> > > > > > > >
> > > > > > > > Signed-off-by: Winston Wen <wentao@xxxxxxxxxxxxx>
> > > > > > > > ---
> > > > > > > > fs/smb/client/connect.c | 7 ++++---
> > > > > > > > 1 file changed, 4 insertions(+), 3 deletions(-)
> > > > > > > >
> > > > > > > > diff --git a/fs/smb/client/connect.c
> > > > > > > > b/fs/smb/client/connect.c index
> > > > > > > > 9d16626e7a66..165ecb222c19 100644 ---
> > > > > > > > a/fs/smb/client/connect.c +++ b/fs/smb/client/connect.c
> > > > > > > > @@ -1963,15 +1963,16 @@ void __cifs_put_smb_ses(struct
> > > > > > > > cifs_ses *ses) spin_unlock(&cifs_tcp_ses_lock);
> > > > > > > > return;
> > > > > > > > }
> > > > > > > > + spin_lock(&ses->ses_lock);
> > > > > > > > + if (ses->ses_status == SES_GOOD)
> > > > > > > > + ses->ses_status = SES_EXITING;
> > > > > > > > + spin_unlock(&ses->ses_lock);
> > > > > > > > spin_unlock(&cifs_tcp_ses_lock);
> > > > > > > >
> > > > > > > > /* ses_count can never go negative */
> > > > > > > > WARN_ON(ses->ses_count < 0);
> > > > > > > >
> > > > > > > > spin_lock(&ses->ses_lock);
> > > > > > > > - if (ses->ses_status == SES_GOOD)
> > > > > > > > - ses->ses_status = SES_EXITING;
> > > > > > > > -
> > > > > > > > if (ses->ses_status == SES_EXITING &&
> > > > > > > > server->ops->logoff) { spin_unlock(&ses->ses_lock);
> > > > > > > > cifs_free_ipc(ses);
> > > > > > > > --
> > > > > > > > 2.40.1
> > > > > > > >
> > > > > > >
> > > > > > > Good catch.
> > > > > > > Looks good to me.
> > > > > > > @Steve French Please CC stable for this one.
> > > > > > >
> > > > > > > --
> > > > > > > Regards,
> > > > > > > Shyam
> > > > > >
> > > > > >
> > > > > >
> > > > > > --
> > > > > > Thanks,
> > > > > >
> > > > > > Steve
> > > > >
> > > > > @Winston Wen I think the following change should be
> > > > > sufficient to fix this issue:
> > > > > diff --git a/fs/smb/client/connect.c b/fs/smb/client/connect.c
> > > > > index 9d16626e7a66..78874eb2537d 100644
> > > > > --- a/fs/smb/client/connect.c
> > > > > +++ b/fs/smb/client/connect.c
> > > > > @@ -1963,10 +1963,11 @@ void __cifs_put_smb_ses(struct
> > > > > cifs_ses *ses) spin_unlock(&cifs_tcp_ses_lock);
> > > > > return;
> > > > > }
> > > > > - spin_unlock(&cifs_tcp_ses_lock);
> > > > >
> > > > > /* ses_count can never go negative */
> > > > > WARN_ON(ses->ses_count < 0);
> > > > > + list_del_init(&ses->smb_ses_list);
> > > > > + spin_unlock(&cifs_tcp_ses_lock);
> > > > >
> > > > > spin_lock(&ses->ses_lock);
> > > > > if (ses->ses_status == SES_GOOD)
> > > > > @@ -1986,9 +1987,6 @@ void __cifs_put_smb_ses(struct cifs_ses
> > > > > *ses) cifs_free_ipc(ses);
> > > > > }
> > > > >
> > > > > - spin_lock(&cifs_tcp_ses_lock);
> > > > > - list_del_init(&ses->smb_ses_list);
> > > > > - spin_unlock(&cifs_tcp_ses_lock);
> > > > >
> > > > > chan_count = ses->chan_count;
> > > > >
> > > > > The bug was that the ses was kept in the smb ses list, even
> > > > > after the ref count had reached 0.
> > > > > With the above change, that should be fixed, and no one
> > > > > should be able to get to the ses from that point.
> > > > >
> > > > > Please let me know if you see a problem with this.
> > > > >
> > > >
> > > > Hi Shyam,
> > > >
> > > > Thanks for the comments! And sorry for my late reply...
> > > >
> > > > It make sense to me that maybe we should remove the session
> > > > from the list once its refcount is reduced to 0 to avoid any
> > > > futher access. In fact, I did try to do this from the
> > > > beginning. But I was not sure if we need to access the session
> > > > from the list in the free process, such as the following:
> > > >
> > > > smb2_check_receive()
> > > > smb2_verify_signature()
> > > > server->ops->calc_signature()
> > > > smb2_calc_signature()
> > > > smb2_find_smb_ses()
> > > > /* scan the list and find the session */
> > > >
> > > > Perhaps we need some refactoring here.
> > >
> > > Yes. The above ses finding is expected to fail during a reconnect.
> >
> > Agreed.
> >
> > >
> > > >
> > > > So I gave up on this approach and did a small fix to make it
> > > > work, but maybe I missed something elsewhere...
> > > >
> > > >
> > > > --
> > > > Thanks,
> > > > Winston
> > >
> > > Attaching the above change as a patch.
> > > It replaces this particular patch in the series.
> >
> > I think this is a better way to fix the problem, the session really
> > should not stay in the list and be found after it has been marked
> > EXITING.
> >
> > >
> > > The other two patches are not strictly necessary with this
> > > change, but don't hurt.
> > >
> >
> > Yes. Feel free to drop them if they are not necessary. And if that's
> > the case, perhaps we should do some cleaning work on other paths to
> > ensure consistency.
>
> I don't really have a strong opinion about this. Even if they stay,
> I'm okay. But curious to know what you mean by the cleaning work on
> other paths here. Do you still think there's more cleanup needed
> around this?
IIRC there are other paths that scan the list and do the
check, like cifs_find_smb_ses(). So I think if they become unnecessary
now after this fix patch, maybe we can also remove them at the same
time to avoid make others confused.
But I also don't have a strong opinion about this. I think we have the
following options and all are okay to me. Which one do you prefer?
- keep/add the check
- remove all checks
- remove all checks and add a WARNING
(I think we shouldn't find a exiting session in the list now.)
>
> >
> > Thanks for your review and comments!
> >
> > >
> > > --
> > > Regards,
> > > Shyam
> >
> >
> > --
> > Thanks,
> > Winston
>
>
>
--
Thanks,
Winston
