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Date:   Mon, 11 Jan 2021 09:52:09 +0800
From:   Can Guo <cang@...eaurora.org>
To:     Bean Huo <huobean@...il.com>
Cc:     asutoshd@...eaurora.org, nguyenb@...eaurora.org,
        hongwus@...eaurora.org, ziqichen@...eaurora.org,
        rnayak@...eaurora.org, linux-scsi@...r.kernel.org,
        kernel-team@...roid.com, saravanak@...gle.com, salyzyn@...gle.com,
        rjw@...ysocki.net, Alim Akhtar <alim.akhtar@...sung.com>,
        Avri Altman <avri.altman@....com>,
        "James E.J. Bottomley" <jejb@...ux.ibm.com>,
        "Martin K. Petersen" <martin.petersen@...cle.com>,
        Stanley Chu <stanley.chu@...iatek.com>,
        Bean Huo <beanhuo@...ron.com>,
        Nitin Rawat <nitirawa@...eaurora.org>,
        Adrian Hunter <adrian.hunter@...el.com>,
        Bart Van Assche <bvanassche@....org>,
        Satya Tangirala <satyat@...gle.com>,
        open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 2/2] scsi: ufs: Protect PM ops and err_handler from user
 access through sysfs

On 2021-01-11 00:13, Bean Huo wrote:
> On Sat, 2021-01-09 at 12:51 +0800, Can Guo wrote:
>> On 2021-01-09 12:45, Can Guo wrote:
>> > On 2021-01-08 19:29, Bean Huo wrote:
>> > > On Wed, 2021-01-06 at 09:20 +0800, Can Guo wrote:
>> > > > Hi Bean,
>> > > >
>> > > > On 2021-01-06 02:38, Bean Huo wrote:
>> > > > > On Tue, 2021-01-05 at 09:07 +0800, Can Guo wrote:
>> > > > > > On 2021-01-05 04:05, Bean Huo wrote:
>> > > > > > > On Sat, 2021-01-02 at 05:59 -0800, Can Guo wrote:
>> > > > > > > > + * @shutting_down: flag to check if shutdown has been
>> > > > > > > > invoked
>> > > > > > >
>> > > > > > > I am not much sure if this flag is need, since once PM
>> > > > > > > going in
>> > > > > > > shutdown path, what will be returnded by
>> > > > > > > pm_runtime_get_sync()?
>> > > > > > >
>> > > > > > > If pm_runtime_get_sync() will fail, just check its
>> > > > > > > return.
>> > > > > > >
>> > > > > >
>> > > > > > That depends. During/after shutdown, for UFS's case only,
>> > > > > > pm_runtime_get_sync(hba->dev) will most likely return 0,
>> > > > > > because it is already RUNTIME_ACTIVE, pm_runtime_get_sync()
>> > > > > > will directly return 0... meaning you cannot count on it.
>> > > > > >
>> > > > > > Check Stanley's change -
>> > > > > > https://lore.kernel.org/patchwork/patch/1341389/
>> > > > > >
>> > > > > > Can Guo.
>> > > > >
>> > > > > Can,
>> > > > >
>> > > > > Thanks for pointing out that.
>> > > > >
>> > > > > Based on my understanding, that patch is redundent. maybe I
>> > > > > misundestood Linux shutdown sequence.
>> > > >
>> > > > Sorry, do you mean Stanley's change is redundant?
>> > >
>> > > yes.
>> > >
>> >
>> > No, it is definitely needed. As Stanley replied you in another
>> > thread, it is not protecting I/Os from user layer, but from
>> > other subsystems during shutdown.
>> >
>> > > >
>> > > > >
>> > > > > I checked the shutdown flow:
>> > > > >
>> > > > > 1. Set the "system_state" variable
>> > > > > 2. Disable usermod to ensure that no user from userspace can
>> > > > > start
>> > > > > a
>> > > > > request
>> > > >
>> > > > I hope it is like what you interpreted, but step #2 only stops
>> > > > UMH(#265)
>> > > > but not all user space activities. Whereas, UMH is for kernel
>> > > > space
>> > > > calling
>> > > > user space.
>> > >
>> > >
>> > > Can,
>> > >
>> > > I did further study and homework on the Linux shutdown in the
>> > > last few
>> > > days. Yes, you are right, usermodehelper_disable() is to prevent
>> > > executing the process from the kernel space.
>> > >
>> > > But I didn't reproduce this "maybe" race issue while shutdown. no
>> > > matter how I torment my system, once Linux shutdown/halt/reboot
>> > > starts,
>> > > nobody can access the sysfs node. I create 10 processes in the
>> > > user
>> > > space and constantly access UFS sysfs node, also, fio is running
>> > > in
>> > > the
>> > > background for the normal data read/write. there is a shutdown
>> > > thread
>> > > that will randomly trigger shutdown/halt/reboot. but no race
>> > > issue
>> > > appears.
>> > >
>> > > I don't know if this is a hypothetical issue(the race between
>> > > shutdown
>> > > flow and sysfs node access), it may not really exist in the Linux
>> > > envriroment. everytime, the shutdonw flow will be:
>> > >
>> > > e10_sync_handler()->e10_svc()->do_e10_svc()->__do_sys_reboot()-
>> > > > kernel_poweroff/kernel_halt()->device_shutdown()-
>> > > > >platform_shutdown()-
>> > > > ufshcd_platform_shutdown()->ufshcd_shutdown().
>> > >
>> > > I think before going into the kernel shutdown, the userspace
>> > > cannot
>> > > issue new requests anymore. otherwise, this would be a big issue.
>> > >
>> > > pm_runtime_get_sync() will return 0 or failure while shutdown?
>> > > the
>> > > answer is not important now, maybe as you said, it is always 0.
>> > > But in
>> > > my testing, it didn't get there the system has been shutdown.
>> > > Which
>> > > means once shutdonw starts, sysfs node access path cannot reach
>> > > pm_runtime_get_sync(). (note, I don't know if sysfs node access
>> > > thread
>> > > has been disabled or not)
>> > >
>> > >
>> > > Responsibly say, I didn't reproduce this issue on my system
>> > > (ubuntu),
>> > > maybe you are using Android. I am not an expert on this topic, if
>> > > you
>> > > have the best idea on how to reproduce this issue. please please
>> > > let
>> > > me
>> > > try. appreciate it!!!!!
>> > >
>> >
>> > When you do a reboot/shutdown/poweroff, how your system behaves
>> > highly
>> > depends on how the reboot cmd is implemented in C code under
>> > /sbin/.
>> >
>> > On Ubuntu, reboot looks like:
>> > $ reboot --help
>> > reboot [OPTIONS...] [ARG]
>> >
>> > Reboot the system.
>> >
>> >       --help      Show this help
>> >       --halt      Halt the machine
>> >    -p --poweroff  Switch off the machine
>> >       --reboot    Reboot the machine
>> >    -f --force     Force immediate halt/power-off/reboot
>> >    -w --wtmp-only Don't halt/power-off/reboot, just write wtmp
>> > record
>> >    -d --no-wtmp   Don't write wtmp record
>> >       --no-wall   Don't send wall message before halt/power-
>> > off/reboot
>> >
>> >
>> > On a pure Linux with a initrd RAM FS built from busybox, reboot
>> > looks
>> > like:
>> > # reboot --help
>> > BusyBox v1.30.1 (2019-05-24 12:53:36 IST) multi-call binary.
>> >
>> > Usage: reboot [-d DELAY] [-n] [-f]
>> >
>> > Reboot the system
>> >
>> >          -d SEC  Delay interval
>> >          -n      Do not sync
>> >          -f      Force (don't go through init)
>> >
>> >
>> > For example, when you work on a pure Linux with a filesystem built
>> > from
>> > busybox, when you hit reboot cmd, halt_main() will be called. And
>> > based
>> > on the reboot options passed to reboot cmd, halt_main() behaves
>> > differently.
>> >
>> > A plain reboot cmd does things like sync filesystem, send SIGKILL
>> > to
>> > all
>> > processes (except for init), remount all filesytem as read-only and
>> > so
>> > on
>> > before invoking linux kernel reboot syscall. In this case, we are
>> > safe.
>> >
>> > However, if you do a "reboot -f", halt_main() directly invokes
>> > reboot().
>> > And with "reboot -f", I can easily reproduce the race condition we
>> > are
>> > talking about here - it is not based on imagination.
>> >
>> > Find the patch I used for replication in the attachment, fix
>> > conflicts
>> > if any. After boot up, the cmd lines I used are
>> >
>> > # while true; do cat /sys/devices/platform/soc@...ufshc*/rpm_lvl;
>> > done
>> > &
>> > # reboot -f
>> >
>> > Can Guo.
>> 
>> Oops... forgot the logs:
>> 
>> #
>> # while true; do cat /sys/devices/platform/soc@...ufshc*/rpm_lvl;
>> done &
>> 3
>> 3
>> 3
>> 3
>> ....
>> # reboot -f
>> 3
>> 3
>> 3
>> ....
>> [   17.959206] sd 0:0:0:5: [sdf] Synchronizing SCSI cache
>> 3
>> [   17.964833] sd 0:0:0:4: [sde] Synchronizing SCSI cache
>> [   17.970224] sd 0:0:0:3: [sdd] Synchronizing SCSI cache
>> [   17.975574] sd 0:0:0:2: [sdc] Synchronizing SCSI cache
>> 3
>> [   17.981034] sd 0:0:0:1: [sdb] Synchronizing SCSI cache
>> [   17.986493] sd 0:0:0:0: [sda] Synchronizing SCSI cache
>> 3
>> [   17.991870] [DEBUG]ufshcd_shutdown: UFS SHUTDOWN START
>> [   17.998902] ------------[ cut here ]------------
>> [   18.003648] kernel BUG at drivers/scsi/ufs/ufs-sysfs.c:62!
>> [   18.009286] Internal error: Oops - BUG: 0 [#1] PREEMPT SMP
>> [   18.034249] pstate: 40c00005 (nZcv daif +PAN +UAO)
>> [   18.039185] pc : rpm_lvl_show+0x38/0x40
>> [   18.043137] lr : dev_attr_show+0x1c/0x58
>> [   18.132552] Call trace:
>> [   18.135076]  rpm_lvl_show+0x38/0x40
>> [   18.138672]  sysfs_kf_seq_show+0xa8/0x140
>> [   18.142802]  kernfs_seq_show+0x28/0x30
>> [   18.146665]  seq_read+0x1d8/0x4b0
>> [   18.150072]  kernfs_fop_read+0x12c/0x1f0
>> [   18.154109]  do_iter_read+0x184/0x1c0
>> [   18.157882]  vfs_readv+0x68/0xb0
>> ....
> 
> Hi Can
> Please forgive me for being verbose.
> 
> The above BUG log is from your BUG_ON() called, not becuase of the race
> betwen shutdown flow and sysfs node access(if I misunderstood, correct
> me). But it tells that the user can still access UFS by sysfs node in
> the "reboot -f" flow since it skips the actual shutdown process, "
> --force" is not a safe way anyway.

What is an "actual shutdown process", is there a standard? I believe it
is free for users to decide what to do before invoking kernel reboot()
syscall. "reboot -f" simply invokes kernel reboot() syscall, safe or not
I don't know, but UFS shold not be the one nailed to the pillar. Do you 
agree?

Can Guo.

> 
> 
> If accessing sysfs nodes, which triggers a UFS UPIU request to
> read/write UFS device descriptors during shutdown flow, there is only
> one issue that sysfs node access failure since UFS device and LINK has
> been shutdown. Strictly speaking, the failure comes after
> ufshcd_set_dev_pwr_mode().
> 
>    __ufshcd_query_descriptor: opcode 0x01 for idn 0 failed, index 0,
> err = -11
> 
> Since the shutdown is oneway process, this failure is not big issue. If
> you meant to avoid this failure for unsafe shutdown, I agree with you,
> But for the race issue, I don't know.
> 
> Bean

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