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Date:   Mon, 11 Jan 2021 09:27:21 +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.
> 

Hi Bean,

Yes, you misunderstood it. I am just giving you an example since rpm_lvl
is the first one shows up in the code. I am showing you that it can 
happen
to any other UFS sysfs nodes. When the BUG_ON I added is triggered, it 
means
that we run into race condition - sysfs node is trying to turn on the 
host
to do something while ufshcd_shutdown() is trying to shut down.

Quotes from Wikipedia:

"A race condition arises in software when a computer program, to operate 
properly, depends on the sequence or timing of the program's processes 
or threads. Critical race conditions cause invalid execution and 
software bugs."

If you are saying race condition is not the right word being used in 
this
patch, please advise a better one.

> 
> 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

You misunderstood it again. You are expecting a simple query cmd error.
But what really matters are NoC issues[1] and OCP[2]. And while/after 
UFS
shutting down, either of them may happen.

[1] When a un-clocked register access issue happens, we call it a NoC 
issue,
meaning you are tring to access a register when clocks are disabled. 
This
leads to system CRASH.

[2] OCP is over current protection. While UFS shutting down, you may
have put UFS regulators to LPM. After that, if you are still trying to
talk to UFS, OCP can happen on VCCQ/VCCQ2. This leads to system CRASH 
too.

> 
> 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.
> 

Easy for you to say. System crash is a big issue to any SoC vendors I 
belive.

Can Guo.

> Bean

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