From: Petr Mladek <pmladek@suse.com> To: Will Deacon <will@kernel.org> Cc: Kees Cook <keescook@chromium.org>, Russell King <linux@armlinux.org.uk>, Sergey Senozhatsky <sergey.senozhatsky@gmail.com>, Steven Rostedt <rostedt@goodmis.org>, Feng Tang <feng.tang@intel.com>, Andrew Morton <akpm@linux-foundation.org>, Greg Kroah-Hartman <gregkh@linuxfoundation.org>, linux-arm-kernel@lists.infradead.org, Ingo Molnar <mingo@redhat.com>, linux-kernel@vger.kernel.org, stable@vger.kernel.org, contact@xogium.me Subject: Re: [PATCH] panic: Ensure preemption is disabled during panic() Date: Fri, 4 Oct 2019 11:11:42 +0200 [thread overview] Message-ID: <20191004091142.57iylai22aqpu6lu@pathway.suse.cz> (raw) In-Reply-To: <20191003205633.w26geqhq67u4ysit@willie-the-truck> On Thu 2019-10-03 21:56:34, Will Deacon wrote: > Hi Kees, > > On Wed, Oct 02, 2019 at 01:58:46PM -0700, Kees Cook wrote: > > On Wed, Oct 02, 2019 at 01:35:38PM +0100, Will Deacon wrote: > > > Calling 'panic()' on a kernel with CONFIG_PREEMPT=y can leave the > > > calling CPU in an infinite loop, but with interrupts and preemption > > > enabled. From this state, userspace can continue to be scheduled, > > > despite the system being "dead" as far as the kernel is concerned. This > > > is easily reproducible on arm64 when booting with "nosmp" on the command > > > line; a couple of shell scripts print out a periodic "Ping" message > > > whilst another triggers a crash by writing to /proc/sysrq-trigger: > > > > > > | sysrq: Trigger a crash > > > | Kernel panic - not syncing: sysrq triggered crash > > > | CPU: 0 PID: 1 Comm: init Not tainted 5.2.15 #1 > > > | Hardware name: linux,dummy-virt (DT) > > > | Call trace: > > > | dump_backtrace+0x0/0x148 > > > | show_stack+0x14/0x20 > > > | dump_stack+0xa0/0xc4 > > > | panic+0x140/0x32c > > > | sysrq_handle_reboot+0x0/0x20 > > > | __handle_sysrq+0x124/0x190 > > > | write_sysrq_trigger+0x64/0x88 > > > | proc_reg_write+0x60/0xa8 > > > | __vfs_write+0x18/0x40 > > > | vfs_write+0xa4/0x1b8 > > > | ksys_write+0x64/0xf0 > > > | __arm64_sys_write+0x14/0x20 > > > | el0_svc_common.constprop.0+0xb0/0x168 > > > | el0_svc_handler+0x28/0x78 > > > | el0_svc+0x8/0xc > > > | Kernel Offset: disabled > > > | CPU features: 0x0002,24002004 > > > | Memory Limit: none > > > | ---[ end Kernel panic - not syncing: sysrq triggered crash ]--- > > > | Ping 2! > > > | Ping 1! > > > | Ping 1! > > > | Ping 2! > > > > > > The issue can also be triggered on x86 kernels if CONFIG_SMP=n, otherwise > > > local interrupts are disabled in 'smp_send_stop()'. > > > > > > Disable preemption in 'panic()' before re-enabling interrupts. > > > > Is this perhaps the correct solution for what commit c39ea0b9dd24 ("panic: > > avoid the extra noise dmesg") was trying to fix? > > Hmm, maybe, although that looks like it's focussed more on irq handling > than preemption. Exactly, the backtrace mentioned in commit c39ea0b9dd24 ("panic: avoid the extra noise dmesg") is printed by wake_up() called from wake_up_klogd_work_func(). It is irq_work. Therefore disabling preemption would not prevent this. > I've deliberately left the irq part alone, since I think > having magic sysrq work via the keyboard interrupt is desirable from the > panic loop. I agree that we should keep sysrq working. One pity thing is that led_panic_blink() in leds/drivers/trigger/ledtrig-panic.c uses workqueues: + led_panic_blink() + led_trigger_event() + led_set_brightness() + schedule_work() It means that it depends on the scheduler. I guess that it does not work in many panic situations. But this patch will always block it. I agree that it is strange that userspace still works at this stage. But does it cause any real problems? Best Regards, Petr
WARNING: multiple messages have this Message-ID (diff)
From: Petr Mladek <pmladek@suse.com> To: Will Deacon <will@kernel.org> Cc: Feng Tang <feng.tang@intel.com>, Kees Cook <keescook@chromium.org>, Greg Kroah-Hartman <gregkh@linuxfoundation.org>, contact@xogium.me, Russell King <linux@armlinux.org.uk>, Steven Rostedt <rostedt@goodmis.org>, linux-kernel@vger.kernel.org, Sergey Senozhatsky <sergey.senozhatsky@gmail.com>, Ingo Molnar <mingo@redhat.com>, stable@vger.kernel.org, Andrew Morton <akpm@linux-foundation.org>, linux-arm-kernel@lists.infradead.org Subject: Re: [PATCH] panic: Ensure preemption is disabled during panic() Date: Fri, 4 Oct 2019 11:11:42 +0200 [thread overview] Message-ID: <20191004091142.57iylai22aqpu6lu@pathway.suse.cz> (raw) In-Reply-To: <20191003205633.w26geqhq67u4ysit@willie-the-truck> On Thu 2019-10-03 21:56:34, Will Deacon wrote: > Hi Kees, > > On Wed, Oct 02, 2019 at 01:58:46PM -0700, Kees Cook wrote: > > On Wed, Oct 02, 2019 at 01:35:38PM +0100, Will Deacon wrote: > > > Calling 'panic()' on a kernel with CONFIG_PREEMPT=y can leave the > > > calling CPU in an infinite loop, but with interrupts and preemption > > > enabled. From this state, userspace can continue to be scheduled, > > > despite the system being "dead" as far as the kernel is concerned. This > > > is easily reproducible on arm64 when booting with "nosmp" on the command > > > line; a couple of shell scripts print out a periodic "Ping" message > > > whilst another triggers a crash by writing to /proc/sysrq-trigger: > > > > > > | sysrq: Trigger a crash > > > | Kernel panic - not syncing: sysrq triggered crash > > > | CPU: 0 PID: 1 Comm: init Not tainted 5.2.15 #1 > > > | Hardware name: linux,dummy-virt (DT) > > > | Call trace: > > > | dump_backtrace+0x0/0x148 > > > | show_stack+0x14/0x20 > > > | dump_stack+0xa0/0xc4 > > > | panic+0x140/0x32c > > > | sysrq_handle_reboot+0x0/0x20 > > > | __handle_sysrq+0x124/0x190 > > > | write_sysrq_trigger+0x64/0x88 > > > | proc_reg_write+0x60/0xa8 > > > | __vfs_write+0x18/0x40 > > > | vfs_write+0xa4/0x1b8 > > > | ksys_write+0x64/0xf0 > > > | __arm64_sys_write+0x14/0x20 > > > | el0_svc_common.constprop.0+0xb0/0x168 > > > | el0_svc_handler+0x28/0x78 > > > | el0_svc+0x8/0xc > > > | Kernel Offset: disabled > > > | CPU features: 0x0002,24002004 > > > | Memory Limit: none > > > | ---[ end Kernel panic - not syncing: sysrq triggered crash ]--- > > > | Ping 2! > > > | Ping 1! > > > | Ping 1! > > > | Ping 2! > > > > > > The issue can also be triggered on x86 kernels if CONFIG_SMP=n, otherwise > > > local interrupts are disabled in 'smp_send_stop()'. > > > > > > Disable preemption in 'panic()' before re-enabling interrupts. > > > > Is this perhaps the correct solution for what commit c39ea0b9dd24 ("panic: > > avoid the extra noise dmesg") was trying to fix? > > Hmm, maybe, although that looks like it's focussed more on irq handling > than preemption. Exactly, the backtrace mentioned in commit c39ea0b9dd24 ("panic: avoid the extra noise dmesg") is printed by wake_up() called from wake_up_klogd_work_func(). It is irq_work. Therefore disabling preemption would not prevent this. > I've deliberately left the irq part alone, since I think > having magic sysrq work via the keyboard interrupt is desirable from the > panic loop. I agree that we should keep sysrq working. One pity thing is that led_panic_blink() in leds/drivers/trigger/ledtrig-panic.c uses workqueues: + led_panic_blink() + led_trigger_event() + led_set_brightness() + schedule_work() It means that it depends on the scheduler. I guess that it does not work in many panic situations. But this patch will always block it. I agree that it is strange that userspace still works at this stage. But does it cause any real problems? Best Regards, Petr _______________________________________________ linux-arm-kernel mailing list linux-arm-kernel@lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-arm-kernel
next prev parent reply other threads:[~2019-10-04 9:11 UTC|newest] Thread overview: 22+ messages / expand[flat|nested] mbox.gz Atom feed top 2019-10-02 12:35 [PATCH] panic: Ensure preemption is disabled during panic() Will Deacon 2019-10-02 12:35 ` Will Deacon 2019-10-02 20:58 ` Kees Cook 2019-10-02 20:58 ` Kees Cook 2019-10-03 20:56 ` Will Deacon 2019-10-03 20:56 ` Will Deacon 2019-10-04 9:11 ` Petr Mladek [this message] 2019-10-04 9:11 ` Petr Mladek 2019-10-04 9:29 ` Russell King - ARM Linux admin 2019-10-04 9:29 ` Russell King - ARM Linux admin 2019-10-04 10:49 ` Will Deacon 2019-10-04 10:49 ` Will Deacon 2019-10-04 11:15 ` Petr Mladek 2019-10-04 11:15 ` Petr Mladek 2019-10-04 13:51 ` Feng Tang 2019-10-04 13:51 ` Feng Tang 2019-10-07 8:02 ` Jiri Kosina 2019-10-07 8:02 ` Jiri Kosina 2019-10-02 21:45 ` Andrew Morton 2019-10-02 21:45 ` Andrew Morton 2019-10-03 20:53 ` Will Deacon 2019-10-03 20:53 ` Will Deacon
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