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Message-ID: <202003241137.A90B14A@keescook>
Date: Tue, 24 Mar 2020 11:37:19 -0700
From: Kees Cook <keescook@...omium.org>
To: "Guilherme G. Piccoli" <gpiccoli@...onical.com>
Cc: linux-kernel@...r.kernel.org, linux-doc@...r.kernel.org,
mcgrof@...nel.org, yzaikin@...gle.com, tglx@...utronix.de,
akpm@...ux-foundation.org, linux-api@...r.kernel.org,
rdunlap@...radead.org, willy@...radead.org, kernel@...ccoli.net
Subject: Re: [PATCH V2] panic: Add sysctl/cmdline to dump all CPUs backtraces
on oops event
On Mon, Mar 23, 2020 at 07:30:35PM -0300, Guilherme G. Piccoli wrote:
> Usually when kernel reach an oops condition, it's a point of no return;
> in case not enough debug information is available in the kernel splat,
> one of the last resorts would be to collect a kernel crash dump and
> analyze it. The problem with this approach is that in order to collect
> the dump, a panic is required (to kexec-load the crash kernel). When
> in an environment of multiple virtual machines, users may prefer to
> try living with the oops, at least until being able to properly
> shutdown their VMs / finish their important tasks.
>
> This patch implements a way to collect a bit more debug details when an
> oops event is reached, by printing all the CPUs backtraces through the
> usage of NMIs (on architectures that support that). The sysctl/kernel
> parameter added (and documented) here was called "oops_all_cpu_backtrace"
> and when set will (as the name suggests) dump all CPUs backtraces.
>
> Far from ideal, this may be the last option though for users that for
> some reason cannot panic on oops. Most of times oopses are clear enough
> to indicate the kernel portion that must be investigated, but in virtual
> environments it's possible to observe hypervisor/KVM issues that could
> lead to oopses shown in other guests CPUs (like virtual APIC crashes).
> This patch hence aims to help debug such complex issues without
> resorting to kdump.
>
> Signed-off-by: Guilherme G. Piccoli <gpiccoli@...onical.com>
Reviewed-by: Kees Cook <keescook@...omium.org>
-Kees
> ---
>
>
> V2: Implemented grammar suggestions from Randy, Andrew and
> Matthew. Thanks in advance for the reviews!
> Cheers,
>
> Guilherme
>
>
> .../admin-guide/kernel-parameters.txt | 8 +++++++
> Documentation/admin-guide/sysctl/kernel.rst | 17 +++++++++++++++
> include/linux/kernel.h | 6 ++++++
> kernel/panic.c | 21 +++++++++++++++++++
> kernel/sysctl.c | 11 ++++++++++
> 5 files changed, 63 insertions(+)
>
> diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
> index 7a14caac6c94..7db622028c00 100644
> --- a/Documentation/admin-guide/kernel-parameters.txt
> +++ b/Documentation/admin-guide/kernel-parameters.txt
> @@ -3333,6 +3333,14 @@
> This will also cause panics on machine check exceptions.
> Useful together with panic=30 to trigger a reboot.
>
> + oops_all_cpu_backtrace=
> + [KNL] Should kernel generate backtraces on all cpus
> + when oops occurs - this should be a last measure resort
> + in case a kdump cannot be collected, for example.
> + Defaults to 0 and can be controlled by the sysctl
> + kernel.oops_all_cpu_backtrace.
> + Format: <integer>
> +
> page_alloc.shuffle=
> [KNL] Boolean flag to control whether the page allocator
> should randomize its free lists. The randomization may
> diff --git a/Documentation/admin-guide/sysctl/kernel.rst b/Documentation/admin-guide/sysctl/kernel.rst
> index 8b4ff69d2348..8660001d3a3e 100644
> --- a/Documentation/admin-guide/sysctl/kernel.rst
> +++ b/Documentation/admin-guide/sysctl/kernel.rst
> @@ -57,6 +57,7 @@ show up in /proc/sys/kernel:
> - msgmnb
> - msgmni
> - nmi_watchdog
> +- oops_all_cpu_backtrace
> - osrelease
> - ostype
> - overflowgid
> @@ -572,6 +573,22 @@ numa_balancing_scan_size_mb is how many megabytes worth of pages are
> scanned for a given scan.
>
>
> +oops_all_cpu_backtrace:
> +================
> +
> +If this option is set, the kernel will send an NMI to all CPUs to dump
> +their backtraces when an oops event occurs. It should be used as a last
> +resort in case a panic cannot be triggered (to protect VMs running, for
> +example) or kdump can't be collected. This file shows up if CONFIG_SMP
> +is enabled.
> +
> +0: Won't show all CPUs backtraces when an oops is detected.
> +This is the default behavior.
> +
> +1: Will non-maskably interrupt all CPUs and dump their backtraces when
> +an oops event is detected.
> +
> +
> osrelease, ostype & version:
> ============================
>
> diff --git a/include/linux/kernel.h b/include/linux/kernel.h
> index 0d9db2a14f44..6cd00257b572 100644
> --- a/include/linux/kernel.h
> +++ b/include/linux/kernel.h
> @@ -513,6 +513,12 @@ static inline u32 int_sqrt64(u64 x)
> }
> #endif
>
> +#ifdef CONFIG_SMP
> +extern unsigned int sysctl_oops_all_cpu_backtrace;
> +#else
> +#define sysctl_oops_all_cpu_backtrace 0
> +#endif /* CONFIG_SMP */
> +
> extern void bust_spinlocks(int yes);
> extern int oops_in_progress; /* If set, an oops, panic(), BUG() or die() is in progress */
> extern int panic_timeout;
> diff --git a/kernel/panic.c b/kernel/panic.c
> index b69ee9e76cb2..73c340418575 100644
> --- a/kernel/panic.c
> +++ b/kernel/panic.c
> @@ -36,6 +36,24 @@
> #define PANIC_TIMER_STEP 100
> #define PANIC_BLINK_SPD 18
>
> +#ifdef CONFIG_SMP
> +/*
> + * Should we dump all CPUs backtraces in an oops event?
> + * Defaults to 0, can be changed either via cmdline or sysctl.
> + */
> +unsigned int __read_mostly sysctl_oops_all_cpu_backtrace;
> +
> +static int __init oops_backtrace_setup(char *str)
> +{
> + int rc = kstrtouint(str, 0, &sysctl_oops_all_cpu_backtrace);
> +
> + if (rc)
> + return rc;
> + return 1;
> +}
> +__setup("oops_all_cpu_backtrace=", oops_backtrace_setup);
> +#endif /* CONFIG_SMP */
> +
> int panic_on_oops = CONFIG_PANIC_ON_OOPS_VALUE;
> static unsigned long tainted_mask =
> IS_ENABLED(CONFIG_GCC_PLUGIN_RANDSTRUCT) ? (1 << TAINT_RANDSTRUCT) : 0;
> @@ -515,6 +533,9 @@ void oops_enter(void)
> /* can't trust the integrity of the kernel anymore: */
> debug_locks_off();
> do_oops_enter_exit();
> +
> + if (sysctl_oops_all_cpu_backtrace)
> + trigger_all_cpu_backtrace();
> }
>
> /*
> diff --git a/kernel/sysctl.c b/kernel/sysctl.c
> index 238f268de486..1ac31d9d5b7e 100644
> --- a/kernel/sysctl.c
> +++ b/kernel/sysctl.c
> @@ -813,6 +813,17 @@ static struct ctl_table kern_table[] = {
> .proc_handler = proc_dointvec,
> },
> #endif
> +#ifdef CONFIG_SMP
> + {
> + .procname = "oops_all_cpu_backtrace",
> + .data = &sysctl_oops_all_cpu_backtrace,
> + .maxlen = sizeof(int),
> + .mode = 0644,
> + .proc_handler = proc_dointvec_minmax,
> + .extra1 = SYSCTL_ZERO,
> + .extra2 = SYSCTL_ONE,
> + },
> +#endif /* CONFIG_SMP */
> {
> .procname = "pid_max",
> .data = &pid_max,
> --
> 2.25.1
>
--
Kees Cook
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