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Message-ID: <CANiDSCs8O5H0WbEMGEg6Q_JOz_hNWyXu5WuXzG5NSgwrGC=1Ow@mail.gmail.com>
Date: Tue, 20 Dec 2022 23:05:09 +0100
From: Ricardo Ribalda <ribalda@...omium.org>
To: Joel Fernandes <joel@...lfernandes.org>
Cc: Philipp Rudo <prudo@...hat.com>,
Eric Biederman <ebiederm@...ssion.com>,
Jonathan Corbet <corbet@....net>, linux-doc@...r.kernel.org,
Steven Rostedt <rostedt@...dmis.org>,
kexec@...ts.infradead.org,
Sergey Senozhatsky <senozhatsky@...omium.org>,
Ross Zwisler <zwisler@...nel.org>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 3/3] kexec: Introduce paramters load_limit_reboot and load_limit_panic
Hi Joel
Thanks for looking into this
On Thu, 15 Dec 2022 at 20:16, Joel Fernandes <joel@...lfernandes.org> wrote:
>
> Hi Ricardo,
>
> On Thu, Dec 08, 2022 at 05:38:02PM +0100, Ricardo Ribalda wrote:
> > Add two parameter to specify how many times a kexec kernel can be loaded.
> >
> > The sysadmin can set different limits for kexec panic and kexec reboot
> > kernels.
> >
> > The value can be modified at runtime via sysfs, but only with a value
> > smaller than the current one (except -1).
> >
> > Signed-off-by: Ricardo Ribalda <ribalda@...omium.org>
> > ---
> > Documentation/admin-guide/kernel-parameters.txt | 14 ++++
> > include/linux/kexec.h | 2 +-
> > kernel/kexec.c | 2 +-
> > kernel/kexec_core.c | 91 ++++++++++++++++++++++++-
> > kernel/kexec_file.c | 2 +-
> > 5 files changed, 106 insertions(+), 5 deletions(-)
> >
> > diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
> > index 42af9ca0127e..2b37d6a20747 100644
> > --- a/Documentation/admin-guide/kernel-parameters.txt
> > +++ b/Documentation/admin-guide/kernel-parameters.txt
> > @@ -2374,6 +2374,20 @@
> > for Movable pages. "nn[KMGTPE]", "nn%", and "mirror"
> > are exclusive, so you cannot specify multiple forms.
> >
> > + kexec_core.load_limit_reboot=
> > + kexec_core.load_limit_panic=
> > + [KNL]
> > + This parameter specifies a limit to the number of times
> > + a kexec kernel can be loaded.
> > + Format: <int>
> > + -1 = Unlimited.
> > + int = Number of times kexec can be called.
> > +
> > + During runtime, this parameter can be modified with a
> > + value smaller than the current one (but not -1).
> > +
> > + Default: -1
> > +
> > kgdbdbgp= [KGDB,HW] kgdb over EHCI usb debug port.
> > Format: <Controller#>[,poll interval]
> > The controller # is the number of the ehci usb debug
> > diff --git a/include/linux/kexec.h b/include/linux/kexec.h
> > index e9e1ab5e8006..3d7d10f7187a 100644
> > --- a/include/linux/kexec.h
> > +++ b/include/linux/kexec.h
> > @@ -407,7 +407,7 @@ extern int kimage_crash_copy_vmcoreinfo(struct kimage *image);
> > extern struct kimage *kexec_image;
> > extern struct kimage *kexec_crash_image;
> >
> > -bool kexec_load_permited(void);
> > +bool kexec_load_permited(bool crash_image);
> >
> > #ifndef kexec_flush_icache_page
> > #define kexec_flush_icache_page(page)
> > diff --git a/kernel/kexec.c b/kernel/kexec.c
> > index d83fc9093aff..2b0856e83fe1 100644
> > --- a/kernel/kexec.c
> > +++ b/kernel/kexec.c
> > @@ -193,7 +193,7 @@ static inline int kexec_load_check(unsigned long nr_segments,
> > int result;
> >
> > /* We only trust the superuser with rebooting the system. */
> > - if (!kexec_load_permited())
> > + if (!kexec_load_permited(flags & KEXEC_ON_CRASH))
>
> nit: permitted.
>
> > return -EPERM;
> >
> > /* Permit LSMs and IMA to fail the kexec */
> > diff --git a/kernel/kexec_core.c b/kernel/kexec_core.c
> > index 18bd90ca9c99..7f9d5288b24b 100644
> > --- a/kernel/kexec_core.c
> > +++ b/kernel/kexec_core.c
> > @@ -952,13 +952,100 @@ static int __init kexec_core_sysctl_init(void)
> > late_initcall(kexec_core_sysctl_init);
> > #endif
> >
> > -bool kexec_load_permited(void)
> > +struct kexec_load_limit {
> > + /* Mutex protects the limit count. */
> > + struct mutex mutex;
> > + int limit;
>
> Can you not just use atomic ops for limit, and get rid of the mutex?
>
> That will simplify the code as well.
I could not find a way to use atomic_t. The operations are not just
counters, but maybe I am missing a better way to do it :)
The current operations:
- permitted():
if (param==-1) {
return -1;
}
if (param =0)
return -1;
param = param -1;
- paramter_set()
new_param = min(-1, new_param);
if (param == -1) {
param = new_param;
return
}
if (new_param == -1) {
return -EINVAL;
}
param = min(new_param, param);
>
> > +};
> > +
> > +struct kexec_load_limit load_limit_reboot = {
> > + .mutex = __MUTEX_INITIALIZER(load_limit_reboot.mutex),
> > + .limit = -1,
> > +};
> > +
> > +struct kexec_load_limit load_limit_panic = {
> > + .mutex = __MUTEX_INITIALIZER(load_limit_panic.mutex),
> > + .limit = -1,
> > +};
> > +
> > +static int param_get_limit(char *buffer, const struct kernel_param *kp)
> > {
> > + int ret;
> > + struct kexec_load_limit *limit = kp->arg;
> > +
> > + mutex_lock(&limit->mutex);
> > + ret = scnprintf(buffer, PAGE_SIZE, "%i\n", limit->limit);
> > + mutex_unlock(&limit->mutex);
> > +
> > + return ret;
> > +}
> > +
> > +static int param_set_limit(const char *buffer, const struct kernel_param *kp)
> > +{
> > + int ret;
> > + struct kexec_load_limit *limit = kp->arg;
> > + int new_val;
> > +
> > + ret = kstrtoint(buffer, 0, &new_val);
> > + if (ret)
> > + return ret;
> > +
> > + new_val = max(-1, new_val);
> > +
> > + mutex_lock(&limit->mutex);
> > +
> > + if (new_val == -1 && limit->limit != -1) {
> > + ret = -EINVAL;
> > + goto done;
> > + }
> > +
> > + if (limit->limit != -1 && new_val > limit->limit) {
> > + ret = -EINVAL;
> > + goto done;
> > + }
> > +
> > + limit->limit = new_val;
> > +
> > +done:
> > + mutex_unlock(&limit->mutex);
> > +
> > + return ret;
> > +}
> > +
> > +static const struct kernel_param_ops load_limit_ops = {
> > + .get = param_get_limit,
> > + .set = param_set_limit,
> > +};
> > +
> > +module_param_cb(load_limit_reboot, &load_limit_ops, &load_limit_reboot, 0644);
> > +MODULE_PARM_DESC(load_limit_reboot, "Maximum attempts to load a kexec reboot kernel");
> > +
> > +module_param_cb(load_limit_panic, &load_limit_ops, &load_limit_panic, 0644);
> > +MODULE_PARM_DESC(load_limit_reboot, "Maximum attempts to load a kexec panic kernel");
> > +
> > +bool kexec_load_permited(bool crash_image)
>
> nit: permitted.
>
> > +{
> > + struct kexec_load_limit *limit;
> > +
> > /*
> > * Only the superuser can use the kexec syscall and if it has not
> > * been disabled.
> > */
> > - return capable(CAP_SYS_BOOT) && !kexec_load_disabled;
> > + if (!capable(CAP_SYS_BOOT) || kexec_load_disabled)
> > + return false;
> > +
> > + /* Check limit counter and decrease it.*/
> > + limit = crash_image ? &load_limit_panic : &load_limit_reboot;
> > + mutex_lock(&limit->mutex);
> > + if (!limit->limit) {
> > + mutex_unlock(&limit->mutex);
> > + return false;
> > + }
> > + if (limit->limit != -1)
> > + limit->limit--;
> > + mutex_unlock(&limit->mutex);
> > +
> > + return true;
>
> IMO, patch 2/3 can be squashed with 3/3 but no strong preference.
>
> thanks,
>
> - Joel
>
>
>
> > }
> >
> > /*
> > diff --git a/kernel/kexec_file.c b/kernel/kexec_file.c
> > index 701147c118d4..61212a9252a6 100644
> > --- a/kernel/kexec_file.c
> > +++ b/kernel/kexec_file.c
> > @@ -330,7 +330,7 @@ SYSCALL_DEFINE5(kexec_file_load, int, kernel_fd, int, initrd_fd,
> > struct kimage **dest_image, *image;
> >
> > /* We only trust the superuser with rebooting the system. */
> > - if (!kexec_load_permited())
> > + if (!kexec_load_permited(flags & KEXEC_FILE_FLAGS))
> > return -EPERM;
> >
> > /* Make sure we have a legal set of flags */
> >
> > --
> > 2.39.0.rc0.267.gcb52ba06e7-goog-b4-0.11.0-dev-696ae
--
Ricardo Ribalda
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