Re: [PATCH v3 3/3] kexec: Introduce parameters load_limit_reboot and load_limit_panic

[Ricardo Ribalda]

 Hi Steven

Thanks for your review!!! Will send a new version.
After giving it a thought... you are right :). setting the current
value should return -EINVAL. We should only return OK if we actually
do something.

On Wed, 21 Dec 2022 at 01:22, Steven Rostedt <rostedt@xxxxxxxxxxx> wrote:
>
> On Tue, 20 Dec 2022 23:05:45 +0100
> Ricardo Ribalda <ribalda@xxxxxxxxxxxx> wrote:
>
> I hate to be the grammar police, but..
>
> > Add two parameter to specify how many times a kexec kernel can be loaded.
>
>    "parameters"
>
> >
> > 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@xxxxxxxxxxxx>
> > ---
> >  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).
>
> Perhaps state:
>                         smaller positive value than the current one or if
>                         current is currently -1.

I find it a bit complicated..
What about:

 During runtime this parameter can be modified with a more restrictive value




>
> > +
> > +                     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 182e0c11b87b..5daf9990d5b8 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_permitted(void);
> > +bool kexec_load_permitted(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 ce1bca874a8d..7aefd134e319 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_permitted())
> > +     if (!kexec_load_permitted(flags & KEXEC_ON_CRASH))
>
> Note, here we have KEXEC_ON_CRASH (see bottom).
>
> >               return -EPERM;
> >
> >       /* Permit LSMs and IMA to fail the kexec */
> > diff --git a/kernel/kexec_core.c b/kernel/kexec_core.c
> > index a1efc70f4158..adf71f2be3ff 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_permitted(void)
> > +struct kexec_load_limit {
> > +     /* Mutex protects the limit count. */
> > +     struct mutex mutex;
> > +     int limit;
> > +};
> > +
> > +struct kexec_load_limit load_limit_reboot = {
>
> Perhaps make the above static?
>
> > +     .mutex = __MUTEX_INITIALIZER(load_limit_reboot.mutex),
> > +     .limit = -1,
> > +};
> > +
> > +struct kexec_load_limit load_limit_panic = {
>
> static?
>
> > +     .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;
>
> Looks better if "int ret;" is after the "limit".
>
> > +
> > +     mutex_lock(&limit->mutex);
> > +     ret = scnprintf(buffer, PAGE_SIZE, "%i\n", limit->limit);
>
> The above string can be at most "-2147483648\n\0"
>
> Which is 13 characters. Why use PAGE_SIZE. Or scnprintf(), and not just
> state:
>
like it!
>         /* buffer is PAGE_SIZE, much larger than what %i can be */
>         ret = sprintf(buffer, "%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);
>
> I wonder if anything less than -1 should be invalid.
>
> > +
> > +     mutex_lock(&limit->mutex);
> > +
> > +     if (new_val == -1 && limit->limit != -1) {
>
> If -1 can't change the value, why allow it to be passed in to begin with.
>
> Perhaps we should only allow sysctl to set positive values? Would make the
> code simpler.
>
> > +             ret = -EINVAL;
> > +             goto done;
> > +     }
> > +
> > +     if (limit->limit != -1 && new_val > limit->limit) {
>
> Since the above documentation said "small than" perhaps ">="?
>
> > +             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");
>
> Wait, why the module params if this can not be a module?
>
> The kernel/kexec.c is decided via CONFIG_KEXEC_CORE which is bool. Either
> builtin or not at all. No module selection possible.
>
> For kernel parameters, we should just use __setup(), right?

Isn't __setup() only kernel parameter and then it cannot be updated on runtime?

What about using late_param_cb? and remove MODULE_PARAM_DESC ?

I think this is how these parameters work

$ ls /sys/module/kernel/parameters/
consoleblank  crash_kexec_post_notifiers  ignore_rlimit_data
initcall_debug  module_blacklist  panic  panic_on_warn  panic_print
pause_on_oops


>
> > +
> > +bool kexec_load_permitted(bool crash_image)
> > +{
> > +     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;
> >  }
> >
> >  /*
> > diff --git a/kernel/kexec_file.c b/kernel/kexec_file.c
> > index 29efa43ea951..6a1d4b07635e 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_permitted())
> > +     if (!kexec_load_permitted(flags & KEXEC_FILE_FLAGS))
>
> Here we have KEXEC_FILE_FLAGS, where above it was KEXCE_FILE_CRASH.
>
> This is confusing to what denotes the "crash_image" boolean. Can we just
> pass in flags and figure it out in the kexec_load_permitted() function?

This is a typo and a bad one!, thanks for catching up!.

It should be KEXEC_ON_CRASH and KEXEC_FILE_ON_CRASH, Of course both
have different values

I could pass the flags and then check for flags & (KEXEC_ON_CRASH |
KEXEC_FILE_ON_CRASH)... but not sure if it is better
>
> -- Steve
>
>
> >               return -EPERM;
> >
> >       /* Make sure we have a legal set of flags */
> >
>


-- 
Ricardo Ribalda