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Message-ID: <202004142007.3C2818CD@keescook>
Date: Tue, 14 Apr 2020 20:08:08 -0700
From: Kees Cook <keescook@...omium.org>
To: Vlastimil Babka <vbabka@...e.cz>
Cc: Andrew Morton <akpm@...ux-foundation.org>,
Luis Chamberlain <mcgrof@...nel.org>,
Iurii Zaikin <yzaikin@...gle.com>,
linux-kernel@...r.kernel.org, linux-api@...r.kernel.org,
linux-mm@...ck.org, Ivan Teterevkov <ivan.teterevkov@...anix.com>,
Michal Hocko <mhocko@...nel.org>,
David Rientjes <rientjes@...gle.com>,
Matthew Wilcox <willy@...radead.org>,
"Eric W . Biederman" <ebiederm@...ssion.com>,
"Guilherme G . Piccoli" <gpiccoli@...onical.com>,
Alexey Dobriyan <adobriyan@...il.com>,
Thomas Gleixner <tglx@...utronix.de>,
Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
Christian Brauner <christian.brauner@...ntu.com>,
Masami Hiramatsu <mhiramat@...nel.org>
Subject: Re: [PATCH v2 1/3] kernel/sysctl: support setting sysctl parameters
from kernel command line
On Tue, Apr 14, 2020 at 01:32:20PM +0200, Vlastimil Babka wrote:
> A recently proposed patch to add vm_swappiness command line parameter in
> addition to existing sysctl [1] made me wonder why we don't have a general
> support for passing sysctl parameters via command line. Googling found only
> somebody else wondering the same [2], but I haven't found any prior discussion
> with reasons why not to do this.
>
> Settings the vm_swappiness issue aside (the underlying issue might be solved in
> a different way), quick search of kernel-parameters.txt shows there are already
> some that exist as both sysctl and kernel parameter - hung_task_panic,
> nmi_watchdog, numa_zonelist_order, traceoff_on_warning. A general mechanism
> would remove the need to add more of those one-offs and might be handy in
> situations where configuration by e.g. /etc/sysctl.d/ is impractical.
>
> Hence, this patch adds a new parse_args() pass that looks for parameters
> prefixed by 'sysctl.' and tries to interpret them as writes to the
> corresponding sys/ files using an temporary in-kernel procfs mount. This
> mechanism was suggested by Eric W. Biederman [3], as it handles all dynamically
> registered sysctl tables, even though we don't handle modular sysctls. Errors
> due to e.g. invalid parameter name or value are reported in the kernel log.
>
> The processing is hooked right before the init process is loaded, as some
> handlers might be more complicated than simple setters and might need some
> subsystems to be initialized. At the moment the init process can be started and
> eventually execute a process writing to /proc/sys/ then it should be also fine
> to do that from the kernel.
>
> Sysctls registered later on module load time are not set by this mechanism -
> it's expected that in such scenarios, setting sysctl values from userspace is
> practical enough.
>
> [1] https://lore.kernel.org/r/BL0PR02MB560167492CA4094C91589930E9FC0@BL0PR02MB5601.namprd02.prod.outlook.com/
> [2] https://unix.stackexchange.com/questions/558802/how-to-set-sysctl-using-kernel-command-line-parameter
> [3] https://lore.kernel.org/r/87bloj2skm.fsf@x220.int.ebiederm.org/
>
> Signed-off-by: Vlastimil Babka <vbabka@...e.cz>
Looks great!
Acked-by: Kees Cook <keescook@...omium.org>
-Kees
> Reviewed-by: Luis Chamberlain <mcgrof@...nel.org>
> ---
> .../admin-guide/kernel-parameters.txt | 9 ++
> fs/proc/proc_sysctl.c | 107 ++++++++++++++++++
> include/linux/sysctl.h | 4 +
> init/main.c | 2 +
> 4 files changed, 122 insertions(+)
>
> diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
> index f2a93c8679e8..fd38f1e8a2bf 100644
> --- a/Documentation/admin-guide/kernel-parameters.txt
> +++ b/Documentation/admin-guide/kernel-parameters.txt
> @@ -4891,6 +4891,15 @@
>
> switches= [HW,M68k]
>
> + sysctl.*= [KNL]
> + Set a sysctl parameter, right before loading the init
> + process, as if the value was written to the respective
> + /proc/sys/... file. Both '.' and '/' are recognized as
> + separators. Unrecognized parameters and invalid values
> + are reported in the kernel log. Sysctls registered
> + later by a loaded module cannot be set this way.
> + Example: sysctl.vm.swappiness=40
> +
> sysfs.deprecated=0|1 [KNL]
> Enable/disable old style sysfs layout for old udev
> on older distributions. When this option is enabled
> diff --git a/fs/proc/proc_sysctl.c b/fs/proc/proc_sysctl.c
> index b6f5d459b087..7804da5f5be0 100644
> --- a/fs/proc/proc_sysctl.c
> +++ b/fs/proc/proc_sysctl.c
> @@ -14,6 +14,7 @@
> #include <linux/mm.h>
> #include <linux/module.h>
> #include <linux/bpf-cgroup.h>
> +#include <linux/mount.h>
> #include "internal.h"
>
> static const struct dentry_operations proc_sys_dentry_operations;
> @@ -1692,3 +1693,109 @@ int __init proc_sys_init(void)
>
> return sysctl_init();
> }
> +
> +/* Set sysctl value passed on kernel command line. */
> +static int process_sysctl_arg(char *param, char *val,
> + const char *unused, void *arg)
> +{
> + char *path;
> + struct vfsmount **proc_mnt = arg;
> + struct file_system_type *proc_fs_type;
> + struct file *file;
> + int len;
> + int err;
> + loff_t pos = 0;
> + ssize_t wret;
> +
> + if (strncmp(param, "sysctl", sizeof("sysctl") - 1))
> + return 0;
> +
> + param += sizeof("sysctl") - 1;
> +
> + if (param[0] != '/' && param[0] != '.')
> + return 0;
> +
> + param++;
> +
> + /*
> + * To set sysctl options, we use a temporary mount of proc, look up the
> + * respective sys/ file and write to it. To avoid mounting it when no
> + * options were given, we mount it only when the first sysctl option is
> + * found. Why not a persistent mount? There are problems with a
> + * persistent mount of proc in that it forces userspace not to use any
> + * proc mount options.
> + */
> + if (!*proc_mnt) {
> + proc_fs_type = get_fs_type("proc");
> + if (!proc_fs_type) {
> + pr_err("Failed to find procfs to set sysctl from command line\n");
> + return 0;
> + }
> + *proc_mnt = kern_mount(proc_fs_type);
> + put_filesystem(proc_fs_type);
> + if (IS_ERR(*proc_mnt)) {
> + pr_err("Failed to mount procfs to set sysctl from command line\n");
> + return 0;
> + }
> + }
> +
> + path = kasprintf(GFP_KERNEL, "sys/%s", param);
> + if (!path)
> + panic("%s: Failed to allocate path for %s\n", __func__, param);
> + strreplace(path, '.', '/');
> +
> + file = file_open_root((*proc_mnt)->mnt_root, *proc_mnt, path, O_WRONLY, 0);
> + if (IS_ERR(file)) {
> + err = PTR_ERR(file);
> + if (err == -ENOENT)
> + pr_err("Failed to set sysctl parameter '%s=%s': parameter not found\n",
> + param, val);
> + else if (err == -EACCES)
> + pr_err("Failed to set sysctl parameter '%s=%s': permission denied (read-only?)\n",
> + param, val);
> + else
> + pr_err("Error %pe opening proc file to set sysctl parameter '%s=%s'\n",
> + file, param, val);
> + goto out;
> + }
> + len = strlen(val);
> + wret = kernel_write(file, val, len, &pos);
> + if (wret < 0) {
> + err = wret;
> + if (err == -EINVAL)
> + pr_err("Failed to set sysctl parameter '%s=%s': invalid value\n",
> + param, val);
> + else
> + pr_err("Error %pe writing to proc file to set sysctl parameter '%s=%s'\n",
> + ERR_PTR(err), param, val);
> + } else if (wret != len) {
> + pr_err("Wrote only %zd bytes of %d writing to proc file %s to set sysctl parameter '%s=%s\n",
> + wret, len, path, param, val);
> + }
> +
> + err = filp_close(file, NULL);
> + if (err)
> + pr_err("Error %pe closing proc file to set sysctl parameter '%s=%s\n",
> + ERR_PTR(err), param, val);
> +out:
> + kfree(path);
> + return 0;
> +}
> +
> +void do_sysctl_args(void)
> +{
> + char *command_line;
> + struct vfsmount *proc_mnt = NULL;
> +
> + command_line = kstrdup(saved_command_line, GFP_KERNEL);
> + if (!command_line)
> + panic("%s: Failed to allocate copy of command line\n", __func__);
> +
> + parse_args("Setting sysctl args", command_line,
> + NULL, 0, -1, -1, &proc_mnt, process_sysctl_arg);
> +
> + if (proc_mnt)
> + kern_unmount(proc_mnt);
> +
> + kfree(command_line);
> +}
> diff --git a/include/linux/sysctl.h b/include/linux/sysctl.h
> index 02fa84493f23..bee62d30ebdb 100644
> --- a/include/linux/sysctl.h
> +++ b/include/linux/sysctl.h
> @@ -206,6 +206,7 @@ struct ctl_table_header *register_sysctl_paths(const struct ctl_path *path,
> void unregister_sysctl_table(struct ctl_table_header * table);
>
> extern int sysctl_init(void);
> +void do_sysctl_args(void);
>
> extern struct ctl_table sysctl_mount_point[];
>
> @@ -236,6 +237,9 @@ static inline void setup_sysctl_set(struct ctl_table_set *p,
> {
> }
>
> +static inline void do_sysctl_args(void)
> +{
> +}
> #endif /* CONFIG_SYSCTL */
>
> int sysctl_max_threads(struct ctl_table *table, int write,
> diff --git a/init/main.c b/init/main.c
> index a48617f2e5e5..7b43118215d6 100644
> --- a/init/main.c
> +++ b/init/main.c
> @@ -1372,6 +1372,8 @@ static int __ref kernel_init(void *unused)
>
> rcu_end_inkernel_boot();
>
> + do_sysctl_args();
> +
> if (ramdisk_execute_command) {
> ret = run_init_process(ramdisk_execute_command);
> if (!ret)
> --
> 2.26.0
>
--
Kees Cook
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