[PATCH v3 1/4] seccomp: Add sysctl to display available actions

[Tyler Hicks]

On 02/15/2017 07:00 PM, Kees Cook wrote:
> On Mon, Feb 13, 2017 at 7:45 PM, Tyler Hicks <tyhicks at canonical.com> wrote:
>> This patch creates a read-only sysctl containing an ordered list of
>> seccomp actions that the kernel supports. The ordering, from left to
>> right, is the lowest action value (kill) to the highest action value
>> (allow). Currently, a read of the sysctl file would return "kill trap
>> errno trace allow". The contents of this sysctl file can be useful for
>> userspace code as well as the system administrator.
>>
>> The path to the sysctl is:
>>
>>   /proc/sys/kernel/seccomp/actions_avail
>>
>> libseccomp and other userspace code can easily determine which actions
>> the current kernel supports. The set of actions supported by the current
>> kernel may be different than the set of action macros found in kernel
>> headers that were installed where the userspace code was built.
>>
>> In addition, this sysctl will allow system administrators to know which
>> actions are supported by the kernel and make it easier to configure
>> exactly what seccomp logs through the audit subsystem. Support for this
>> level of logging configuration will come in a future patch.
>>
>> Signed-off-by: Tyler Hicks <tyhicks at canonical.com>
>> ---
>>  Documentation/prctl/seccomp_filter.txt | 16 ++++++++++
>>  Documentation/sysctl/kernel.txt        |  1 +
>>  kernel/seccomp.c                       | 55 ++++++++++++++++++++++++++++++++++
>>  3 files changed, 72 insertions(+)
>>
>> diff --git a/Documentation/prctl/seccomp_filter.txt b/Documentation/prctl/seccomp_filter.txt
>> index 1e469ef..a5554ff 100644
>> --- a/Documentation/prctl/seccomp_filter.txt
>> +++ b/Documentation/prctl/seccomp_filter.txt
>> @@ -166,7 +166,23 @@ The samples/seccomp/ directory contains both an x86-specific example
>>  and a more generic example of a higher level macro interface for BPF
>>  program generation.
>>
>> +Sysctls
>> +-------
>> +
>> +Seccomp's sysctl files can be found in the /proc/sys/kernel/seccomp/
>> +directory. Here's a description of each file in that directory:
>> +
>> +actions_avail:
>> +       A read-only ordered list of seccomp return values (refer to the
>> +       SECCOMP_RET_* macros above) in string form. The ordering, from
>> +       left-to-right, is the least permissive return value to the most
>> +       permissive return value.
>>
>> +       The list represents the set of seccomp return values supported
>> +       by the kernel. A userspace program may use this list to
>> +       determine if the actions found in the seccomp.h, when the
>> +       program was built, differs from the set of actions actually
>> +       supported in the current running kernel.
>>
>>  Adding architecture support
>>  -----------------------
>> diff --git a/Documentation/sysctl/kernel.txt b/Documentation/sysctl/kernel.txt
>> index a32b4b7..56f9b29 100644
>> --- a/Documentation/sysctl/kernel.txt
>> +++ b/Documentation/sysctl/kernel.txt
>> @@ -74,6 +74,7 @@ show up in /proc/sys/kernel:
>>  - reboot-cmd                  [ SPARC only ]
>>  - rtsig-max
>>  - rtsig-nr
>> +- seccomp/                    ==> Documentation/prctl/seccomp_filter.txt
>>  - sem
>>  - sem_next_id                [ sysv ipc ]
>>  - sg-big-buff                 [ generic SCSI device (sg) ]
>> diff --git a/kernel/seccomp.c b/kernel/seccomp.c
>> index f7ce79a..e36dfe9 100644
>> --- a/kernel/seccomp.c
>> +++ b/kernel/seccomp.c
>> @@ -16,10 +16,12 @@
>>  #include <linux/atomic.h>
>>  #include <linux/audit.h>
>>  #include <linux/compat.h>
>> +#include <linux/kmemleak.h>
>>  #include <linux/sched.h>
>>  #include <linux/seccomp.h>
>>  #include <linux/slab.h>
>>  #include <linux/syscalls.h>
>> +#include <linux/sysctl.h>
>>
>>  #ifdef CONFIG_HAVE_ARCH_SECCOMP_FILTER
>>  #include <asm/syscall.h>
>> @@ -905,3 +907,56 @@ long seccomp_get_filter(struct task_struct *task, unsigned long filter_off,
>>         return ret;
>>  }
>>  #endif
>> +
>> +#ifdef CONFIG_SYSCTL
>> +
>> +/* Human readable action names for friendly sysctl interaction */
>> +#define SECCOMP_RET_KILL_NAME          "kill"
>> +#define SECCOMP_RET_TRAP_NAME          "trap"
>> +#define SECCOMP_RET_ERRNO_NAME         "errno"
>> +#define SECCOMP_RET_TRACE_NAME         "trace"
>> +#define SECCOMP_RET_ALLOW_NAME         "allow"
>> +
>> +static char seccomp_actions_avail[] = SECCOMP_RET_KILL_NAME    " "
>> +                                     SECCOMP_RET_TRAP_NAME     " "
>> +                                     SECCOMP_RET_ERRNO_NAME    " "
>> +                                     SECCOMP_RET_TRACE_NAME    " "
>> +                                     SECCOMP_RET_ALLOW_NAME;
>> +
>> +static struct ctl_path seccomp_sysctl_path[] = {
>> +       { .procname = "kernel", },
>> +       { .procname = "seccomp", },
>> +       { }
>> +};
>> +
>> +static struct ctl_table seccomp_sysctl_table[] = {
>> +       {
>> +               .procname       = "actions_avail",
>> +               .data           = &seccomp_actions_avail,
>> +               .maxlen         = sizeof(seccomp_actions_avail),
>> +               .mode           = 0444,
>> +               .proc_handler   = proc_dostring,
>> +       },
>> +       { }
>> +};
>> +
>> +static int __init seccomp_sysctl_init(void)
>> +{
>> +       struct ctl_table_header *hdr;
>> +
>> +       hdr = register_sysctl_paths(seccomp_sysctl_path, seccomp_sysctl_table);
>> +       if (!hdr)
>> +               pr_warn("seccomp: sysctl registration failed\n");
>> +       else
>> +               kmemleak_not_leak(hdr);
>> +
>> +       return 0;
>> +}
>> +
>> +#else /* CONFIG_SYSCTL */
>> +
>> +static __init int seccomp_sysctl_init(void) { return 0; }
>> +
>> +#endif /* CONFIG_SYSCTL */
>> +
>> +device_initcall(seccomp_sysctl_init)
> 
> Can the device_initcall() just live in the CONFIG_SYSCTL #ifdef to
> avoid the #else and stub?

Yes. That'll be a nice cleanup.

Tyler

> 
> -Kees
> 


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