Re: [PATCH v5 1/6] seccomp: Sysctl to display available actions

From: Tyler Hicks
Date: Thu Aug 03 2017 - 20:46:52 EST

On 08/03/2017 11:37 AM, Kees Cook wrote:
> On Fri, Jul 28, 2017 at 1:55 PM, Tyler Hicks <tyhicks@xxxxxxxxxxxxx> 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@xxxxxxxxxxxxx>
>> ---
>>
>> * Changes since v4:
>> - move device_initcall() into CONFIG_SYSCTL ifdef
>> - mark the seccomp_actions_avail string as const
>> - adjust for new reStructuredText format
>>
>> Documentation/sysctl/kernel.txt | 1 +
>> Documentation/userspace-api/seccomp_filter.rst | 16 ++++++++
>> kernel/seccomp.c | 51 ++++++++++++++++++++++++++
>> 3 files changed, 68 insertions(+)
>>
>> diff --git a/Documentation/sysctl/kernel.txt b/Documentation/sysctl/kernel.txt
>> index bac23c1..995c42c 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/userspace-api/seccomp_filter.rst
>> - sem
>> - sem_next_id [ sysv ipc ]
>> - sg-big-buff [ generic SCSI device (sg) ]
>> diff --git a/Documentation/userspace-api/seccomp_filter.rst b/Documentation/userspace-api/seccomp_filter.rst
>> index f71eb5e..35fc7cb 100644
>> --- a/Documentation/userspace-api/seccomp_filter.rst
>> +++ b/Documentation/userspace-api/seccomp_filter.rst
>> @@ -169,7 +169,23 @@ 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/kernel/seccomp.c b/kernel/seccomp.c
>> index 98b59b5..6bff068 100644
>> --- a/kernel/seccomp.c
>> +++ b/kernel/seccomp.c
>> @@ -17,11 +17,13 @@
>> #include <linux/audit.h>
>> #include <linux/compat.h>
>> #include <linux/coredump.h>
>> +#include <linux/kmemleak.h>
>> #include <linux/sched.h>
>> #include <linux/sched/task_stack.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>
>> @@ -922,3 +924,52 @@ 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 const 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 = (void *) &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;
>> +}
>> +
>> +device_initcall(seccomp_sysctl_init)
>> +
>> +#endif /* CONFIG_SYSCTL */
>
> Looks good. And for the record, the BPF return values, while not
> checked in seccomp_check_filter(), are part of ABI and the kernel will
> behave differently for unexpected values. For example, an older kernel
> encountering the future SECCOMP_RET_LOG will treat it as
> SECCOMP_RET_KILL since it's missing from the switch statement in
> __seccomp_filter().
>
> A question about patch ordering: should the new seccomp action
> introspection patch maybe follow this one, so they're together in the
> series (they provide the same information)?

That would be fine. I'll move it to patch #2.

Tyler

>
> -Kees
>


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