Messages in this thread | | | Subject | Re: [PATCH v2 1/4] seccomp: Add sysctl to display available actions | From | Tyler Hicks <> | Date | Tue, 7 Feb 2017 18:25:11 -0600 |
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On 02/07/2017 06:03 PM, Kees Cook wrote: > On Thu, Feb 2, 2017 at 9:37 PM, Tyler Hicks <tyhicks@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. > > This is certainly good: having a discoverable way to detect filter > capabilities. I do wonder if it'd still be easier to just expose the > max_log sysctl as a numeric value, since the SECCOMP_RET_* values are > all part of uapi, so we can't escape their values...
I was very torn on whether to use a numeric or string representation here. The reason I decided on string representation is because I think these sysctls are mostly aimed for admins and numeric representations wouldn't be easy to use. I considered added a utility to libseccomp but, since the kernel code to do a string representation was so simple, I went with doing it in the kernel.
Another possibility is exposing the SECCOMP_RET_*_NAME macros as part of the uapi.
> > > >> >> 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@canonical.com> >> --- >> kernel/seccomp.c | 50 ++++++++++++++++++++++++++++++++++++++++++++++++++ >> 1 file changed, 50 insertions(+) >> >> diff --git a/kernel/seccomp.c b/kernel/seccomp.c >> index f7ce79a..919ad9f 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 +907,51 @@ long seccomp_get_filter(struct task_struct *task, unsigned long filter_off, >> return ret; >> } >> #endif >> + >> +#ifdef CONFIG_SYSCTL >> + >> +#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); >> + kmemleak_not_leak(hdr); > > Will kmemleak complain about this if hdr is saved to a global (or not > saved at all)? Also, something should be reported in the failure > case...
I have to admit to blindly following the example set by sysctl_init() in kernel/sysctl.c. I can test what kmemleak will/won't complain about and report back (tomorrow at the earliest).
Tyler
> >> + return 0; >> +} >> + >> +#else /* CONFIG_SYSCTL */ >> + >> +static __init int seccomp_sysctl_init(void) { return 0; } >> + >> +#endif /* CONFIG_SYSCTL */ >> + >> +device_initcall(seccomp_sysctl_init) >> -- >> 2.7.4 >> > > -Kees >
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