On 08/03/2017 11:37 AM, Kees Cook wrote: > On Fri, Jul 28, 2017 at 1:55 PM, Tyler Hicks 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 >> --- >> >> * 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 >> #include >> #include >> +#include >> #include >> #include >> #include >> #include >> #include >> +#include >> >> #ifdef CONFIG_HAVE_ARCH_SECCOMP_FILTER >> #include >> @@ -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 >