Messages in this thread | | | Subject | Re: [PATCH v2 3/4] seccomp: Create an action to log before allowing | From | Tyler Hicks <> | Date | Fri, 10 Feb 2017 18:01:07 -0600 |
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On 02/07/2017 06:33 PM, Kees Cook wrote: > On Thu, Feb 2, 2017 at 9:37 PM, Tyler Hicks <tyhicks@canonical.com> wrote: >> Add a new action, SECCOMP_RET_LOG, that logs a syscall before allowing >> the syscall. At the implementation level, this action is identical to >> the existing SECCOMP_RET_ALLOW action. However, it can be very useful when >> initially developing a seccomp filter for an application. The developer >> can set the default action to be SECCOMP_RET_LOG, maybe mark any >> obviously needed syscalls with SECCOMP_RET_ALLOW, and then put the >> application through its paces. A list of syscalls that triggered the >> default action (SECCOMP_RET_LOG) can be easily gleaned from the logs and >> that list can be used to build the syscall whitelist. Finally, the >> developer can change the default action to the desired value. >> >> This provides a more friendly experience than seeing the application get >> killed, then updating the filter and rebuilding the app, seeing the >> application get killed due to a different syscall, then updating the >> filter and rebuilding the app, etc. >> >> The functionality is similar to what's supported by the various LSMs. >> SELinux has permissive mode, AppArmor has complain mode, SMACK has >> bring-up mode, etc. >> >> SECCOMP_RET_LOG is given a lower value than SECCOMP_RET_ALLOW so that >> "allow" can be written to the max_action_to_log sysctl in order to get a >> list of logged actions without the, potentially larger, set of allowed >> actions. >> >> Signed-off-by: Tyler Hicks <tyhicks@canonical.com> >> --- >> Documentation/prctl/seccomp_filter.txt | 6 ++++++ >> include/uapi/linux/seccomp.h | 1 + >> kernel/seccomp.c | 4 ++++ >> 3 files changed, 11 insertions(+) >> >> diff --git a/Documentation/prctl/seccomp_filter.txt b/Documentation/prctl/seccomp_filter.txt >> index 1e469ef..ba55a91 100644 >> --- a/Documentation/prctl/seccomp_filter.txt >> +++ b/Documentation/prctl/seccomp_filter.txt >> @@ -138,6 +138,12 @@ SECCOMP_RET_TRACE: >> allow use of ptrace, even of other sandboxed processes, without >> extreme care; ptracers can use this mechanism to escape.) >> >> +SECCOMP_RET_LOG: >> + Results in the system call being executed after it is logged. This >> + should be used by application developers to learn which syscalls their >> + application needs without having to iterate through multiple test and >> + development cycles to build the list. >> + >> SECCOMP_RET_ALLOW: >> Results in the system call being executed. >> >> diff --git a/include/uapi/linux/seccomp.h b/include/uapi/linux/seccomp.h >> index 0f238a4..67f72cd 100644 >> --- a/include/uapi/linux/seccomp.h >> +++ b/include/uapi/linux/seccomp.h >> @@ -29,6 +29,7 @@ >> #define SECCOMP_RET_TRAP 0x00030000U /* disallow and force a SIGSYS */ >> #define SECCOMP_RET_ERRNO 0x00050000U /* returns an errno */ >> #define SECCOMP_RET_TRACE 0x7ff00000U /* pass to a tracer or disallow */ >> +#define SECCOMP_RET_LOG 0x7ffe0000U /* allow after logging */ > > This adds to UAPI, so it'd be good to think for a moment about how > this would work on older kernels: right now, if someone tried to use > this RET_LOG on an old kernel, it'll get treated like RET_KILL. Is > this sane?
It is not sane for userspace code to blindly attempt to use a new feature on an old kernel. One of the main motivations of the actions_avail sysctl is to allow userspace to be smart about what the current kernel supports.
I'll be adding logic (requested by Paul) to libseccomp that checks this sysctl when SECOMP_RET_LOG is attempted to be used. Programs that don't use libseccomp will have to do something similar.
> > I'm also trying to figure out if there is some other solution to this, > but they all involve tests against an otherwise RET_ALLOW case, which > I want to avoid. :) > > So, I think, for now, this looks good, but I'd prefer this be > 0x7ffc0000U, just to make sure we have not painted ourselves into a > numerical corner if we for some reason ever need to put something > between RET_ALLOW and RET_LOG.
That makes sense. I'll do that in v3. > >> #define SECCOMP_RET_ALLOW 0x7fff0000U /* allow */ >> >> /* Masks for the return value sections. */ >> diff --git a/kernel/seccomp.c b/kernel/seccomp.c >> index 548fb89..8627481 100644 >> --- a/kernel/seccomp.c >> +++ b/kernel/seccomp.c >> @@ -650,6 +650,7 @@ static int __seccomp_filter(int this_syscall, const struct seccomp_data *sd, >> >> return 0; >> >> + case SECCOMP_RET_LOG: > > Given my protective feelings about the RET_ALLOW case, can you make > this a fully separate case statement? I'd rather have RET_ALLOW be > distinctly separate.
Sure! It actually has to be two different cases now that we're doing the hot path approach for SECCOMP_RET_ALLOW.
Tyler
> >> case SECCOMP_RET_ALLOW: >> seccomp_log(this_syscall, 0, action); >> return 0; >> @@ -934,6 +935,7 @@ long seccomp_get_filter(struct task_struct *task, unsigned long filter_off, >> #define SECCOMP_RET_TRAP_NAME "trap" >> #define SECCOMP_RET_ERRNO_NAME "errno" >> #define SECCOMP_RET_TRACE_NAME "trace" >> +#define SECCOMP_RET_LOG_NAME "log" >> #define SECCOMP_RET_ALLOW_NAME "allow" >> >> /* Largest strlen() of all action names */ >> @@ -943,6 +945,7 @@ static char seccomp_actions_avail[] = SECCOMP_RET_KILL_NAME " " >> SECCOMP_RET_TRAP_NAME " " >> SECCOMP_RET_ERRNO_NAME " " >> SECCOMP_RET_TRACE_NAME " " >> + SECCOMP_RET_LOG_NAME " " >> SECCOMP_RET_ALLOW_NAME; >> >> struct seccomp_action_name { >> @@ -955,6 +958,7 @@ static struct seccomp_action_name seccomp_action_names[] = { >> { SECCOMP_RET_TRAP, SECCOMP_RET_TRAP_NAME }, >> { SECCOMP_RET_ERRNO, SECCOMP_RET_ERRNO_NAME }, >> { SECCOMP_RET_TRACE, SECCOMP_RET_TRACE_NAME }, >> + { SECCOMP_RET_LOG, SECCOMP_RET_LOG_NAME }, >> { SECCOMP_RET_ALLOW, SECCOMP_RET_ALLOW_NAME }, >> { } >> }; >> -- >> 2.7.4 >> > > -Kees >
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