Re: [PATCH] audit: Don't spam logs with SECCOMP_KILL/RET_ERRNO by default

[Eric Paris <eparis@redhat.com>]

Just an FYI originally the idea was to follow the pattern of logging
set by core dumps see kernel/auditsc.c::audit_core_dumps(). Which is
gated by audit_enable but not anything else. I believe at that time the
only option was kill, which meant, much like the core dumper, spam was
not a likely result given the initiator is killed.

I'm all for a way to shut up unsolicited audit messages, especially
seccomp with errno or trap. I think it would be best to default 'KILL'
to on and everything else to off. I'm no so sure a sysctl is the right
way though. Enabling more forms of 'seccomp audit' should really be a
part of the audit policy.

(p.s. I think the action should be part of the seccomp message, as
right now all we know is that Andi's message isn't KILL since the
sig=0)

-Eric


On Mon, 2016-04-11 at 09:30 -0400, Paul Moore wrote:
> On Mon, Apr 11, 2016 at 12:13 AM, Andi Kleen <andi@firstfloor.org>
> wrote:
> > 
> > From: Andi Kleen <ak@linux.intel.com>
> > 
> > When I run chrome on my opensuse system every time I open
> > a new tab the system log is spammed with:
> > 
> > audit[16857]: SECCOMP auid=1000 uid=1000 gid=100 ses=1 pid=16857
> > comm="chrome" exe="/opt/google/chrome/chrome" sig=0 arch=c000003e
> > syscall=273 compat=0 ip=0x7fe27c11a444 code=0x50000
> > 
> > This happens because chrome uses SECCOMP for its sandbox,
> > and for some reason always reaches a SECCOMP_KILL or more likely
> > SECCOMP_RET_ERRNO in the rule set.
> > 
> > The seccomp auditing was originally added by Eric with
> > 
> >     commit 85e7bac33b8d5edafc4e219c7dfdb3d48e0b4e31
> >     Author: Eric Paris <eparis@redhat.com>
> >     Date:   Tue Jan 3 14:23:05 2012 -0500
> > 
> >         seccomp: audit abnormal end to a process due to seccomp
> > 
> >         The audit system likes to collect information about
> > processes that end
> >         abnormally (SIGSEGV) as this may me useful intrusion
> > detection information.
> >         This patch adds audit support to collect information when
> > seccomp
> >         forces a task to exit because of misbehavior in a similar
> > way.
> > 
> > I don't have any other syscall auditing enabled,
> > just the standard user space auditing used by the systemd
> > and PAM userland. So basic auditing is alwas enabled,
> > but no other kernel auditing.
> > 
> > Add a sysctl to enable this unconditional behavior with default
> > to off. This replaces an earlier patch that simply checked
> > whether syscall auditing was on, but Paul Moore preferred
> > this more elaborate approach.
> > 
> > Signed-off-by: Andi Kleen <ak@linux.intel.com>
> > ---
> >  Documentation/sysctl/kernel.txt |  9 +++++++++
> >  include/linux/audit.h           |  4 +++-
> >  kernel/seccomp.c                |  4 ++++
> >  kernel/sysctl.c                 | 11 +++++++++++
> >  4 files changed, 27 insertions(+), 1 deletion(-)
> Quick response as I'm traveling the next few days and
> time/connectivity will be spotty ... thanks for sending an updated
> patch, some initial thoughts:
> 
> * My thinking was that the sysctl knob could be a threshold value
> such
> that setting it to 0x00030000 would only log TRAP and KILL.
> * With the sysctl tunable defaulting to no-logging there is no need
> to
> check for audit_enabled, further, checking for audit_enabled would
> prevent logging to dmesg/syslog which I believe is valuable (you may
> not).
> * A bit nitpicky, but considering the possibility of logging to
> dmesg/syslog when auditing is disabled, I think
> "seccomp-log-threshold" or similar would be a better sysctl name.
> 
> > 
> > diff --git a/Documentation/sysctl/kernel.txt
> > b/Documentation/sysctl/kernel.txt
> > index 57653a4..abc6ef9 100644
> > --- a/Documentation/sysctl/kernel.txt
> > +++ b/Documentation/sysctl/kernel.txt
> > @@ -21,6 +21,7 @@ show up in /proc/sys/kernel:
> >  - acct
> >  - acpi_video_flags
> >  - auto_msgmni
> > +- audit_log_seccomp
> >  - bootloader_type           [ X86 only ]
> >  - bootloader_version        [ X86 only ]
> >  - callhome                  [ S390 only ]
> > @@ -129,6 +130,14 @@ upon memory add/remove or upon ipc namespace
> > creation/removal.
> >  Echoing "1" into this file enabled msgmni automatic recomputing.
> >  Echoing "0" turned it off. auto_msgmni default value was 1.
> > 
> > +==============================================================
> > +
> > +audit_log_seccomp
> > +
> > +When this variable is set to 1 every
> > SECCOMP_KILL/SECCOMP_RET_ERRNO
> > +results in an audit log. This is generally a bad idea because
> > +it leads to a audit message every time Chrome opens a new tab.
> > +Defaults to 0.
> > 
> >  ==============================================================
> > 
> > diff --git a/include/linux/audit.h b/include/linux/audit.h
> > index e38e3fc..c7787ba 100644
> > --- a/include/linux/audit.h
> > +++ b/include/linux/audit.h
> > @@ -315,9 +315,11 @@ static inline void audit_inode_child(struct
> > inode *parent,
> >  }
> >  void audit_core_dumps(long signr);
> > 
> > +extern int audit_log_seccomp;
> > +
> >  static inline void audit_seccomp(unsigned long syscall, long
> > signr, int code)
> >  {
> > -       if (!audit_enabled)
> > +       if (!audit_enabled || !audit_log_seccomp)
> >                 return;
> > 
> >         /* Force a record to be reported if a signal was delivered.
> > */
> > diff --git a/kernel/seccomp.c b/kernel/seccomp.c
> > index e1e5a35..09a8b03 100644
> > --- a/kernel/seccomp.c
> > +++ b/kernel/seccomp.c
> > @@ -25,6 +25,10 @@
> >  #include <asm/syscall.h>
> >  #endif
> > 
> > +#ifdef CONFIG_AUDIT
> > +int audit_log_seccomp __read_mostly = 0;
> > +#endif
> > +
> >  #ifdef CONFIG_SECCOMP_FILTER
> >  #include <linux/filter.h>
> >  #include <linux/pid.h>
> > diff --git a/kernel/sysctl.c b/kernel/sysctl.c
> > index 725587f..0c7611e 100644
> > --- a/kernel/sysctl.c
> > +++ b/kernel/sysctl.c
> > @@ -65,6 +65,7 @@
> >  #include <linux/sched/sysctl.h>
> >  #include <linux/kexec.h>
> >  #include <linux/bpf.h>
> > +#include <linux/audit.h>
> > 
> >  #include <asm/uaccess.h>
> >  #include <asm/processor.h>
> > @@ -529,6 +530,16 @@ static struct ctl_table kern_table[] = {
> >                 .proc_handler   = proc_dointvec,
> >         },
> >  #endif
> > +#ifdef CONFIG_AUDIT
> > +       {
> > +               .procname       = "audit-log-seccomp",
> > +               .data           = &audit_log_seccomp,
> > +               .maxlen         = sizeof(int),
> > +               .mode           = 0644,
> > +               .proc_handler   = proc_dointvec,
> > +       },
> > +
> > +#endif
> >         {
> >                 .procname       = "print-fatal-signals",
> >                 .data           = &print_fatal_signals,
> > --
> > 2.7.4
> > 
> 
>