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

[Paul Moore]

On Mon, Apr 11, 2016 at 12:13 AM, Andi Kleen <andi@xxxxxxxxxxxxxx> wrote:
> From: Andi Kleen <ak@xxxxxxxxxxxxxxx>
>
> 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@xxxxxxxxxx>
>     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@xxxxxxxxxxxxxxx>
> ---
>  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
>



-- 
paul moore
www.paul-moore.com