[Qemu-devel] [PATCH v4 0/4] seccomp fixes

[Qemu-devel] [PATCH v4 0/4] seccomp fixes

[Marc-André Lureau]

Hi,

This series fixes 2 issues with -sandbox:

- The seccomp action SCMP_ACT_KILL results in immediate termination of
  the thread that made the bad system call. However, qemu being
  multi-threaded, it keeps running. There is no easy way for parent
  process / management layer (libvirt) to know about that situation.

  Instead, the default SIGSYS handler when invoked with SCMP_ACT_TRAP
  will terminate the program and core dump.

  This may not be the most secure solution, but probably better than
  just killing the offending thread. SCMP_ACT_KILL_PROCESS has been
  added in Linux 4.14 to improve the situation, which I propose to use
  by default if available.

  Related to:
  https://bugzilla.redhat.com/show_bug.cgi?id=1594456

- The seccomp filter isn't applied to all threads. We can solve the
  issue by using SECCOMP_FILTER_FLAG_TSYNC since libseccomp 2.2.0 and
  kernel >= 3.17.

v3:
- modify qemu_seccomp() to set errno=ENOSYS
- add patch "seccomp: set the seccomp filter to all threads"

v2:
- fix clang unused inline warning
- add acked-by/r-b tags

Marc-André Lureau (4):
  seccomp: use SIGSYS signal instead of killing the thread
  seccomp: prefer SCMP_ACT_KILL_PROCESS if available
  configure: require libseccomp 2.2.0
  seccomp: set the seccomp filter to all threads

 qemu-seccomp.c | 36 +++++++++++++++++++++++++++++++++++-
 configure      |  7 ++-----
 2 files changed, 37 insertions(+), 6 deletions(-)

-- 
2.18.0.547.g1d89318c48

[Qemu-devel] [PATCH v4 1/4] seccomp: use SIGSYS signal instead of killing the thread

[Marc-André Lureau]

The seccomp action SCMP_ACT_KILL results in immediate termination of
the thread that made the bad system call. However, qemu being
multi-threaded, it keeps running. There is no easy way for parent
process / management layer (libvirt) to know about that situation.

Instead, the default SIGSYS handler when invoked with SCMP_ACT_TRAP
will terminate the program and core dump.

This may not be the most secure solution, but probably better than
just killing the offending thread. SCMP_ACT_KILL_PROCESS has been
added in Linux 4.14 to improve the situation, which I propose to use
by default if available in the next patch.

Related to:
https://bugzilla.redhat.com/show_bug.cgi?id=1594456

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Acked-by: Eduardo Otubo <otubo@redhat.com>
---
 qemu-seccomp.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/qemu-seccomp.c b/qemu-seccomp.c
index 9cd8eb9499..b117a92559 100644
--- a/qemu-seccomp.c
+++ b/qemu-seccomp.c
@@ -125,7 +125,7 @@ static int seccomp_start(uint32_t seccomp_opts)
             continue;
         }
 
-        rc = seccomp_rule_add_array(ctx, SCMP_ACT_KILL, blacklist[i].num,
+        rc = seccomp_rule_add_array(ctx, SCMP_ACT_TRAP, blacklist[i].num,
                                     blacklist[i].narg, blacklist[i].arg_cmp);
         if (rc < 0) {
             goto seccomp_return;
-- 
2.18.0.547.g1d89318c48

[Qemu-devel] [PATCH v4 2/4] seccomp: prefer SCMP_ACT_KILL_PROCESS if available

[Marc-André Lureau]

The upcoming libseccomp release should have SCMP_ACT_KILL_PROCESS
action (https://github.com/seccomp/libseccomp/issues/96).

SCMP_ACT_KILL_PROCESS is preferable to immediately terminate the
offending process, rather than having the SIGSYS handler running.

Use SECCOMP_GET_ACTION_AVAIL to check availability of kernel support,
as libseccomp will fallback on SCMP_ACT_KILL otherwise, and we still
prefer SCMP_ACT_TRAP.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Acked-by: Eduardo Otubo <otubo@redhat.com>
---
 qemu-seccomp.c | 31 ++++++++++++++++++++++++++++++-
 1 file changed, 30 insertions(+), 1 deletion(-)

diff --git a/qemu-seccomp.c b/qemu-seccomp.c
index b117a92559..f0c833f3ca 100644
--- a/qemu-seccomp.c
+++ b/qemu-seccomp.c
@@ -20,6 +20,7 @@
 #include <sys/prctl.h>
 #include <seccomp.h>
 #include "sysemu/seccomp.h"
+#include <linux/seccomp.h>
 
 /* For some architectures (notably ARM) cacheflush is not supported until
  * libseccomp 2.2.3, but configure enforces that we are using a more recent
@@ -107,12 +108,40 @@ static const struct QemuSeccompSyscall blacklist[] = {
     { SCMP_SYS(sched_get_priority_min), QEMU_SECCOMP_SET_RESOURCECTL },
 };
 
+static inline __attribute__((unused)) int
+qemu_seccomp(unsigned int operation, unsigned int flags, void *args)
+{
+#ifdef __NR_seccomp
+    return syscall(__NR_seccomp, operation, flags, args);
+#else
+    errno = ENOSYS;
+    return -1;
+#endif
+}
+
+static uint32_t qemu_seccomp_get_kill_action(void)
+{
+#if defined(SECCOMP_GET_ACTION_AVAIL) && defined(SCMP_ACT_KILL_PROCESS) && \
+    defined(SECCOMP_RET_KILL_PROCESS)
+    {
+        uint32_t action = SECCOMP_RET_KILL_PROCESS;
+
+        if (qemu_seccomp(SECCOMP_GET_ACTION_AVAIL, 0, &action) == 0) {
+            return SCMP_ACT_KILL_PROCESS;
+        }
+    }
+#endif
+
+    return SCMP_ACT_TRAP;
+}
+
 
 static int seccomp_start(uint32_t seccomp_opts)
 {
     int rc = 0;
     unsigned int i = 0;
     scmp_filter_ctx ctx;
+    uint32_t action = qemu_seccomp_get_kill_action();
 
     ctx = seccomp_init(SCMP_ACT_ALLOW);
     if (ctx == NULL) {
@@ -125,7 +154,7 @@ static int seccomp_start(uint32_t seccomp_opts)
             continue;
         }
 
-        rc = seccomp_rule_add_array(ctx, SCMP_ACT_TRAP, blacklist[i].num,
+        rc = seccomp_rule_add_array(ctx, action, blacklist[i].num,
                                     blacklist[i].narg, blacklist[i].arg_cmp);
         if (rc < 0) {
             goto seccomp_return;
-- 
2.18.0.547.g1d89318c48

[Qemu-devel] [PATCH v4 3/4] configure: require libseccomp 2.2.0

[Marc-André Lureau]

The following patch is going to require TSYNC, which is only available
since libseccomp 2.2.0.

libseccomp 2.2.0 was released February 12, 2015.

According to repology, libseccomp version in different distros:

  RHEL-7: 2.3.1
  Debian (Stretch): 2.3.1
  OpenSUSE Leap 15: 2.3.2
  Ubuntu (Xenial):  2.3.1

This will drop support for -sandbox on:

  Debian (Jessie): 2.1.1 (but 2.2.3 in backports)

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
---
 configure | 7 ++-----
 1 file changed, 2 insertions(+), 5 deletions(-)

diff --git a/configure b/configure
index e7bddc04b0..5fc2915096 100755
--- a/configure
+++ b/configure
@@ -2228,13 +2228,10 @@ fi
 ##########################################
 # libseccomp check
 
+libseccomp_minver="2.2.0"
 if test "$seccomp" != "no" ; then
     case "$cpu" in
-    i386|x86_64)
-        libseccomp_minver="2.1.0"
-        ;;
-    mips)
-        libseccomp_minver="2.2.0"
+    i386|x86_64|mips)
         ;;
     arm|aarch64)
         libseccomp_minver="2.2.3"
-- 
2.18.0.547.g1d89318c48

Re: [Qemu-devel] [PATCH v4 3/4] configure: require libseccomp 2.2.0

[Daniel P. Berrangé]

On Wed, Aug 22, 2018 at 07:02:49PM +0200, Marc-André Lureau wrote:
> The following patch is going to require TSYNC, which is only available
> since libseccomp 2.2.0.
> 
> libseccomp 2.2.0 was released February 12, 2015.
> 
> According to repology, libseccomp version in different distros:
> 
>   RHEL-7: 2.3.1
>   Debian (Stretch): 2.3.1
>   OpenSUSE Leap 15: 2.3.2
>   Ubuntu (Xenial):  2.3.1
> 
> This will drop support for -sandbox on:
> 
>   Debian (Jessie): 2.1.1 (but 2.2.3 in backports)
>

Diverging slightly from our normal distro target set, but
acceptable given that this is a security feature which is
impossible to correctly implement with older versions.

> Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
> ---
>  configure | 7 ++-----
>  1 file changed, 2 insertions(+), 5 deletions(-)

Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>


Regards,
Daniel
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[Qemu-devel] [PATCH v4 4/4] seccomp: set the seccomp filter to all threads

[Marc-André Lureau]

When using "-seccomp on", the seccomp policy is only applied to the
main thread, the vcpu worker thread and other worker threads created
after seccomp policy is applied; the seccomp policy is not applied to
e.g. the RCU thread because it is created before the seccomp policy is
applied and SECCOMP_FILTER_FLAG_TSYNC isn't used.

This can be verified with
for task in /proc/`pidof qemu`/task/*; do cat $task/status | grep Secc ; done
Seccomp:	2
Seccomp:	0
Seccomp:	0
Seccomp:	2
Seccomp:	2
Seccomp:	2

Starting with libseccomp 2.2.0 and kernel >= 3.17, we can use
seccomp_attr_set(ctx, > SCMP_FLTATR_CTL_TSYNC, 1) to update the policy
on all threads.

libseccomp requirement was bumped to 2.2.0 in previous patch.
libseccomp should fail to set the filter if it can't honour
SCMP_FLTATR_CTL_TSYNC (untested), and thus -sandbox will now fail on
kernel < 3.17.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
---
 qemu-seccomp.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/qemu-seccomp.c b/qemu-seccomp.c
index f0c833f3ca..4729eb107f 100644
--- a/qemu-seccomp.c
+++ b/qemu-seccomp.c
@@ -149,6 +149,11 @@ static int seccomp_start(uint32_t seccomp_opts)
         goto seccomp_return;
     }
 
+    rc = seccomp_attr_set(ctx, SCMP_FLTATR_CTL_TSYNC, 1);
+    if (rc != 0) {
+        goto seccomp_return;
+    }
+
     for (i = 0; i < ARRAY_SIZE(blacklist); i++) {
         if (!(seccomp_opts & blacklist[i].set)) {
             continue;
-- 
2.18.0.547.g1d89318c48

Re: [Qemu-devel] [PATCH v4 4/4] seccomp: set the seccomp filter to all threads

[Daniel P. Berrangé]

On Wed, Aug 22, 2018 at 07:02:50PM +0200, Marc-André Lureau wrote:
> When using "-seccomp on", the seccomp policy is only applied to the
> main thread, the vcpu worker thread and other worker threads created
> after seccomp policy is applied; the seccomp policy is not applied to
> e.g. the RCU thread because it is created before the seccomp policy is
> applied and SECCOMP_FILTER_FLAG_TSYNC isn't used.
> 
> This can be verified with
> for task in /proc/`pidof qemu`/task/*; do cat $task/status | grep Secc ; done
> Seccomp:	2
> Seccomp:	0
> Seccomp:	0
> Seccomp:	2
> Seccomp:	2
> Seccomp:	2
> 
> Starting with libseccomp 2.2.0 and kernel >= 3.17, we can use
> seccomp_attr_set(ctx, > SCMP_FLTATR_CTL_TSYNC, 1) to update the policy
> on all threads.
> 
> libseccomp requirement was bumped to 2.2.0 in previous patch.
> libseccomp should fail to set the filter if it can't honour
> SCMP_FLTATR_CTL_TSYNC (untested), and thus -sandbox will now fail on
> kernel < 3.17.
> 
> Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
> ---
>  qemu-seccomp.c | 5 +++++
>  1 file changed, 5 insertions(+)

Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>

Regards,
Daniel
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Re: [Qemu-devel] [PATCH v4 0/4] seccomp fixes

[Eduardo Otubo]

[-- Attachment #1: Type: text/plain, Size: 1694 bytes --]

On 22/08/2018 - 19:02:46, Marc-André Lureau wrote:
> Hi,
> 
> This series fixes 2 issues with -sandbox:
> 
> - The seccomp action SCMP_ACT_KILL results in immediate termination of
>   the thread that made the bad system call. However, qemu being
>   multi-threaded, it keeps running. There is no easy way for parent
>   process / management layer (libvirt) to know about that situation.
> 
>   Instead, the default SIGSYS handler when invoked with SCMP_ACT_TRAP
>   will terminate the program and core dump.
> 
>   This may not be the most secure solution, but probably better than
>   just killing the offending thread. SCMP_ACT_KILL_PROCESS has been
>   added in Linux 4.14 to improve the situation, which I propose to use
>   by default if available.
> 
>   Related to:
>   https://bugzilla.redhat.com/show_bug.cgi?id=1594456
> 
> - The seccomp filter isn't applied to all threads. We can solve the
>   issue by using SECCOMP_FILTER_FLAG_TSYNC since libseccomp 2.2.0 and
>   kernel >= 3.17.
> 
> v3:
> - modify qemu_seccomp() to set errno=ENOSYS
> - add patch "seccomp: set the seccomp filter to all threads"
> 
> v2:
> - fix clang unused inline warning
> - add acked-by/r-b tags
> 
> Marc-André Lureau (4):
>   seccomp: use SIGSYS signal instead of killing the thread
>   seccomp: prefer SCMP_ACT_KILL_PROCESS if available
>   configure: require libseccomp 2.2.0
>   seccomp: set the seccomp filter to all threads
> 
>  qemu-seccomp.c | 36 +++++++++++++++++++++++++++++++++++-
>  configure      |  7 ++-----
>  2 files changed, 37 insertions(+), 6 deletions(-)
> 
> -- 
> 2.18.0.547.g1d89318c48
> 
Acked-by: Eduardo Otubo <otubo@redhat.com>

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