[PATCH v1 4/4] um: Add seccomp support

Mickaël Salaün
2015-12-21T00:18:49Z

This brings SECCOMP_MODE_STRICT and SECCOMP_MODE_FILTER support through
prctl(2) and seccomp(2) to User-mode Linux for i386 and x86_64
subarchitectures.

secure_computing() is called first in handle_syscall() so that the
syscall emulation will be aborted quickly if matching a seccomp rule.

This is inspired from Meredydd Luff's patch
(https://gerrit.chromium.org/gerrit/21425).

Signed-off-by: Mickaël Salaün <mic@digikod.net>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Jeff Dike <jdike@addtoit.com>
Cc: Richard Weinberger <richard@nod.at>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Kees Cook <keescook@chromium.org>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Will Drewry <wad@chromium.org>
Cc: Chris Metcalf <cmetcalf@ezchip.com>
Cc: Michael Ellerman <mpe@ellerman.id.au>
Cc: James Hogan <james.hogan@imgtec.com>
Cc: linux-doc@vger.kernel.org
Cc: linux-kernel@vger.kernel.org
Cc: user-mode-linux-devel@lists.sourceforge.net
Cc: user-mode-linux-user@lists.sourceforge.net
Cc: Meredydd Luff <meredydd@senatehouse.org>
Cc: David Drysdale <drysdale@google.com>
---
.../features/seccomp/seccomp-filter/arch-support.txt | 2 +-
arch/um/Kconfig.common | 1 +
arch/um/Kconfig.um | 16 ++++++++++++++++
arch/um/include/asm/thread_info.h | 2 ++
arch/um/kernel/skas/syscall.c | 9 +++++++++
5 files changed, 29 insertions(+), 1 deletion(-)

diff --git a/Documentation/features/seccomp/seccomp-filter/arch-support.txt b/Documentation/features/seccomp/seccomp-filter/arch-support.txt
index 76d39d6..4f66ec1 100644
--- a/Documentation/features/seccomp/seccomp-filter/arch-support.txt
+++ b/Documentation/features/seccomp/seccomp-filter/arch-support.txt
@@ -33,7 +33,7 @@
| sh: | TODO |
| sparc: | TODO |
| tile: | ok |
- | um: | TODO |
+ | um: | ok |
| unicore32: | TODO |
| x86: | ok |
| xtensa: | TODO |
diff --git a/arch/um/Kconfig.common b/arch/um/Kconfig.common
index d195a87..cc00134 100644
--- a/arch/um/Kconfig.common
+++ b/arch/um/Kconfig.common
@@ -2,6 +2,7 @@ config UML
bool
default y
select HAVE_ARCH_AUDITSYSCALL
+ select HAVE_ARCH_SECCOMP_FILTER
select HAVE_UID16
select HAVE_FUTEX_CMPXCHG if FUTEX
select GENERIC_IRQ_SHOW
diff --git a/arch/um/Kconfig.um b/arch/um/Kconfig.um
index 28a9885..4b2ed58 100644
--- a/arch/um/Kconfig.um
+++ b/arch/um/Kconfig.um
@@ -104,3 +104,19 @@ config PGTABLE_LEVELS
int
default 3 if 3_LEVEL_PGTABLES
default 2
+
+config SECCOMP
+ def_bool y
+ prompt "Enable seccomp to safely compute untrusted bytecode"
+ ---help---
+ This kernel feature is useful for number crunching applications
+ that may need to compute untrusted bytecode during their
+ execution. By using pipes or other transports made available to
+ the process as file descriptors supporting the read/write
+ syscalls, it's possible to isolate those applications in
+ their own address space using seccomp. Once seccomp is
+ enabled via prctl(PR_SET_SECCOMP), it cannot be disabled
+ and the task is only allowed to execute a few safe syscalls
+ defined by each seccomp mode.
+
+ If unsure, say Y.
diff --git a/arch/um/include/asm/thread_info.h b/arch/um/include/asm/thread_info.h
index 53968aa..053baff 100644
--- a/arch/um/include/asm/thread_info.h
+++ b/arch/um/include/asm/thread_info.h
@@ -62,11 +62,13 @@ static inline struct thread_info *current_thread_info(void)
#define TIF_SYSCALL_AUDIT 6
#define TIF_RESTORE_SIGMASK 7
#define TIF_NOTIFY_RESUME 8
+#define TIF_SECCOMP 9 /* secure computing */

#define _TIF_SYSCALL_TRACE (1 << TIF_SYSCALL_TRACE)
#define _TIF_SIGPENDING (1 << TIF_SIGPENDING)
#define _TIF_NEED_RESCHED (1 << TIF_NEED_RESCHED)
#define _TIF_MEMDIE (1 << TIF_MEMDIE)
#define _TIF_SYSCALL_AUDIT (1 << TIF_SYSCALL_AUDIT)
+#define _TIF_SECCOMP (1 << TIF_SECCOMP)

#endif
diff --git a/arch/um/kernel/skas/syscall.c b/arch/um/kernel/skas/syscall.c
index 65f0d1a..af567da 100644
--- a/arch/um/kernel/skas/syscall.c
+++ b/arch/um/kernel/skas/syscall.c
@@ -5,6 +5,7 @@

#include <linux/kernel.h>
#include <linux/ptrace.h>
+#include <linux/seccomp.h>
#include <kern_util.h>
#include <sysdep/ptrace.h>
#include <sysdep/ptrace_user.h>
@@ -20,6 +21,14 @@ void handle_syscall(struct uml_pt_regs *r)
/* Save the syscall register. */
UPT_SYSCALL_NR(r) = PT_SYSCALL_NR(r->gp);

+ /* Do the secure computing check first; failures should be fast. */
+ if (secure_computing() == -1) {
+ /* Do not put secure_computing() into syscall_trace_enter() to
+ * avoid forced syscall return value.
+ */
+ return;
+ }
+
if (syscall_trace_enter(regs)) {
result = -ENOSYS;
goto out;
--
2.6.4