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Subject[PATCH v8 4/8] seccomp: add SECCOMP_RET_ERRNO
Date
This change adds the SECCOMP_RET_ERRNO as a valid return value from a
seccomp filter. Additionally, it makes the first use of the lower
16-bits for storing a filter-supplied errno. 16-bits is more than
enough for the errno-base.h calls.

Returning errors instead of immediately terminating processes that
violate seccomp policy allow for broader use of this functionality
for kernel attack surface reduction. For example, a linux container
could maintain a whitelist of pre-existing system calls but drop
all new ones with errnos. This would keep a logically static attack
surface while providing errnos that may allow for graceful failure
without the downside of do_exit() on a bad call.

v8: - update Kconfig to note new need for syscall_set_return_value.
- reordered such that TRAP behavior follows on later.
- made the for loop a little less indent-y
v7: - introduced

Signed-off-by: Will Drewry <wad@chromium.org>
(cherry picked from commit e90e1a5389d0ce3a667640121b0a90538014a16c)
---
arch/Kconfig | 5 ++++-
include/linux/seccomp.h | 20 +++++++++++++++-----
kernel/seccomp.c | 42 ++++++++++++++++++++++++++++++------------
3 files changed, 49 insertions(+), 18 deletions(-)
diff --git a/arch/Kconfig b/arch/Kconfig
index c6ba1db..3f3052b 100644
--- a/arch/Kconfig
+++ b/arch/Kconfig
@@ -203,7 +203,10 @@ config HAVE_ARCH_SECCOMP_FILTER
bool
help
This symbol should be selected by an architecure if it provides
- asm/syscall.h, specifically syscall_get_arguments().
+ asm/syscall.h, specifically syscall_get_arguments() and
+ syscall_set_return_value(). Additionally, its system call
+ entry path must respect a return value of -1 from
+ __secure_computing_int() and/or secure_computing().

config SECCOMP_FILTER
def_bool y
diff --git a/include/linux/seccomp.h b/include/linux/seccomp.h
index 2bee1f7..879ece2 100644
--- a/include/linux/seccomp.h
+++ b/include/linux/seccomp.h
@@ -12,16 +12,20 @@

/*
* BPF programs may return a 32-bit value.
- * The bottom 16-bits are reserved for future use.
+ * The bottom 16-bits are for optional related return data.
* The upper 16-bits are ordered from least permissive values to most.
*
* The ordering ensures that a min_t() over composed return values always
* selects the least permissive choice.
*/
-#define SECCOMP_RET_MASK 0xffff0000U
#define SECCOMP_RET_KILL 0x00000000U /* kill the task immediately */
+#define SECCOMP_RET_ERRNO 0x00030000U /* returns an errno */
#define SECCOMP_RET_ALLOW 0x7fff0000U /* allow */

+/* Masks for accessing the above values. */
+#define SECCOMP_RET_ACTION 0xffff0000U
+#define SECCOMP_RET_DATA 0x0000ffffU
+
/* Format of the data the BPF program executes over. */
struct seccomp_data {
int nr;
@@ -57,11 +61,17 @@ struct seccomp {
struct seccomp_filter *filter;
};

-extern void __secure_computing(int);
-static inline void secure_computing(int this_syscall)
+/*
+ * Direct callers to __secure_computing should be updated as
+ * CONFIG_HAVE_ARCH_SECCOMP_FILTER propagates.
+ */
+extern void __secure_computing(int) __deprecated;
+extern int __secure_computing_int(int);
+static inline int secure_computing(int this_syscall)
{
if (unlikely(test_thread_flag(TIF_SECCOMP)))
- __secure_computing(this_syscall);
+ return __secure_computing_int(this_syscall);
+ return 0;
}

extern long prctl_get_seccomp(void);
diff --git a/kernel/seccomp.c b/kernel/seccomp.c
index 14d1869..55d000d 100644
--- a/kernel/seccomp.c
+++ b/kernel/seccomp.c
@@ -137,25 +137,22 @@ static void *bpf_pointer(const void *nr, int off, unsigned int size, void *buf)
static u32 seccomp_run_filters(int syscall)
{
struct seccomp_filter *f;
- const struct bpf_load_fns loaders = { bpf_pointer, bpf_length };
- u32 ret = SECCOMP_RET_KILL;
+ const struct bpf_load_fns fns = { bpf_pointer, bpf_length };
+ u32 ret = SECCOMP_RET_ALLOW;
const void *sc_ptr = (const void *)(uintptr_t)syscall;

/* It's not possible for the filter to be NULL here. */
#ifdef CONFIG_COMPAT
if (current->seccomp.filter->compat != !!(is_compat_task()))
- return ret;
+ return SECCOMP_RET_KILL;
#endif

/*
* All filters are evaluated in order of youngest to oldest. The lowest
* BPF return value always takes priority.
*/
- for (f = current->seccomp.filter; f; f = f->prev) {
- ret = bpf_run_filter(sc_ptr, f->insns, &loaders);
- if (ret != SECCOMP_RET_ALLOW)
- break;
- }
+ for (f = current->seccomp.filter; f; f = f->prev)
+ ret = min_t(u32, ret, bpf_run_filter(sc_ptr, f->insns, &fns));
return ret;
}

@@ -314,6 +311,13 @@ static int mode1_syscalls_32[] = {

void __secure_computing(int this_syscall)
{
+ /* Filter calls should never use this function. */
+ BUG_ON(current->seccomp.mode == SECCOMP_MODE_FILTER);
+ __secure_computing_int(this_syscall);
+}
+
+int __secure_computing_int(int this_syscall)
+{
int mode = current->seccomp.mode;
int *syscall;

@@ -326,15 +330,28 @@ void __secure_computing(int this_syscall)
#endif
do {
if (*syscall == this_syscall)
- return;
+ return 0;
} while (*++syscall);
break;
#ifdef CONFIG_SECCOMP_FILTER
- case SECCOMP_MODE_FILTER:
- if (seccomp_run_filters(this_syscall) == SECCOMP_RET_ALLOW)
- return;
+ case SECCOMP_MODE_FILTER: {
+ u32 action = seccomp_run_filters(this_syscall);
+ switch (action & SECCOMP_RET_ACTION) {
+ case SECCOMP_RET_ERRNO:
+ /* Set the low-order 16-bits as a errno. */
+ syscall_set_return_value(current, task_pt_regs(current),
+ -(action & SECCOMP_RET_DATA),
+ 0);
+ return -1;
+ case SECCOMP_RET_ALLOW:
+ return 0;
+ case SECCOMP_RET_KILL:
+ default:
+ break;
+ }
seccomp_filter_log_failure(this_syscall);
break;
+ }
#endif
default:
BUG();
@@ -345,6 +362,7 @@ void __secure_computing(int this_syscall)
#endif
audit_seccomp(this_syscall);
do_exit(SIGKILL);
+ return -1; /* never reached */
}

long prctl_get_seccomp(void)
--
1.7.5.4


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