[PATCH v2 seccomp 4/6] seccomp/cache: Lookup syscall allowlist for fast path

YiFei Zhu
2020-09-24T12:44:51Z

From: YiFei Zhu <yifeifz2@illinois.edu>

The fast (common) path for seccomp should be that the filter permits
the syscall to pass through, and failing seccomp is expected to be
an exceptional case; it is not expected for userspace to call a
denylisted syscall over and over.

This first finds the current allow bitmask by iterating through
syscall_arches[] array and comparing it to the one in struct
seccomp_data; this loop is expected to be unrolled. It then
does a test_bit against the bitmask. If the bit is set, then
there is no need to run the full filter; it returns
SECCOMP_RET_ALLOW immediately.

Co-developed-by: Dimitrios Skarlatos <dskarlat@cs.cmu.edu>
Signed-off-by: Dimitrios Skarlatos <dskarlat@cs.cmu.edu>
Signed-off-by: YiFei Zhu <yifeifz2@illinois.edu>
---
kernel/seccomp.c | 37 +++++++++++++++++++++++++++++++++++++
1 file changed, 37 insertions(+)

diff --git a/kernel/seccomp.c b/kernel/seccomp.c
index 20d33378a092..ac0266b6d18a 100644
--- a/kernel/seccomp.c
+++ b/kernel/seccomp.c
@@ -167,6 +167,12 @@ static inline void seccomp_cache_inherit(struct seccomp_filter *sfilter,
const struct seccomp_filter *prev)
{
}
+
+static inline bool seccomp_cache_check(const struct seccomp_filter *sfilter,
+ const struct seccomp_data *sd)
+{
+ return false;
+}
#endif /* CONFIG_SECCOMP_CACHE_NR_ONLY */

/**
@@ -321,6 +327,34 @@ static int seccomp_check_filter(struct sock_filter *filter, unsigned int flen)
return 0;
}

+#ifdef CONFIG_SECCOMP_CACHE_NR_ONLY
+/**
+ * seccomp_cache_check - lookup seccomp cache
+ * @sfilter: The seccomp filter
+ * @sd: The seccomp data to lookup the cache with
+ *
+ * Returns true if the seccomp_data is cached and allowed.
+ */
+static bool seccomp_cache_check(const struct seccomp_filter *sfilter,
+ const struct seccomp_data *sd)
+{
+ int syscall_nr = sd->nr;
+ int arch;
+
+ if (unlikely(syscall_nr < 0 || syscall_nr >= NR_syscalls))
+ return false;
+
+ for (arch = 0; arch < ARRAY_SIZE(syscall_arches); arch++) {
+ if (likely(syscall_arches[arch] == sd->arch))
+ return test_bit(syscall_nr,
+ sfilter->cache.syscall_ok[arch]);
+ }
+
+ WARN_ON_ONCE(true);
+ return false;
+}
+#endif /* CONFIG_SECCOMP_CACHE_NR_ONLY */
+
/**
* seccomp_run_filters - evaluates all seccomp filters against @sd
* @sd: optional seccomp data to be passed to filters
@@ -343,6 +377,9 @@ static u32 seccomp_run_filters(const struct seccomp_data *sd,
if (WARN_ON(f == NULL))
return SECCOMP_RET_KILL_PROCESS;

+ if (seccomp_cache_check(f, sd))
+ return SECCOMP_RET_ALLOW;
+
/*
* All filters in the list are evaluated and the lowest BPF return
* value always takes priority (ignoring the DATA).
--
2.28.0