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Subject[PATCH v2 1/4] seccomp: Provide matching filter for introspection
Date
Both the upcoming logging improvements and changes to RET_KILL will need
to know which filter a given seccomp return value originated from. In
order to delay logic processing of result until after the seccomp loop,
this adds a single pointer assignment on matches. This will allow both
log and RET_KILL logic to work off the filter rather than doing more
expensive tests inside the time-critical run_filters loop.

Running tight cycles of getpid() with filters attached shows no measurable
difference in speed.

Suggested-by: Tyler Hicks <tyhicks@canonical.com>
Signed-off-by: Kees Cook <keescook@chromium.org>
Reviewed-by: Tyler Hicks <tyhicks@canonical.com>
---
kernel/seccomp.c | 18 +++++++++++++++---
1 file changed, 15 insertions(+), 3 deletions(-)

diff --git a/kernel/seccomp.c b/kernel/seccomp.c
index 98b59b5db90b..1f3347fc2605 100644
--- a/kernel/seccomp.c
+++ b/kernel/seccomp.c
@@ -171,10 +171,14 @@ static int seccomp_check_filter(struct sock_filter *filter, unsigned int flen)
/**
* seccomp_run_filters - evaluates all seccomp filters against @sd
* @sd: optional seccomp data to be passed to filters
+ * @match: stores struct seccomp_filter that resulted in the return value,
+ * unless filter returned SECCOMP_RET_ALLOW, in which case it will
+ * be unchanged.
*
* Returns valid seccomp BPF response codes.
*/
-static u32 seccomp_run_filters(const struct seccomp_data *sd)
+static u32 seccomp_run_filters(const struct seccomp_data *sd,
+ struct seccomp_filter **match)
{
struct seccomp_data sd_local;
u32 ret = SECCOMP_RET_ALLOW;
@@ -198,8 +202,10 @@ static u32 seccomp_run_filters(const struct seccomp_data *sd)
for (; f; f = f->prev) {
u32 cur_ret = BPF_PROG_RUN(f->prog, sd);

- if ((cur_ret & SECCOMP_RET_ACTION) < (ret & SECCOMP_RET_ACTION))
+ if ((cur_ret & SECCOMP_RET_ACTION) < (ret & SECCOMP_RET_ACTION)) {
ret = cur_ret;
+ *match = f;
+ }
}
return ret;
}
@@ -566,6 +572,7 @@ static int __seccomp_filter(int this_syscall, const struct seccomp_data *sd,
const bool recheck_after_trace)
{
u32 filter_ret, action;
+ struct seccomp_filter *match = NULL;
int data;

/*
@@ -574,7 +581,7 @@ static int __seccomp_filter(int this_syscall, const struct seccomp_data *sd,
*/
rmb();

- filter_ret = seccomp_run_filters(sd);
+ filter_ret = seccomp_run_filters(sd, &match);
data = filter_ret & SECCOMP_RET_DATA;
action = filter_ret & SECCOMP_RET_ACTION;

@@ -638,6 +645,11 @@ static int __seccomp_filter(int this_syscall, const struct seccomp_data *sd,
return 0;

case SECCOMP_RET_ALLOW:
+ /*
+ * Note that the "match" filter will always be NULL for
+ * this action since SECCOMP_RET_ALLOW is the starting
+ * state in seccomp_run_filters().
+ */
return 0;

case SECCOMP_RET_KILL:
--
2.7.4
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