| Message ID | 10f91a367ec4fcdea7fc3f086de3f5f13a4a7436.1602431034.git.yifeifz2@illinois.edu (mailing list archive) |
|---|---|
| State | Not Applicable |
| Headers | show |
| Series | seccomp: Add bitmap cache of constant allow filter results | expand |
On Sun, Oct 11, 2020 at 5:48 PM YiFei Zhu <zhuyifei1999@gmail.com> wrote: > The overhead of running Seccomp filters has been part of some past > discussions [1][2][3]. Oftentimes, the filters have a large number > of instructions that check syscall numbers one by one and jump based > on that. Some users chain BPF filters which further enlarge the > overhead. A recent work [6] comprehensively measures the Seccomp > overhead and shows that the overhead is non-negligible and has a > non-trivial impact on application performance. > > We observed some common filters, such as docker's [4] or > systemd's [5], will make most decisions based only on the syscall > numbers, and as past discussions considered, a bitmap where each bit > represents a syscall makes most sense for these filters. > > 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. > > When it can be concluded that an allow must occur for the given > architecture and syscall pair (this determination is introduced in > the next commit), seccomp will immediately allow the syscall, > bypassing further BPF execution. > > Each architecture number has its own bitmap. The architecture > number in seccomp_data is checked against the defined architecture > number constant before proceeding to test the bit against the > bitmap with the syscall number as the index of the bit in the > bitmap, and if the bit is set, seccomp returns allow. The bitmaps > are all clear in this patch and will be initialized in the next > commit. > > When only one architecture exists, the check against architecture > number is skipped, suggested by Kees Cook [7]. > > [1] https://lore.kernel.org/linux-security-module/c22a6c3cefc2412cad00ae14c1371711@huawei.com/T/ > [2] https://lore.kernel.org/lkml/202005181120.971232B7B@keescook/T/ > [3] https://github.com/seccomp/libseccomp/issues/116 > [4] https://github.com/moby/moby/blob/ae0ef82b90356ac613f329a8ef5ee42ca923417d/profiles/seccomp/default.json > [5] https://github.com/systemd/systemd/blob/6743a1caf4037f03dc51a1277855018e4ab61957/src/shared/seccomp-util.c#L270 > [6] Draco: Architectural and Operating System Support for System Call Security > https://tianyin.github.io/pub/draco.pdf, MICRO-53, Oct. 2020 > [7] https://lore.kernel.org/bpf/202010091614.8BB0EB64@keescook/ > > 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> Reviewed-by: Jann Horn <jannh@google.com>
diff --git a/kernel/seccomp.c b/kernel/seccomp.c index ae6b40cc39f4..d67a8b61f2bf 100644 --- a/kernel/seccomp.c +++ b/kernel/seccomp.c @@ -143,6 +143,34 @@ struct notification { struct list_head notifications; }; +#ifdef SECCOMP_ARCH_NATIVE +/** + * struct action_cache - per-filter cache of seccomp actions per + * arch/syscall pair + * + * @allow_native: A bitmap where each bit represents whether the + * filter will always allow the syscall, for the + * native architecture. + * @allow_compat: A bitmap where each bit represents whether the + * filter will always allow the syscall, for the + * compat architecture. + */ +struct action_cache { + DECLARE_BITMAP(allow_native, SECCOMP_ARCH_NATIVE_NR); +#ifdef SECCOMP_ARCH_COMPAT + DECLARE_BITMAP(allow_compat, SECCOMP_ARCH_COMPAT_NR); +#endif +}; +#else +struct action_cache { }; + +static inline bool seccomp_cache_check_allow(const struct seccomp_filter *sfilter, + const struct seccomp_data *sd) +{ + return false; +} +#endif /* SECCOMP_ARCH_NATIVE */ + /** * struct seccomp_filter - container for seccomp BPF programs * @@ -298,6 +326,52 @@ static int seccomp_check_filter(struct sock_filter *filter, unsigned int flen) return 0; } +#ifdef SECCOMP_ARCH_NATIVE +static inline bool seccomp_cache_check_allow_bitmap(const void *bitmap, + size_t bitmap_size, + int syscall_nr) +{ + if (unlikely(syscall_nr < 0 || syscall_nr >= bitmap_size)) + return false; + syscall_nr = array_index_nospec(syscall_nr, bitmap_size); + + return test_bit(syscall_nr, bitmap); +} + +/** + * seccomp_cache_check_allow - 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 inline bool seccomp_cache_check_allow(const struct seccomp_filter *sfilter, + const struct seccomp_data *sd) +{ + int syscall_nr = sd->nr; + const struct action_cache *cache = &sfilter->cache; + +#ifndef SECCOMP_ARCH_COMPAT + /* A native-only architecture doesn't need to check sd->arch. */ + return seccomp_cache_check_allow_bitmap(cache->allow_native, + SECCOMP_ARCH_NATIVE_NR, + syscall_nr); +#else + if (likely(sd->arch == SECCOMP_ARCH_NATIVE)) + return seccomp_cache_check_allow_bitmap(cache->allow_native, + SECCOMP_ARCH_NATIVE_NR, + syscall_nr); + if (likely(sd->arch == SECCOMP_ARCH_COMPAT)) + return seccomp_cache_check_allow_bitmap(cache->allow_compat, + SECCOMP_ARCH_COMPAT_NR, + syscall_nr); +#endif /* SECCOMP_ARCH_COMPAT */ + + WARN_ON_ONCE(true); + return false; +} +#endif /* SECCOMP_ARCH_NATIVE */ + /** * seccomp_run_filters - evaluates all seccomp filters against @sd * @sd: optional seccomp data to be passed to filters @@ -320,6 +394,9 @@ static u32 seccomp_run_filters(const struct seccomp_data *sd, if (WARN_ON(f == NULL)) return SECCOMP_RET_KILL_PROCESS; + if (seccomp_cache_check_allow(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).