| Message ID | 1a40458d081ce0d5423eb0282210055496e28774.1602263422.git.yifeifz2@illinois.edu (mailing list archive) |
|---|---|
| State | Superseded |
| Headers | show |
| Series | seccomp: Add bitmap cache of constant allow filter results | expand |
On Fri, Oct 9, 2020 at 7:15 PM YiFei Zhu <zhuyifei1999@gmail.com> wrote: > > From: YiFei Zhu <yifeifz2@illinois.edu> > > SECCOMP_CACHE will only operate on syscalls that do not access > any syscall arguments or instruction pointer. To facilitate > this we need a static analyser to know whether a filter will > return allow regardless of syscall arguments for a given > architecture number / syscall number pair. This is implemented > here with a pseudo-emulator, and stored in a per-filter bitmap. > > In order to build this bitmap at filter attach time, each filter is > emulated for every syscall (under each possible architecture), and > checked for any accesses of struct seccomp_data that are not the "arch" > nor "nr" (syscall) members. If only "arch" and "nr" are examined, and > the program returns allow, then we can be sure that the filter must > return allow independent from syscall arguments. > > Nearly all seccomp filters are built from these cBPF instructions: > > BPF_LD | BPF_W | BPF_ABS > BPF_JMP | BPF_JEQ | BPF_K > BPF_JMP | BPF_JGE | BPF_K > BPF_JMP | BPF_JGT | BPF_K > BPF_JMP | BPF_JSET | BPF_K > BPF_JMP | BPF_JA > BPF_RET | BPF_K > BPF_ALU | BPF_AND | BPF_K > > Each of these instructions are emulated. Any weirdness or loading > from a syscall argument will cause the emulator to bail. > > The emulation is also halted if it reaches a return. In that case, > if it returns an SECCOMP_RET_ALLOW, the syscall is marked as good. > > Emulator structure and comments are from Kees [1] and Jann [2]. > > Emulation is done at attach time. If a filter depends on more > filters, and if the dependee does not guarantee to allow the > syscall, then we skip the emulation of this syscall. > > [1] https://lore.kernel.org/lkml/20200923232923.3142503-5-keescook@chromium.org/ > [2] https://lore.kernel.org/lkml/CAG48ez1p=dR_2ikKq=xVxkoGg0fYpTBpkhJSv1w-6BG=76PAvw@mail.gmail.com/ [...] > @@ -682,6 +693,150 @@ seccomp_prepare_user_filter(const char __user *user_filter) > return filter; > } > > +#ifdef SECCOMP_ARCH_NATIVE > +/** > + * seccomp_is_const_allow - check if filter is constant allow with given data > + * @fprog: The BPF programs > + * @sd: The seccomp data to check against, only syscall number are arch > + * number are considered constant. nit: s/syscall number are arch number/syscall number and arch number/ > + */ > +static bool seccomp_is_const_allow(struct sock_fprog_kern *fprog, > + struct seccomp_data *sd) > +{ > + unsigned int insns; > + unsigned int reg_value = 0; > + unsigned int pc; > + bool op_res; > + > + if (WARN_ON_ONCE(!fprog)) > + return false; > + > + insns = bpf_classic_proglen(fprog); bpf_classic_proglen() is defined as: #define bpf_classic_proglen(fprog) (fprog->len * sizeof(fprog->filter[0])) so this is wrong - what you want is the number of instructions in the program, what you actually have is the size of the program in bytes. Please instead check for `pc < fprog->len` in the loop condition. > + for (pc = 0; pc < insns; pc++) { > + struct sock_filter *insn = &fprog->filter[pc]; > + u16 code = insn->code; > + u32 k = insn->k; [...] > + } > + > + /* ran off the end of the filter?! */ > + WARN_ON(1); > + return false; > +}
On Fri, Oct 09, 2020 at 11:30:18PM +0200, Jann Horn wrote: > On Fri, Oct 9, 2020 at 7:15 PM YiFei Zhu <zhuyifei1999@gmail.com> wrote: > > > > From: YiFei Zhu <yifeifz2@illinois.edu> > > > > SECCOMP_CACHE will only operate on syscalls that do not access > > any syscall arguments or instruction pointer. To facilitate > > this we need a static analyser to know whether a filter will > > return allow regardless of syscall arguments for a given > > architecture number / syscall number pair. This is implemented > > here with a pseudo-emulator, and stored in a per-filter bitmap. > > > > In order to build this bitmap at filter attach time, each filter is > > emulated for every syscall (under each possible architecture), and > > checked for any accesses of struct seccomp_data that are not the "arch" > > nor "nr" (syscall) members. If only "arch" and "nr" are examined, and > > the program returns allow, then we can be sure that the filter must > > return allow independent from syscall arguments. > > > > Nearly all seccomp filters are built from these cBPF instructions: > > > > BPF_LD | BPF_W | BPF_ABS > > BPF_JMP | BPF_JEQ | BPF_K > > BPF_JMP | BPF_JGE | BPF_K > > BPF_JMP | BPF_JGT | BPF_K > > BPF_JMP | BPF_JSET | BPF_K > > BPF_JMP | BPF_JA > > BPF_RET | BPF_K > > BPF_ALU | BPF_AND | BPF_K > > > > Each of these instructions are emulated. Any weirdness or loading > > from a syscall argument will cause the emulator to bail. > > > > The emulation is also halted if it reaches a return. In that case, > > if it returns an SECCOMP_RET_ALLOW, the syscall is marked as good. > > > > Emulator structure and comments are from Kees [1] and Jann [2]. > > > > Emulation is done at attach time. If a filter depends on more > > filters, and if the dependee does not guarantee to allow the > > syscall, then we skip the emulation of this syscall. > > > > [1] https://lore.kernel.org/lkml/20200923232923.3142503-5-keescook@chromium.org/ > > [2] https://lore.kernel.org/lkml/CAG48ez1p=dR_2ikKq=xVxkoGg0fYpTBpkhJSv1w-6BG=76PAvw@mail.gmail.com/ > [...] > > @@ -682,6 +693,150 @@ seccomp_prepare_user_filter(const char __user *user_filter) > > return filter; > > } > > > > +#ifdef SECCOMP_ARCH_NATIVE > > +/** > > + * seccomp_is_const_allow - check if filter is constant allow with given data > > + * @fprog: The BPF programs > > + * @sd: The seccomp data to check against, only syscall number are arch > > + * number are considered constant. > > nit: s/syscall number are arch number/syscall number and arch number/ > > > + */ > > +static bool seccomp_is_const_allow(struct sock_fprog_kern *fprog, > > + struct seccomp_data *sd) > > +{ > > + unsigned int insns; > > + unsigned int reg_value = 0; > > + unsigned int pc; > > + bool op_res; > > + > > + if (WARN_ON_ONCE(!fprog)) > > + return false; > > + > > + insns = bpf_classic_proglen(fprog); > > bpf_classic_proglen() is defined as: > > #define bpf_classic_proglen(fprog) (fprog->len * sizeof(fprog->filter[0])) > > so this is wrong - what you want is the number of instructions in the > program, what you actually have is the size of the program in bytes. > Please instead check for `pc < fprog->len` in the loop condition. Oh yes, good catch. I had this wrong in my v1.
diff --git a/kernel/seccomp.c b/kernel/seccomp.c index 73f6b6e9a3b0..51032b41fe59 100644 --- a/kernel/seccomp.c +++ b/kernel/seccomp.c @@ -169,6 +169,10 @@ static inline bool seccomp_cache_check_allow(const struct seccomp_filter *sfilte { return false; } + +static inline void seccomp_cache_prepare(struct seccomp_filter *sfilter) +{ +} #endif /* SECCOMP_ARCH_NATIVE */ /** @@ -187,6 +191,7 @@ static inline bool seccomp_cache_check_allow(const struct seccomp_filter *sfilte * this filter after reaching 0. The @users count is always smaller * or equal to @refs. Hence, reaching 0 for @users does not mean * the filter can be freed. + * @cache: cache of arch/syscall mappings to actions * @log: true if all actions except for SECCOMP_RET_ALLOW should be logged * @prev: points to a previously installed, or inherited, filter * @prog: the BPF program to evaluate @@ -208,6 +213,7 @@ struct seccomp_filter { refcount_t refs; refcount_t users; bool log; + struct action_cache cache; struct seccomp_filter *prev; struct bpf_prog *prog; struct notification *notif; @@ -616,7 +622,12 @@ static struct seccomp_filter *seccomp_prepare_filter(struct sock_fprog *fprog) { struct seccomp_filter *sfilter; int ret; - const bool save_orig = IS_ENABLED(CONFIG_CHECKPOINT_RESTORE); + const bool save_orig = +#if defined(CONFIG_CHECKPOINT_RESTORE) || defined(SECCOMP_ARCH_NATIVE) + true; +#else + false; +#endif if (fprog->len == 0 || fprog->len > BPF_MAXINSNS) return ERR_PTR(-EINVAL); @@ -682,6 +693,150 @@ seccomp_prepare_user_filter(const char __user *user_filter) return filter; } +#ifdef SECCOMP_ARCH_NATIVE +/** + * seccomp_is_const_allow - check if filter is constant allow with given data + * @fprog: The BPF programs + * @sd: The seccomp data to check against, only syscall number are arch + * number are considered constant. + */ +static bool seccomp_is_const_allow(struct sock_fprog_kern *fprog, + struct seccomp_data *sd) +{ + unsigned int insns; + unsigned int reg_value = 0; + unsigned int pc; + bool op_res; + + if (WARN_ON_ONCE(!fprog)) + return false; + + insns = bpf_classic_proglen(fprog); + for (pc = 0; pc < insns; pc++) { + struct sock_filter *insn = &fprog->filter[pc]; + u16 code = insn->code; + u32 k = insn->k; + + switch (code) { + case BPF_LD | BPF_W | BPF_ABS: + switch (k) { + case offsetof(struct seccomp_data, nr): + reg_value = sd->nr; + break; + case offsetof(struct seccomp_data, arch): + reg_value = sd->arch; + break; + default: + /* can't optimize (non-constant value load) */ + return false; + } + break; + case BPF_RET | BPF_K: + /* reached return with constant values only, check allow */ + return k == SECCOMP_RET_ALLOW; + case BPF_JMP | BPF_JA: + pc += insn->k; + break; + case BPF_JMP | BPF_JEQ | BPF_K: + case BPF_JMP | BPF_JGE | BPF_K: + case BPF_JMP | BPF_JGT | BPF_K: + case BPF_JMP | BPF_JSET | BPF_K: + switch (BPF_OP(code)) { + case BPF_JEQ: + op_res = reg_value == k; + break; + case BPF_JGE: + op_res = reg_value >= k; + break; + case BPF_JGT: + op_res = reg_value > k; + break; + case BPF_JSET: + op_res = !!(reg_value & k); + break; + default: + /* can't optimize (unknown jump) */ + return false; + } + + pc += op_res ? insn->jt : insn->jf; + break; + case BPF_ALU | BPF_AND | BPF_K: + reg_value &= k; + break; + default: + /* can't optimize (unknown insn) */ + return false; + } + } + + /* ran off the end of the filter?! */ + WARN_ON(1); + return false; +} + +static void seccomp_cache_prepare_bitmap(struct seccomp_filter *sfilter, + void *bitmap, const void *bitmap_prev, + size_t bitmap_size, int arch) +{ + struct sock_fprog_kern *fprog = sfilter->prog->orig_prog; + struct seccomp_data sd; + int nr; + + if (bitmap_prev) { + /* The new filter must be as restrictive as the last. */ + bitmap_copy(bitmap, bitmap_prev, bitmap_size); + } else { + /* Before any filters, all syscalls are always allowed. */ + bitmap_fill(bitmap, bitmap_size); + } + + for (nr = 0; nr < bitmap_size; nr++) { + /* No bitmap change: not a cacheable action. */ + if (!test_bit(nr, bitmap)) + continue; + + sd.nr = nr; + sd.arch = arch; + + /* No bitmap change: continue to always allow. */ + if (seccomp_is_const_allow(fprog, &sd)) + continue; + + /* + * Not a cacheable action: always run filters. + * atomic clear_bit() not needed, filter not visible yet. + */ + __clear_bit(nr, bitmap); + } +} + +/** + * seccomp_cache_prepare - emulate the filter to find cachable syscalls + * @sfilter: The seccomp filter + * + * Returns 0 if successful or -errno if error occurred. + */ +static void seccomp_cache_prepare(struct seccomp_filter *sfilter) +{ + struct action_cache *cache = &sfilter->cache; + const struct action_cache *cache_prev = + sfilter->prev ? &sfilter->prev->cache : NULL; + + seccomp_cache_prepare_bitmap(sfilter, cache->allow_native, + cache_prev ? cache_prev->allow_native : NULL, + SECCOMP_ARCH_NATIVE_NR, + SECCOMP_ARCH_NATIVE); + +#ifdef SECCOMP_ARCH_COMPAT + seccomp_cache_prepare_bitmap(sfilter, cache->allow_compat, + cache_prev ? cache_prev->allow_compat : NULL, + SECCOMP_ARCH_COMPAT_NR, + SECCOMP_ARCH_COMPAT); +#endif /* SECCOMP_ARCH_COMPAT */ +} +#endif /* SECCOMP_ARCH_NATIVE */ + /** * seccomp_attach_filter: validate and attach filter * @flags: flags to change filter behavior @@ -731,6 +886,7 @@ static long seccomp_attach_filter(unsigned int flags, * task reference. */ filter->prev = current->seccomp.filter; + seccomp_cache_prepare(filter); current->seccomp.filter = filter; atomic_inc(¤t->seccomp.filter_count);