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@xxxxxxxxx> wrote: > > > > From: YiFei Zhu <yifeifz2@xxxxxxxxxxxx> > > > > 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@xxxxxxxxxxxx/ > > [2] https://lore.kernel.org/lkml/CAG48ez1p=dR_2ikKq=xVxkoGg0fYpTBpkhJSv1w-6BG=76PAvw@xxxxxxxxxxxxxx/ > [...] > > @@ -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. -- Kees Cook
