Re: [PATCH v4 seccomp 2/5] seccomp/cache: Add "emulator" to check if filter is constant allow

[Jann Horn]

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.

> +       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;
> +}