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

[Kees Cook]

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