Re: [PATCH 1/2] riscv: add support for SECCOMP incl. filters

[Kees Cook]

On Thu, Dec 6, 2018 at 7:02 AM David Abdurachmanov
<david.abdurachmanov@xxxxxxxxx> wrote:
>
> The patch adds support for SECCOMP and SECCOMP_FILTER (BPF).
>
> Signed-off-by: David Abdurachmanov <david.abdurachmanov@xxxxxxxxx>
> ---
>  arch/riscv/Kconfig                   | 14 ++++++++++++++
>  arch/riscv/include/asm/thread_info.h |  5 ++++-
>  arch/riscv/kernel/entry.S            | 27 +++++++++++++++++++++++++--
>  arch/riscv/kernel/ptrace.c           |  8 ++++++++
>  4 files changed, 51 insertions(+), 3 deletions(-)
>
> diff --git a/arch/riscv/Kconfig b/arch/riscv/Kconfig
> index a4f48f757204..49cd8e251547 100644
> --- a/arch/riscv/Kconfig
> +++ b/arch/riscv/Kconfig
> @@ -29,6 +29,7 @@ config RISCV
>         select GENERIC_SMP_IDLE_THREAD
>         select GENERIC_ATOMIC64 if !64BIT || !RISCV_ISA_A
>         select HAVE_ARCH_AUDITSYSCALL
> +       select HAVE_ARCH_SECCOMP_FILTER
>         select HAVE_MEMBLOCK_NODE_MAP
>         select HAVE_DMA_CONTIGUOUS
>         select HAVE_FUTEX_CMPXCHG if FUTEX
> @@ -228,6 +229,19 @@ menu "Kernel features"
>
>  source "kernel/Kconfig.hz"
>
> +config SECCOMP
> +       bool "Enable seccomp to safely compute untrusted bytecode"
> +       help
> +         This kernel feature is useful for number crunching applications
> +         that may need to compute untrusted bytecode during their
> +         execution. By using pipes or other transports made available to
> +         the process as file descriptors supporting the read/write
> +         syscalls, it's possible to isolate those applications in
> +         their own address space using seccomp. Once seccomp is
> +         enabled via prctl(PR_SET_SECCOMP), it cannot be disabled
> +         and the task is only allowed to execute a few safe syscalls
> +         defined by each seccomp mode.
> +
>  endmenu
>
>  menu "Boot options"
> diff --git a/arch/riscv/include/asm/thread_info.h b/arch/riscv/include/asm/thread_info.h
> index 1c9cc8389928..1fd6e4130cab 100644
> --- a/arch/riscv/include/asm/thread_info.h
> +++ b/arch/riscv/include/asm/thread_info.h
> @@ -81,6 +81,7 @@ struct thread_info {
>  #define TIF_MEMDIE             5       /* is terminating due to OOM killer */
>  #define TIF_SYSCALL_TRACEPOINT  6       /* syscall tracepoint instrumentation */
>  #define TIF_SYSCALL_AUDIT      7       /* syscall auditing */
> +#define TIF_SECCOMP                    8       /* syscall secure computing */
>
>  #define _TIF_SYSCALL_TRACE     (1 << TIF_SYSCALL_TRACE)
>  #define _TIF_NOTIFY_RESUME     (1 << TIF_NOTIFY_RESUME)
> @@ -88,11 +89,13 @@ struct thread_info {
>  #define _TIF_NEED_RESCHED      (1 << TIF_NEED_RESCHED)
>  #define _TIF_SYSCALL_TRACEPOINT        (1 << TIF_SYSCALL_TRACEPOINT)
>  #define _TIF_SYSCALL_AUDIT     (1 << TIF_SYSCALL_AUDIT)
> +#define _TIF_SECCOMP           (1 << TIF_SECCOMP)
>
>  #define _TIF_WORK_MASK \
>         (_TIF_NOTIFY_RESUME | _TIF_SIGPENDING | _TIF_NEED_RESCHED)
>
>  #define _TIF_SYSCALL_WORK \
> -       (_TIF_SYSCALL_TRACE | _TIF_SYSCALL_TRACEPOINT | _TIF_SYSCALL_AUDIT)
> +       (_TIF_SYSCALL_TRACE | _TIF_SYSCALL_TRACEPOINT | _TIF_SYSCALL_AUDIT \
> +        _TIF_SECCOMP )
>
>  #endif /* _ASM_RISCV_THREAD_INFO_H */
> diff --git a/arch/riscv/kernel/entry.S b/arch/riscv/kernel/entry.S
> index 355166f57205..e88ccbfa61ee 100644
> --- a/arch/riscv/kernel/entry.S
> +++ b/arch/riscv/kernel/entry.S
> @@ -207,8 +207,25 @@ check_syscall_nr:
>         /* Check to make sure we don't jump to a bogus syscall number. */
>         li t0, __NR_syscalls
>         la s0, sys_ni_syscall
> -       /* Syscall number held in a7 */
> -       bgeu a7, t0, 1f
> +       /*
> +        * The tracer can change syscall number to valid/invalid value.
> +        * We use syscall_set_nr helper in syscall_trace_enter thus we
> +        * cannot trust the current value in a7 and have to reload from
> +        * the current task pt_regs.
> +        */
> +       REG_L a7, PT_A7(sp)
> +       /*
> +        * Syscall number held in a7.
> +        * If syscall number is above allowed value, redirect to ni_syscall.
> +        */
> +       bge a7, t0, 1f
> +       /*
> +        * Check if syscall is rejected by tracer or seccomp, i.e., a7 == -1.
> +        * If yes, we pretend it was executed.
> +        */
> +       li t1, -1
> +       beq a7, t1, ret_from_syscall_rejected
> +       /* Call syscall */
>         la s0, sys_call_table
>         slli t0, a7, RISCV_LGPTR
>         add s0, s0, t0
> @@ -219,6 +236,12 @@ check_syscall_nr:
>  ret_from_syscall:
>         /* Set user a0 to kernel a0 */
>         REG_S a0, PT_A0(sp)
> +       /*
> +        * We didn't execute the actual syscall.
> +        * Seccomp already set return value for the current task pt_regs.
> +        * (If it was configured with SECCOMP_RET_ERRNO/TRACE)
> +        */
> +ret_from_syscall_rejected:
>         /* Trace syscalls, but only if requested by the user. */
>         REG_L t0, TASK_TI_FLAGS(tp)
>         andi t0, t0, _TIF_SYSCALL_WORK
> diff --git a/arch/riscv/kernel/ptrace.c b/arch/riscv/kernel/ptrace.c
> index c1b51539c3e2..598e48b8ca2b 100644
> --- a/arch/riscv/kernel/ptrace.c
> +++ b/arch/riscv/kernel/ptrace.c
> @@ -160,6 +160,14 @@ void do_syscall_trace_enter(struct pt_regs *regs)
>                 if (tracehook_report_syscall_entry(regs))
>                         syscall_set_nr(current, regs, -1);
>
> +       /*
> +        * Do the secure computing after ptrace; failures should be fast.
> +        * If this fails we might have return value in a0 from seccomp
> +        * (via SECCOMP_RET_ERRNO/TRACE).
> +        */
> +       if (secure_computing(NULL) == -1)
> +               syscall_set_nr(current, regs, -1);

On a -1 return, this should return immediately -- it should not
continue to process trace_sys_enter(), etc.

-Kees

> +
>  #ifdef CONFIG_HAVE_SYSCALL_TRACEPOINTS
>         if (test_thread_flag(TIF_SYSCALL_TRACEPOINT))
>                 trace_sys_enter(regs, syscall_get_nr(current, regs));
> --
> 2.19.2
>


-- 
Kees Cook