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

From: David Abdurachmanov
Date: Thu Dec 06 2018 - 12:11:00 EST

On Thu, Dec 6, 2018 at 5:47 PM Kees Cook <keescook@xxxxxxxxxxxx> wrote:
>
> 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.

Ops! No idea how I missed that.
Will fix it.

> -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