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Subject[PATCH 1/2] riscv: add support for SECCOMP incl. filters
Date
The patch adds support for SECCOMP and SECCOMP_FILTER (BPF).

Signed-off-by: David Abdurachmanov <david.abdurachmanov@gmail.com>
---
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);
+
#ifdef CONFIG_HAVE_SYSCALL_TRACEPOINTS
if (test_thread_flag(TIF_SYSCALL_TRACEPOINT))
trace_sys_enter(regs, syscall_get_nr(current, regs));
--
2.19.2
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