This splits syscall_trace_enter into syscall_trace_enter_phase1 and
syscall_trace_enter_phase2. Only phase 2 has full pt_regs, and only
phase 2 is permitted to modify any of pt_regs except for orig_ax.
The intent is that phase 1 can be called from the syscall fast path.
In this implementation, phase1 can handle any combination of
TIF_NOHZ (RCU context tracking), TIF_SECCOMP, and TIF_SYSCALL_AUDIT,
unless seccomp requests a ptrace event, in which case phase2 is
forced.
In principle, this could yield a big speedup for TIF_NOHZ as well as
for TIF_SECCOMP if syscall exit work were similarly split up.
Signed-off-by: Andy Lutomirski <luto@xxxxxxxxxxxxxx>
---
arch/x86/include/asm/ptrace.h | 5 ++
arch/x86/kernel/ptrace.c | 151 ++++++++++++++++++++++++++++++++++++------
2 files changed, 135 insertions(+), 21 deletions(-)
diff --git a/arch/x86/include/asm/ptrace.h b/arch/x86/include/asm/ptrace.h
index 6205f0c..86fc2bb 100644
--- a/arch/x86/include/asm/ptrace.h
+++ b/arch/x86/include/asm/ptrace.h
@@ -75,6 +75,11 @@ convert_ip_to_linear(struct task_struct *child, struct pt_regs *regs);
extern void send_sigtrap(struct task_struct *tsk, struct pt_regs *regs,
int error_code, int si_code);
+
+extern unsigned long syscall_trace_enter_phase1(struct pt_regs *, u32 arch);
+extern long syscall_trace_enter_phase2(struct pt_regs *, u32 arch,
+ unsigned long phase1_result);
+
extern long syscall_trace_enter(struct pt_regs *);
extern void syscall_trace_leave(struct pt_regs *);
diff --git a/arch/x86/kernel/ptrace.c b/arch/x86/kernel/ptrace.c
index bbf338a..5da2672 100644
--- a/arch/x86/kernel/ptrace.c
+++ b/arch/x86/kernel/ptrace.c
@@ -1441,20 +1441,126 @@ void send_sigtrap(struct task_struct *tsk, struct pt_regs *regs,
force_sig_info(SIGTRAP, &info, tsk);
}
+static void do_audit_syscall_entry(struct pt_regs *regs, u32 arch)
+{
+#ifdef CONFIG_X86_64
+ if (arch == AUDIT_ARCH_X86_64) {
+ audit_syscall_entry(arch, regs->orig_ax, regs->di,
+ regs->si, regs->dx, regs->r10);
+ } else
+#endif
+ {
+ audit_syscall_entry(arch, regs->orig_ax, regs->bx,
+ regs->cx, regs->dx, regs->si);
+ }
+}
+
/*
- * We must return the syscall number to actually look up in the table.
- * This can be -1L to skip running any syscall at all.
+ * We can return 0 to resume the syscall or anything else to go to phase
+ * 2. If we resume the syscall, we need to put something appropriate in
+ * regs->orig_ax.
+ *
+ * NB: We don't have full pt_regs here, but regs->orig_ax and regs->ax
+ * are fully functional.
+ *
+ * For phase 2's benefit, our return value is:
+ * 0: resume the syscall
+ * 1: go to phase 2; no seccomp phase 2 needed
+ * anything else: go to phase 2; pass return value to seccomp
*/
-long syscall_trace_enter(struct pt_regs *regs)
+unsigned long syscall_trace_enter_phase1(struct pt_regs *regs, u32 arch)
{
- long ret = 0;
+ unsigned long ret = 0;
+ u32 work;
+
+ BUG_ON(regs != task_pt_regs(current));
+
+ work = ACCESS_ONCE(current_thread_info()->flags) &
+ _TIF_WORK_SYSCALL_ENTRY;
/*
* If TIF_NOHZ is set, we are required to call user_exit() before
* doing anything that could touch RCU.
*/
- if (test_thread_flag(TIF_NOHZ))
+ if (work & _TIF_NOHZ) {
user_exit();
+ work &= ~TIF_NOHZ;
+ }
+
+#ifdef CONFIG_SECCOMP
+ /*
+ * Do seccomp first -- it should minimize exposure of other
+ * code, and keeping seccomp fast is probably more valuable
+ * than the rest of this.
+ */
+ if (work & _TIF_SECCOMP) {
+ struct seccomp_data sd;
+
+ sd.arch = arch;
+ sd.nr = regs->orig_ax;
+ sd.instruction_pointer = regs->ip;
+#ifdef CONFIG_X86_64
+ if (arch == AUDIT_ARCH_X86_64) {
+ sd.args[0] = regs->di;
+ sd.args[1] = regs->si;
+ sd.args[2] = regs->dx;
+ sd.args[3] = regs->r10;
+ sd.args[4] = regs->r8;
+ sd.args[5] = regs->r9;
+ } else
+#endif
+ {
+ sd.args[0] = regs->bx;
+ sd.args[1] = regs->cx;
+ sd.args[2] = regs->dx;
+ sd.args[3] = regs->si;
+ sd.args[4] = regs->di;
+ sd.args[5] = regs->bp;
+ }
+
+ BUILD_BUG_ON(SECCOMP_PHASE1_OK != 0);
+ BUILD_BUG_ON(SECCOMP_PHASE1_SKIP != 1);
+
+ ret = seccomp_phase1(&sd);
+ if (ret == SECCOMP_PHASE1_SKIP) {
+ regs->orig_ax = -1;
+ ret = 0;
+ } else if (ret != SECCOMP_PHASE1_OK) {
+ return ret; /* Go directly to phase 2 */
+ }
+
+ work &= ~_TIF_SECCOMP;
+ }
+#endif
+
+ /* Do our best to finish without phase 2. */
+ if (work == 0)
+ return ret; /* seccomp and/or nohz only (ret == 0 here) */
+
+#ifdef CONFIG_AUDITSYSCALL
+ if (work == _TIF_SYSCALL_AUDIT) {
+ /*
+ * If there is no more work to be done except auditing,
+ * then audit in phase 1. Phase 2 always audits, so, if
+ * we audit here, then we can't go on to phase 2.
+ */
+ do_audit_syscall_entry(regs, arch);
+ return 0;
+ }
+#endif
+
+ return 1; /* Something is enabled that we can't handle in phase 1 */
+}
+
+/* Returns the syscall nr to run (which should match regs->orig_ax). */
+long syscall_trace_enter_phase2(struct pt_regs *regs, u32 arch,
+ unsigned long phase1_result)
+{
+ long ret = 0;
+ u32 work = ACCESS_ONCE(current_thread_info()->flags) &
+ _TIF_WORK_SYSCALL_ENTRY;
+
+ BUG_ON(regs != task_pt_regs(current));
/*
* If we stepped into a sysenter/syscall insn, it trapped in
@@ -1463,17 +1569,20 @@ long syscall_trace_enter(struct pt_regs *regs)
* do_debug() and we need to set it again to restore the user
* state. If we entered on the slow path, TF was already set.
*/
- if (test_thread_flag(TIF_SINGLESTEP))
+ if (work & _TIF_SINGLESTEP)
regs->flags |= X86_EFLAGS_TF;
- /* do the secure computing check first */
- if (secure_computing()) {
+ /*
+ * Call seccomp_phase2 before running the other hooks so that
+ * they can see any changes made by a seccomp tracer.
+ */
+ if (phase1_result > 1 && seccomp_phase2(phase1_result)) {
/* seccomp failures shouldn't expose any additional code. */
ret = -1L;
goto out;
}
- if (unlikely(test_thread_flag(TIF_SYSCALL_EMU)))
+ if (unlikely(work & _TIF_SYSCALL_EMU))
ret = -1L;
if ((ret || test_thread_flag(TIF_SYSCALL_TRACE)) &&
@@ -1483,23 +1592,23 @@ long syscall_trace_enter(struct pt_regs *regs)
if (unlikely(test_thread_flag(TIF_SYSCALL_TRACEPOINT)))
trace_sys_enter(regs, regs->orig_ax);
- if (is_ia32_task())
- audit_syscall_entry(AUDIT_ARCH_I386,
- regs->orig_ax,
- regs->bx, regs->cx,
- regs->dx, regs->si);
-#ifdef CONFIG_X86_64
- else
- audit_syscall_entry(AUDIT_ARCH_X86_64,
- regs->orig_ax,
- regs->di, regs->si,
- regs->dx, regs->r10);
-#endif
+ do_audit_syscall_entry(regs, arch);
out:
return ret ?: regs->orig_ax;
}
+long syscall_trace_enter(struct pt_regs *regs)
+{
+ u32 arch = is_ia32_task() ? AUDIT_ARCH_I386 : AUDIT_ARCH_X86_64;
+ unsigned long phase1_result = syscall_trace_enter_phase1(regs, arch);
+
+ if (phase1_result == 0)
+ return regs->orig_ax;
+ else
+ return syscall_trace_enter_phase2(regs, arch, phase1_result);
+}
+
void syscall_trace_leave(struct pt_regs *regs)
{
bool step;
--
1.9.3
