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Commit e0ffbaab authored by Andy Lutomirski's avatar Andy Lutomirski Committed by H. Peter Anvin
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x86: Split syscall_trace_enter into two phases 



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: default avatarAndy Lutomirski <luto@amacapital.net>
Link: http://lkml.kernel.org/r/2df320a600020fda055fccf2b668145729dd0c04.1409954077.git.luto@amacapital.net


Signed-off-by: default avatarH. Peter Anvin <hpa@linux.intel.com>
parent fd143b21

arch/x86/include/asm/ptrace.h

+5 −0
Original line number Diff line number Diff line
@@ -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 *);

arch/x86/kernel/ptrace.c

+133 −24
Original line number Diff line number Diff line
@@ -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 +1569,21 @@ 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()) {

#ifdef CONFIG_SECCOMP

	/*

	 * 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;

		return -1;

	}

#endif



	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 +1593,22 @@ 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;