Commit 21d375b6 authored by Andy Lutomirski's avatar Andy Lutomirski Committed by Thomas Gleixner
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x86/entry/64: Remove the SYSCALL64 fast path 



The SYCALLL64 fast path was a nice, if small, optimization back in the good
old days when syscalls were actually reasonably fast.  Now there is PTI to
slow everything down, and indirect branches are verboten, making everything
messier.  The retpoline code in the fast path is particularly nasty.

Just get rid of the fast path. The slow path is barely slower.

[ tglx: Split out the 'push all extra regs' part ]

Signed-off-by: default avatarAndy Lutomirski <luto@kernel.org>
Signed-off-by: default avatarThomas Gleixner <tglx@linutronix.de>
Acked-by: default avatarIngo Molnar <mingo@kernel.org>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Kernel Hardening <kernel-hardening@lists.openwall.com>
Link: https://lkml.kernel.org/r/462dff8d4d64dfbfc851fbf3130641809d980ecd.1517164461.git.luto@kernel.org
parent 9471eee9

arch/x86/entry/entry_64.S

+0 −117
Original line number Diff line number Diff line
@@ -241,86 +241,11 @@ GLOBAL(entry_SYSCALL_64_after_hwframe) 


	TRACE_IRQS_OFF



	/*

	 * If we need to do entry work or if we guess we'll need to do

	 * exit work, go straight to the slow path.

	 */

	movq	PER_CPU_VAR(current_task), %r11

	testl	$_TIF_WORK_SYSCALL_ENTRY|_TIF_ALLWORK_MASK, TASK_TI_flags(%r11)

	jnz	entry_SYSCALL64_slow_path



entry_SYSCALL_64_fastpath:

	/*

	 * Easy case: enable interrupts and issue the syscall.  If the syscall

	 * needs pt_regs, we'll call a stub that disables interrupts again

	 * and jumps to the slow path.

	 */

	TRACE_IRQS_ON

	ENABLE_INTERRUPTS(CLBR_NONE)

#if __SYSCALL_MASK == ~0

	cmpq	$__NR_syscall_max, %rax

#else

	andl	$__SYSCALL_MASK, %eax

	cmpl	$__NR_syscall_max, %eax

#endif

	ja	1f				/* return -ENOSYS (already in pt_regs->ax) */

	movq	%r10, %rcx



	/*

	 * This call instruction is handled specially in stub_ptregs_64.

	 * It might end up jumping to the slow path.  If it jumps, RAX

	 * and all argument registers are clobbered.

	 */

#ifdef CONFIG_RETPOLINE

	movq	sys_call_table(, %rax, 8), %rax

	call	__x86_indirect_thunk_rax

#else

	call	*sys_call_table(, %rax, 8)

#endif

.Lentry_SYSCALL_64_after_fastpath_call:



	movq	%rax, RAX(%rsp)

1:



	/*

	 * If we get here, then we know that pt_regs is clean for SYSRET64.

	 * If we see that no exit work is required (which we are required

	 * to check with IRQs off), then we can go straight to SYSRET64.

	 */

	DISABLE_INTERRUPTS(CLBR_ANY)

	TRACE_IRQS_OFF

	movq	PER_CPU_VAR(current_task), %r11

	testl	$_TIF_ALLWORK_MASK, TASK_TI_flags(%r11)

	jnz	1f



	LOCKDEP_SYS_EXIT

	TRACE_IRQS_ON		/* user mode is traced as IRQs on */

	movq	RIP(%rsp), %rcx

	movq	EFLAGS(%rsp), %r11

	addq	$6*8, %rsp	/* skip extra regs -- they were preserved */

	UNWIND_HINT_EMPTY

	jmp	.Lpop_c_regs_except_rcx_r11_and_sysret



1:

	/*

	 * The fast path looked good when we started, but something changed

	 * along the way and we need to switch to the slow path.  Calling

	 * raise(3) will trigger this, for example.  IRQs are off.

	 */

	TRACE_IRQS_ON

	ENABLE_INTERRUPTS(CLBR_ANY)

	SAVE_EXTRA_REGS

	movq	%rsp, %rdi

	call	syscall_return_slowpath	/* returns with IRQs disabled */

	jmp	return_from_SYSCALL_64



entry_SYSCALL64_slow_path:

	/* IRQs are off. */

	SAVE_EXTRA_REGS

	movq	%rsp, %rdi

	call	do_syscall_64		/* returns with IRQs disabled */



return_from_SYSCALL_64:

	TRACE_IRQS_IRETQ		/* we're about to change IF */



	/*
@@ -393,7 +318,6 @@ syscall_return_via_sysret: 
	/* rcx and r11 are already restored (see code above) */

	UNWIND_HINT_EMPTY

	POP_EXTRA_REGS

.Lpop_c_regs_except_rcx_r11_and_sysret:

	popq	%rsi	/* skip r11 */

	popq	%r10

	popq	%r9
@@ -424,47 +348,6 @@ syscall_return_via_sysret: 
	USERGS_SYSRET64

END(entry_SYSCALL_64)



ENTRY(stub_ptregs_64)

	/*

	 * Syscalls marked as needing ptregs land here.

	 * If we are on the fast path, we need to save the extra regs,

	 * which we achieve by trying again on the slow path.  If we are on

	 * the slow path, the extra regs are already saved.

	 *

	 * RAX stores a pointer to the C function implementing the syscall.

	 * IRQs are on.

	 */

	cmpq	$.Lentry_SYSCALL_64_after_fastpath_call, (%rsp)

	jne	1f



	/*

	 * Called from fast path -- disable IRQs again, pop return address

	 * and jump to slow path

	 */

	DISABLE_INTERRUPTS(CLBR_ANY)

	TRACE_IRQS_OFF

	popq	%rax

	UNWIND_HINT_REGS extra=0

	jmp	entry_SYSCALL64_slow_path



1:

	JMP_NOSPEC %rax				/* Called from C */

END(stub_ptregs_64)



.macro ptregs_stub func

ENTRY(ptregs_\func)

	UNWIND_HINT_FUNC

	leaq	\func(%rip), %rax

	jmp	stub_ptregs_64

END(ptregs_\func)

.endm



/* Instantiate ptregs_stub for each ptregs-using syscall */

#define __SYSCALL_64_QUAL_(sym)

#define __SYSCALL_64_QUAL_ptregs(sym) ptregs_stub sym

#define __SYSCALL_64(nr, sym, qual) __SYSCALL_64_QUAL_##qual(sym)

#include <asm/syscalls_64.h>



/*

 * %rdi: prev task

 * %rsi: next task

arch/x86/entry/syscall_64.c

+2 −5
Original line number Diff line number Diff line
@@ -7,14 +7,11 @@ 
#include <asm/asm-offsets.h>

#include <asm/syscall.h>



#define __SYSCALL_64_QUAL_(sym) sym

#define __SYSCALL_64_QUAL_ptregs(sym) ptregs_##sym



#define __SYSCALL_64(nr, sym, qual) extern asmlinkage long __SYSCALL_64_QUAL_##qual(sym)(unsigned long, unsigned long, unsigned long, unsigned long, unsigned long, unsigned long);

#define __SYSCALL_64(nr, sym, qual) extern asmlinkage long sym(unsigned long, unsigned long, unsigned long, unsigned long, unsigned long, unsigned long);

#include <asm/syscalls_64.h>

#undef __SYSCALL_64



#define __SYSCALL_64(nr, sym, qual) [nr] = __SYSCALL_64_QUAL_##qual(sym),

#define __SYSCALL_64(nr, sym, qual) [nr] = sym,



extern long sys_ni_syscall(unsigned long, unsigned long, unsigned long, unsigned long, unsigned long, unsigned long);

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