[tip:x86/asm] x86/entry/32: Re-implement SYSENTER using the new C path

From: tip-bot for Andy Lutomirski
Date: Fri Oct 09 2015 - 09:14:10 EST

Commit-ID: 5f310f739b4cc343f3f087681e41bbc2f0ce902d
Gitweb: http://git.kernel.org/tip/5f310f739b4cc343f3f087681e41bbc2f0ce902d
Author: Andy Lutomirski <luto@xxxxxxxxxx>
AuthorDate: Mon, 5 Oct 2015 17:48:15 -0700
Committer: Ingo Molnar <mingo@xxxxxxxxxx>
CommitDate: Fri, 9 Oct 2015 09:41:10 +0200

x86/entry/32: Re-implement SYSENTER using the new C path

Signed-off-by: Andy Lutomirski <luto@xxxxxxxxxx>
Cc: Andy Lutomirski <luto@xxxxxxxxxxxxxx>
Cc: Borislav Petkov <bp@xxxxxxxxx>
Cc: Brian Gerst <brgerst@xxxxxxxxx>
Cc: Denys Vlasenko <dvlasenk@xxxxxxxxxx>
Cc: H. Peter Anvin <hpa@xxxxxxxxx>
Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: linux-kernel@xxxxxxxxxxxxxxx
Link: http://lkml.kernel.org/r/5b99659e8be70f3dd10cd8970a5c90293d9ad9a7.1444091585.git.luto@xxxxxxxxxx
Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>
---
arch/x86/entry/common.c | 17 +++-
arch/x86/entry/entry_32.S | 132 ++++++++-----------------------
arch/x86/entry/vdso/vdso32/system_call.S | 2 +
3 files changed, 51 insertions(+), 100 deletions(-)

diff --git a/arch/x86/entry/common.c b/arch/x86/entry/common.c
index 88dc5ba..0ed023d 100644
--- a/arch/x86/entry/common.c
+++ b/arch/x86/entry/common.c
@@ -363,7 +363,7 @@ __visible void do_int80_syscall_32(struct pt_regs *regs)
syscall_return_slowpath(regs);
}

-/* Returns 0 to return using IRET or 1 to return using SYSRETL. */
+/* Returns 0 to return using IRET or 1 to return using SYSEXIT/SYSRETL. */
__visible long do_fast_syscall_32(struct pt_regs *regs)
{
/*
@@ -417,7 +417,20 @@ __visible long do_fast_syscall_32(struct pt_regs *regs)
regs->ip == landing_pad &&
(regs->flags & (X86_EFLAGS_RF | X86_EFLAGS_TF)) == 0;
#else
- return 0;
+ /*
+ * Opportunistic SYSEXIT: if possible, try to return using SYSEXIT.
+ *
+ * Unlike 64-bit opportunistic SYSRET, we can't check that CX == IP,
+ * because the ECX fixup above will ensure that this is essentially
+ * never the case.
+ *
+ * We don't allow syscalls at all from VM86 mode, but we still
+ * need to check VM, because we might be returning from sys_vm86.
+ */
+ return static_cpu_has(X86_FEATURE_SEP) &&
+ regs->cs == __USER_CS && regs->ss == __USER_DS &&
+ regs->ip == landing_pad &&
+ (regs->flags & (X86_EFLAGS_RF | X86_EFLAGS_TF | X86_EFLAGS_VM)) == 0;
#endif
}
#endif
diff --git a/arch/x86/entry/entry_32.S b/arch/x86/entry/entry_32.S
index 02881e5..c1c7c63 100644
--- a/arch/x86/entry/entry_32.S
+++ b/arch/x86/entry/entry_32.S
@@ -287,76 +287,47 @@ need_resched:
END(resume_kernel)
#endif

-/*
- * SYSENTER_RETURN points to after the SYSENTER instruction
- * in the vsyscall page. See vsyscall-sysentry.S, which defines
- * the symbol.
- */
-
# SYSENTER call handler stub
ENTRY(entry_SYSENTER_32)
movl TSS_sysenter_sp0(%esp), %esp
sysenter_past_esp:
+ pushl $__USER_DS /* pt_regs->ss */
+ pushl %ecx /* pt_regs->cx */
+ pushfl /* pt_regs->flags (except IF = 0) */
+ orl $X86_EFLAGS_IF, (%esp) /* Fix IF */
+ pushl $__USER_CS /* pt_regs->cs */
+ pushl $0 /* pt_regs->ip = 0 (placeholder) */
+ pushl %eax /* pt_regs->orig_ax */
+ SAVE_ALL pt_regs_ax=$-ENOSYS /* save rest */
+
/*
- * Interrupts are disabled here, but we can't trace it until
- * enough kernel state to call TRACE_IRQS_OFF can be called - but
- * we immediately enable interrupts at that point anyway.
- */
- pushl $__USER_DS
- pushl %ebp
- pushfl
- orl $X86_EFLAGS_IF, (%esp)
- pushl $__USER_CS
- /*
- * Push current_thread_info()->sysenter_return to the stack.
- * A tiny bit of offset fixup is necessary: TI_sysenter_return
- * is relative to thread_info, which is at the bottom of the
- * kernel stack page. 4*4 means the 4 words pushed above;
- * TOP_OF_KERNEL_STACK_PADDING takes us to the top of the stack;
- * and THREAD_SIZE takes us to the bottom.
+ * User mode is traced as though IRQs are on, and SYSENTER
+ * turned them off.
*/
- pushl ((TI_sysenter_return) - THREAD_SIZE + TOP_OF_KERNEL_STACK_PADDING + 4*4)(%esp)
-
- pushl %eax
- SAVE_ALL
- ENABLE_INTERRUPTS(CLBR_NONE)
-
-/*
- * Load the potential sixth argument from user stack.
- * Careful about security.
- */
- cmpl $__PAGE_OFFSET-3, %ebp
- jae syscall_fault
- ASM_STAC
-1: movl (%ebp), %ebp
- ASM_CLAC
- movl %ebp, PT_EBP(%esp)
- _ASM_EXTABLE(1b, syscall_fault)
-
- GET_THREAD_INFO(%ebp)
-
- testl $_TIF_WORK_SYSCALL_ENTRY, TI_flags(%ebp)
- jnz syscall_trace_entry
-sysenter_do_call:
- cmpl $(NR_syscalls), %eax
- jae sysenter_badsys
- call *sys_call_table(, %eax, 4)
-sysenter_after_call:
- movl %eax, PT_EAX(%esp)
- LOCKDEP_SYS_EXIT
- DISABLE_INTERRUPTS(CLBR_ANY)
TRACE_IRQS_OFF
- movl TI_flags(%ebp), %ecx
- testl $_TIF_ALLWORK_MASK, %ecx
- jnz syscall_exit_work_irqs_off
-sysenter_exit:
-/* if something modifies registers it must also disable sysexit */
- movl PT_EIP(%esp), %edx
- movl PT_OLDESP(%esp), %ecx
- xorl %ebp, %ebp
- TRACE_IRQS_ON
+
+ movl %esp, %eax
+ call do_fast_syscall_32
+ testl %eax, %eax
+ jz .Lsyscall_32_done
+
+/* Opportunistic SYSEXIT */
+ TRACE_IRQS_ON /* User mode traces as IRQs on. */
+ movl PT_EIP(%esp), %edx /* pt_regs->ip */
+ movl PT_OLDESP(%esp), %ecx /* pt_regs->sp */
+ popl %ebx /* pt_regs->bx */
+ addl $2*4, %esp /* skip pt_regs->cx and pt_regs->dx */
+ popl %esi /* pt_regs->si */
+ popl %edi /* pt_regs->di */
+ popl %ebp /* pt_regs->bp */
+ popl %eax /* pt_regs->ax */
1: mov PT_FS(%esp), %fs
PTGS_TO_GS
+
+ /*
+ * Return back to the vDSO, which will pop ecx and edx.
+ * Don't bother with DS and ES (they already contain __USER_DS).
+ */
ENABLE_INTERRUPTS_SYSEXIT

.pushsection .fixup, "ax"
@@ -371,7 +342,7 @@ ENDPROC(entry_SYSENTER_32)
ENTRY(entry_INT80_32)
ASM_CLAC
pushl %eax /* pt_regs->orig_ax */
- SAVE_ALL pt_regs_ax=$-ENOSYS /* save rest, load -ENOSYS into ax */
+ SAVE_ALL pt_regs_ax=$-ENOSYS /* save rest */

/*
* User mode is traced as though IRQs are on, and the interrupt gate
@@ -381,6 +352,7 @@ ENTRY(entry_INT80_32)

movl %esp, %eax
call do_int80_syscall_32
+.Lsyscall_32_done:

restore_all:
TRACE_IRQS_IRET
@@ -457,42 +429,6 @@ ldt_ss:
#endif
ENDPROC(entry_INT80_32)

- # perform syscall exit tracing
- ALIGN
-syscall_trace_entry:
- movl $-ENOSYS, PT_EAX(%esp)
- movl %esp, %eax
- call syscall_trace_enter
- /* What it returned is what we'll actually use. */
- cmpl $(NR_syscalls), %eax
- jnae syscall_call
- jmp syscall_exit
-END(syscall_trace_entry)
-
- # perform syscall exit tracing
- ALIGN
-syscall_exit_work_irqs_off:
- TRACE_IRQS_ON
- ENABLE_INTERRUPTS(CLBR_ANY)
-
-syscall_exit_work:
- movl %esp, %eax
- call syscall_return_slowpath
- jmp restore_all
-END(syscall_exit_work)
-
-syscall_fault:
- ASM_CLAC
- GET_THREAD_INFO(%ebp)
- movl $-EFAULT, PT_EAX(%esp)
- jmp resume_userspace
-END(syscall_fault)
-
-sysenter_badsys:
- movl $-ENOSYS, %eax
- jmp sysenter_after_call
-END(sysenter_badsys)
-
.macro FIXUP_ESPFIX_STACK
/*
* Switch back for ESPFIX stack to the normal zerobased stack
diff --git a/arch/x86/entry/vdso/vdso32/system_call.S b/arch/x86/entry/vdso/vdso32/system_call.S
index 00157ca..93bd845 100644
--- a/arch/x86/entry/vdso/vdso32/system_call.S
+++ b/arch/x86/entry/vdso/vdso32/system_call.S
@@ -34,6 +34,8 @@ __kernel_vsyscall:
/* If SYSENTER (Intel) or SYSCALL32 (AMD) is available, use it. */
ALTERNATIVE_2 "", "sysenter", X86_FEATURE_SYSENTER32, \
"syscall", X86_FEATURE_SYSCALL32
+#else
+ ALTERNATIVE "", "sysenter", X86_FEATURE_SEP
#endif

/* Enter using int $0x80 */
--
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