From 539f5113650068ba221197f190267ab727296ef5 Mon Sep 17 00:00:00 2001 From: Andy Lutomirski Date: Tue, 9 Jun 2015 12:36:01 -0700 Subject: [PATCH] x86/asm/entry/64: Disentangle error_entry/exit gsbase/ebx/usermode code The error_entry/error_exit code to handle gsbase and whether we return to user mdoe was a mess: - error_sti was misnamed. In particular, it did not enable interrupts. - Error handling for gs_change was hopelessly tangled the normal usermode path. Separate it out. This saves a branch in normal entries from kernel mode. - The comments were bad. Fix it up. As a nice side effect, there's now a code path that happens on error entries from user mode. We'll use it soon. Signed-off-by: Andy Lutomirski Cc: Andrew Morton Cc: Andy Lutomirski Cc: Borislav Petkov Cc: Brian Gerst Cc: Denys Vlasenko Cc: H. Peter Anvin Cc: Linus Torvalds Cc: Peter Zijlstra Cc: Thomas Gleixner Link: http://lkml.kernel.org/r/f1be898ab93360169fb845ab85185948832209ee.1433878454.git.luto@kernel.org [ Prettified it, clarified comments some more. ] Signed-off-by: Ingo Molnar --- arch/x86/entry/entry_64.S | 39 +++++++++++++++++++++++++++++++-------- 1 file changed, 31 insertions(+), 8 deletions(-) diff --git a/arch/x86/entry/entry_64.S b/arch/x86/entry/entry_64.S index bd97161..3bb2c43 100644 --- a/arch/x86/entry/entry_64.S +++ b/arch/x86/entry/entry_64.S @@ -1135,7 +1135,7 @@ END(paranoid_exit) /* * Save all registers in pt_regs, and switch gs if needed. - * Return: ebx=0: need swapgs on exit, ebx=1: otherwise + * Return: EBX=0: came from user mode; EBX=1: otherwise */ ENTRY(error_entry) cld @@ -1144,9 +1144,11 @@ ENTRY(error_entry) xorl %ebx, %ebx testb $3, CS+8(%rsp) jz error_kernelspace -error_swapgs: + + /* We entered from user mode */ SWAPGS -error_sti: + +error_entry_done: TRACE_IRQS_OFF ret @@ -1165,8 +1167,15 @@ error_kernelspace: cmpq %rax, RIP+8(%rsp) je bstep_iret cmpq $gs_change, RIP+8(%rsp) - je error_swapgs - jmp error_sti + jne error_entry_done + + /* + * hack: gs_change can fail with user gsbase. If this happens, fix up + * gsbase and proceed. We'll fix up the exception and land in + * gs_change's error handler with kernel gsbase. + */ + SWAPGS + jmp error_entry_done bstep_iret: /* Fix truncated RIP */ @@ -1174,16 +1183,30 @@ bstep_iret: /* fall through */ error_bad_iret: + /* + * We came from an IRET to user mode, so we have user gsbase. + * Switch to kernel gsbase: + */ SWAPGS + + /* + * Pretend that the exception came from user mode: set up pt_regs + * as if we faulted immediately after IRET and clear EBX so that + * error_exit knows that we will be returning to user mode. + */ mov %rsp, %rdi call fixup_bad_iret mov %rax, %rsp - decl %ebx /* Return to usergs */ - jmp error_sti + decl %ebx + jmp error_entry_done END(error_entry) -/* On entry, ebx is "no swapgs" flag (1: don't need swapgs, 0: need it) */ +/* + * On entry, EBS is a "return to kernel mode" flag: + * 1: already in kernel mode, don't need SWAPGS + * 0: user gsbase is loaded, we need SWAPGS and standard preparation for return to usermode + */ ENTRY(error_exit) movl %ebx, %eax RESTORE_EXTRA_REGS -- 2.7.4