* [PATCH] x86/asm/entry/64: Disentangle error_entry/exit gsbase/ebx/usermode code
@ 2015-06-09 19:36 Andy Lutomirski
2015-06-10 7:10 ` [tip:x86/asm] x86/asm/entry/64: Disentangle error_entry/ exit " tip-bot for Andy Lutomirski
0 siblings, 1 reply; 2+ messages in thread
From: Andy Lutomirski @ 2015-06-09 19:36 UTC (permalink / raw
To: x86
Cc: linux-kernel@vger.kernel.org, Denys Vlasenko, Borislav Petkov,
Andy Lutomirski
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 <luto@kernel.org>
---
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 bd97161f90cb..3462a4f09425 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 zero ebx so that
+ * error_exit knows that we will 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, ebx is "return to kernel" flag
+ * 1: don't need swapgs
+ * 0: need swapgs and standard preparation for return to usermode
+ */
ENTRY(error_exit)
movl %ebx, %eax
RESTORE_EXTRA_REGS
--
2.4.2
^ permalink raw reply related [flat|nested] 2+ messages in thread
* [tip:x86/asm] x86/asm/entry/64: Disentangle error_entry/ exit gsbase/ebx/usermode code
2015-06-09 19:36 [PATCH] x86/asm/entry/64: Disentangle error_entry/exit gsbase/ebx/usermode code Andy Lutomirski
@ 2015-06-10 7:10 ` tip-bot for Andy Lutomirski
0 siblings, 0 replies; 2+ messages in thread
From: tip-bot for Andy Lutomirski @ 2015-06-10 7:10 UTC (permalink / raw
To: linux-tip-commits
Cc: linux-kernel, torvalds, akpm, peterz, tglx, mingo, luto, dvlasenk,
luto, brgerst, bp, hpa
Commit-ID: 539f5113650068ba221197f190267ab727296ef5
Gitweb: http://git.kernel.org/tip/539f5113650068ba221197f190267ab727296ef5
Author: Andy Lutomirski <luto@kernel.org>
AuthorDate: Tue, 9 Jun 2015 12:36:01 -0700
Committer: Ingo Molnar <mingo@kernel.org>
CommitDate: Wed, 10 Jun 2015 08:47:57 +0200
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 <luto@kernel.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Brian Gerst <brgerst@gmail.com>
Cc: Denys Vlasenko <dvlasenk@redhat.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/f1be898ab93360169fb845ab85185948832209ee.1433878454.git.luto@kernel.org
[ Prettified it, clarified comments some more. ]
Signed-off-by: Ingo Molnar <mingo@kernel.org>
---
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
^ permalink raw reply related [flat|nested] 2+ messages in thread
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2015-06-09 19:36 [PATCH] x86/asm/entry/64: Disentangle error_entry/exit gsbase/ebx/usermode code Andy Lutomirski
2015-06-10 7:10 ` [tip:x86/asm] x86/asm/entry/64: Disentangle error_entry/ exit " tip-bot for Andy Lutomirski
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