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Message-Id: <5bf75d3abce25ec615048075705f120b331aaf0b.1432253793.git.luto@kernel.org>
Date: Thu, 21 May 2015 17:17:32 -0700
From: Andy Lutomirski <luto@...nel.org>
To: x86@...nel.org, "H. Peter Anvin" <hpa@...or.com>,
"H.J. Lu" <hjl.tools@...il.com>
Cc: Borislav Petkov <bp@...en8.de>, Jan Beulich <JBeulich@...e.com>,
Binutils <binutils@...rceware.org>,
"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
Andy Lutomirski <luto@...nel.org>
Subject: [PATCH] x86: Stop relying on magic jmp behavior for early_idt_handlers
The early_idt_handlers asm code generates an array of entry points
spaced nine bytes apart. It's not really clear from that code
or from the places that reference it what's going on, and the code
only works in the first place because gas never generates two-byte
jmp instructions when jumping to global labels.
Clean up the code to generate the correct array stride explicitly.
This should be considerably more robust against screw-ups, as gas
will warn if a .fill directive has a negative count. Using '. =' to
advance would have been even more robust (it would generate an
actual error if it tried to move backwards), but it would pad with
nulls, confusing anyone who tries to disassemble the code. The new
scheme should be much clearer to future readers.
Binutils may start relaxing jumps to non-weak labels. If so, this
change will fix our build, and we may need to backport this change.
Before, on x86_64:
0000000000000000 <early_idt_handlers>:
0: 6a 00 pushq $0x0
2: 6a 00 pushq $0x0
4: e9 00 00 00 00 jmpq 9 <early_idt_handlers+0x9>
5: R_X86_64_PC32 early_idt_handler-0x4
...
48: 66 90 xchg %ax,%ax
4a: 6a 08 pushq $0x8
4c: e9 00 00 00 00 jmpq 51 <early_idt_handlers+0x51>
4d: R_X86_64_PC32 early_idt_handler-0x4
...
117: 6a 00 pushq $0x0
119: 6a 1f pushq $0x1f
11b: e9 00 00 00 00 jmpq 120 <early_idt_handler>
11c: R_X86_64_PC32 early_idt_handler-0x4
After:
0000000000000000 <early_idt_handlers>:
0: 6a 00 pushq $0x0
2: 6a 00 pushq $0x0
4: e9 14 01 00 00 jmpq 11d <early_idt_handler>
...
48: 6a 08 pushq $0x8
4a: e9 d1 00 00 00 jmpq 120 <early_idt_handler>
4f: cc int3
50: cc int3
...
117: 6a 00 pushq $0x0
119: 6a 1f pushq $0x1f
11b: eb 03 jmp 120 <early_idt_handler>
11d: cc int3
11e: cc int3
11f: cc int3
Signed-off-by: Andy Lutomirski <luto@...nel.org>
---
arch/x86/include/asm/segment.h | 13 +++++++++++--
arch/x86/kernel/head_32.S | 12 ++++++------
arch/x86/kernel/head_64.S | 11 ++++++-----
3 files changed, 23 insertions(+), 13 deletions(-)
diff --git a/arch/x86/include/asm/segment.h b/arch/x86/include/asm/segment.h
index 5a9856eb12ba..4bbe0eb24d7e 100644
--- a/arch/x86/include/asm/segment.h
+++ b/arch/x86/include/asm/segment.h
@@ -231,12 +231,21 @@
#define TLS_SIZE (GDT_ENTRY_TLS_ENTRIES* 8)
#ifdef __KERNEL__
-#ifndef __ASSEMBLY__
-extern const char early_idt_handlers[NUM_EXCEPTION_VECTORS][2+2+5];
+/*
+ * early_idt_handlers is an array of entry points. For simplicity, it's
+ * a real array. We allocate nine bytes for each entry: two one-byte
+ * push instructions and a five-byte jump in the worst case.
+ */
+#define EARLY_IDT_HANDLER_STRIDE 9
+#ifndef __ASSEMBLY__
+extern const char early_idt_handlers[NUM_EXCEPTION_VECTORS][EARLY_IDT_HANDLER_STRIDE];
#ifdef CONFIG_TRACING
# define trace_early_idt_handlers early_idt_handlers
#endif
+#endif
+
+#ifndef __ASSEMBLY__
/*
* Load a segment. Fall back on loading the zero
diff --git a/arch/x86/kernel/head_32.S b/arch/x86/kernel/head_32.S
index d031bad9e07e..43154cd4ad62 100644
--- a/arch/x86/kernel/head_32.S
+++ b/arch/x86/kernel/head_32.S
@@ -492,7 +492,7 @@ setup_once:
movl %eax,4(%edi)
/* interrupt gate, dpl=0, present */
movl $(0x8E000000 + __KERNEL_CS),2(%edi)
- addl $9,%eax
+ addl $EARLY_IDT_HANDLER_STRIDE,%eax
addl $8,%edi
loop 1b
@@ -524,6 +524,7 @@ setup_once:
andl $0,setup_once_ref /* Once is enough, thanks */
ret
+/* Build the early_idt_handlers array */
ENTRY(early_idt_handlers)
# 36(%esp) %eflags
# 32(%esp) %cs
@@ -531,19 +532,18 @@ ENTRY(early_idt_handlers)
# 24(%rsp) error code
i = 0
.rept NUM_EXCEPTION_VECTORS
- .if (EXCEPTION_ERRCODE_MASK >> i) & 1
- ASM_NOP2
- .else
+ .fill early_idt_handlers + i * EARLY_IDT_HANDLER_STRIDE - ., 1, 0xcc
+ .ifeq (EXCEPTION_ERRCODE_MASK >> i) & 1
pushl $0 # Dummy error code, to make stack frame uniform
.endif
pushl $i # 20(%esp) Vector number
jmp early_idt_handler
i = i + 1
.endr
+ .fill early_idt_handlers + i * EARLY_IDT_HANDLER_STRIDE - ., 1, 0xcc
ENDPROC(early_idt_handlers)
- /* This is global to keep gas from relaxing the jumps */
-ENTRY(early_idt_handler)
+early_idt_handler:
cld
cmpl $2,(%esp) # X86_TRAP_NMI
diff --git a/arch/x86/kernel/head_64.S b/arch/x86/kernel/head_64.S
index ae6588b301c2..c1874059aadf 100644
--- a/arch/x86/kernel/head_64.S
+++ b/arch/x86/kernel/head_64.S
@@ -320,6 +320,7 @@ ENDPROC(start_cpu0)
bad_address:
jmp bad_address
+/* Build the early_idt_handlers array */
__INIT
.globl early_idt_handlers
early_idt_handlers:
@@ -329,18 +330,18 @@ early_idt_handlers:
# 80(%rsp) error code
i = 0
.rept NUM_EXCEPTION_VECTORS
- .if (EXCEPTION_ERRCODE_MASK >> i) & 1
- ASM_NOP2
- .else
+ .fill early_idt_handlers + i * EARLY_IDT_HANDLER_STRIDE - ., 1, 0xcc
+ .ifeq (EXCEPTION_ERRCODE_MASK >> i) & 1
pushq $0 # Dummy error code, to make stack frame uniform
.endif
pushq $i # 72(%rsp) Vector number
jmp early_idt_handler
i = i + 1
.endr
+ .fill early_idt_handlers + i * EARLY_IDT_HANDLER_STRIDE - ., 1, 0xcc
+ENDPROC(early_idt_handlers)
-/* This is global to keep gas from relaxing the jumps */
-ENTRY(early_idt_handler)
+early_idt_handler:
cld
cmpl $2,(%rsp) # X86_TRAP_NMI
--
2.3.0
--
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