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Subject[PATCH v5 04/12] x86/retpoline/entry: Convert entry assembler indirect jumps
Date
Convert indirect jumps in core 32/64bit entry assembler code to use
non-speculative sequences when CONFIG_RETPOLINE is enabled.

Don't use NOSPEC_CALL in entry_SYSCALL_64_fastpath because the return
address after the 'call' instruction must be *precisely* at the
.Lentry_SYSCALL_64_after_fastpath label for stub_ptregs_64 to work,
and the use of alternatives will mess that up unless we play horrid
games to prepend with NOPs and make the variants the same length. It's
not worth it; in the case where we ALTERNATIVE out the retpoline, the
first instruction at __x86.indirect_thunk.rax is going to be a bare
jmp *%rax anyway.

Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>
---
arch/x86/entry/entry_32.S | 5 +++--
arch/x86/entry/entry_64.S | 12 +++++++++---
2 files changed, 12 insertions(+), 5 deletions(-)

diff --git a/arch/x86/entry/entry_32.S b/arch/x86/entry/entry_32.S
index ace8f32..cf9ef33 100644
--- a/arch/x86/entry/entry_32.S
+++ b/arch/x86/entry/entry_32.S
@@ -44,6 +44,7 @@
#include <asm/asm.h>
#include <asm/smap.h>
#include <asm/frame.h>
+#include <asm/nospec-branch.h>

.section .entry.text, "ax"

@@ -290,7 +291,7 @@ ENTRY(ret_from_fork)

/* kernel thread */
1: movl %edi, %eax
- call *%ebx
+ NOSPEC_CALL %ebx
/*
* A kernel thread is allowed to return here after successfully
* calling do_execve(). Exit to userspace to complete the execve()
@@ -919,7 +920,7 @@ common_exception:
movl %ecx, %es
TRACE_IRQS_OFF
movl %esp, %eax # pt_regs pointer
- call *%edi
+ NOSPEC_CALL %edi
jmp ret_from_exception
END(common_exception)

diff --git a/arch/x86/entry/entry_64.S b/arch/x86/entry/entry_64.S
index ed31d00..2a2bb98 100644
--- a/arch/x86/entry/entry_64.S
+++ b/arch/x86/entry/entry_64.S
@@ -37,6 +37,7 @@
#include <asm/pgtable_types.h>
#include <asm/export.h>
#include <asm/frame.h>
+#include <asm/nospec-branch.h>
#include <linux/err.h>

#include "calling.h"
@@ -187,7 +188,7 @@ ENTRY(entry_SYSCALL_64_trampoline)
*/
pushq %rdi
movq $entry_SYSCALL_64_stage2, %rdi
- jmp *%rdi
+ NOSPEC_JMP %rdi
END(entry_SYSCALL_64_trampoline)

.popsection
@@ -266,7 +267,12 @@ entry_SYSCALL_64_fastpath:
* 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)
@@ -438,7 +444,7 @@ ENTRY(stub_ptregs_64)
jmp entry_SYSCALL64_slow_path

1:
- jmp *%rax /* Called from C */
+ NOSPEC_JMP %rax /* Called from C */
END(stub_ptregs_64)

.macro ptregs_stub func
@@ -517,7 +523,7 @@ ENTRY(ret_from_fork)
1:
/* kernel thread */
movq %r12, %rdi
- call *%rbx
+ NOSPEC_CALL %rbx
/*
* A kernel thread is allowed to return here after successfully
* calling do_execve(). Exit to userspace to complete the execve()
--
2.7.4
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