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Subject[PATCH 4.14 037/159] x86/entry/64: Move SWAPGS into the common IRET-to-usermode path
Date
4.14-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Andy Lutomirski <luto@kernel.org>

commit 8a055d7f411d41755ce30db5bb65b154777c4b78 upstream.

All of the code paths that ended up doing IRET to usermode did
SWAPGS immediately beforehand. Move the SWAPGS into the common
code.

Signed-off-by: Andy Lutomirski <luto@kernel.org>
Cc: Borislav Petkov <bpetkov@suse.de>
Cc: Brian Gerst <brgerst@gmail.com>
Cc: Dave Hansen <dave.hansen@intel.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/27fd6f45b7cd640de38fb9066fd0349bcd11f8e1.1509609304.git.luto@kernel.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
arch/x86/entry/entry_64.S | 32 ++++++++++++++------------------
arch/x86/entry/entry_64_compat.S | 3 +--
2 files changed, 15 insertions(+), 20 deletions(-)

--- a/arch/x86/entry/entry_64.S
+++ b/arch/x86/entry/entry_64.S
@@ -250,12 +250,14 @@ return_from_SYSCALL_64:

/*
* Try to use SYSRET instead of IRET if we're returning to
- * a completely clean 64-bit userspace context.
+ * a completely clean 64-bit userspace context. If we're not,
+ * go to the slow exit path.
*/
movq RCX(%rsp), %rcx
movq RIP(%rsp), %r11
- cmpq %rcx, %r11 /* RCX == RIP */
- jne opportunistic_sysret_failed
+
+ cmpq %rcx, %r11 /* SYSRET requires RCX == RIP */
+ jne swapgs_restore_regs_and_return_to_usermode

/*
* On Intel CPUs, SYSRET with non-canonical RCX/RIP will #GP
@@ -273,14 +275,14 @@ return_from_SYSCALL_64:

/* If this changed %rcx, it was not canonical */
cmpq %rcx, %r11
- jne opportunistic_sysret_failed
+ jne swapgs_restore_regs_and_return_to_usermode

cmpq $__USER_CS, CS(%rsp) /* CS must match SYSRET */
- jne opportunistic_sysret_failed
+ jne swapgs_restore_regs_and_return_to_usermode

movq R11(%rsp), %r11
cmpq %r11, EFLAGS(%rsp) /* R11 == RFLAGS */
- jne opportunistic_sysret_failed
+ jne swapgs_restore_regs_and_return_to_usermode

/*
* SYSCALL clears RF when it saves RFLAGS in R11 and SYSRET cannot
@@ -301,12 +303,12 @@ return_from_SYSCALL_64:
* would never get past 'stuck_here'.
*/
testq $(X86_EFLAGS_RF|X86_EFLAGS_TF), %r11
- jnz opportunistic_sysret_failed
+ jnz swapgs_restore_regs_and_return_to_usermode

/* nothing to check for RSP */

cmpq $__USER_DS, SS(%rsp) /* SS must match SYSRET */
- jne opportunistic_sysret_failed
+ jne swapgs_restore_regs_and_return_to_usermode

/*
* We win! This label is here just for ease of understanding
@@ -319,10 +321,6 @@ syscall_return_via_sysret:
movq RSP(%rsp), %rsp
UNWIND_HINT_EMPTY
USERGS_SYSRET64
-
-opportunistic_sysret_failed:
- SWAPGS
- jmp restore_regs_and_return_to_usermode
END(entry_SYSCALL_64)

ENTRY(stub_ptregs_64)
@@ -423,8 +421,7 @@ ENTRY(ret_from_fork)
movq %rsp, %rdi
call syscall_return_slowpath /* returns with IRQs disabled */
TRACE_IRQS_ON /* user mode is traced as IRQS on */
- SWAPGS
- jmp restore_regs_and_return_to_usermode
+ jmp swapgs_restore_regs_and_return_to_usermode

1:
/* kernel thread */
@@ -612,9 +609,8 @@ GLOBAL(retint_user)
mov %rsp,%rdi
call prepare_exit_to_usermode
TRACE_IRQS_IRETQ
- SWAPGS

-GLOBAL(restore_regs_and_return_to_usermode)
+GLOBAL(swapgs_restore_regs_and_return_to_usermode)
#ifdef CONFIG_DEBUG_ENTRY
/* Assert that pt_regs indicates user mode. */
testl $3, CS(%rsp)
@@ -622,6 +618,7 @@ GLOBAL(restore_regs_and_return_to_usermo
ud2
1:
#endif
+ SWAPGS
RESTORE_EXTRA_REGS
RESTORE_C_REGS
REMOVE_PT_GPREGS_FROM_STACK 8
@@ -1343,8 +1340,7 @@ ENTRY(nmi)
* Return back to user mode. We must *not* do the normal exit
* work, because we don't want to enable interrupts.
*/
- SWAPGS
- jmp restore_regs_and_return_to_usermode
+ jmp swapgs_restore_regs_and_return_to_usermode

.Lnmi_from_kernel:
/*
--- a/arch/x86/entry/entry_64_compat.S
+++ b/arch/x86/entry/entry_64_compat.S
@@ -337,8 +337,7 @@ ENTRY(entry_INT80_compat)

/* Go back to user mode. */
TRACE_IRQS_ON
- SWAPGS
- jmp restore_regs_and_return_to_usermode
+ jmp swapgs_restore_regs_and_return_to_usermode
END(entry_INT80_compat)

ENTRY(stub32_clone)

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