[PATCH v3 3/5] x86/enter: Use IBRS on syscall and interrupts

From: Tim Chen
Date: Tue Jan 09 2018 - 21:47:44 EST

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Set IBRS upon kernel entrance via syscall and interrupts. Clear it
upon exit. IBRS protects against unsafe indirect branching predictions
in the kernel.

The NMI interrupt save/restore of IBRS state was based on Andrea
Arcangeli's implementation.
Here's an explanation by Dave Hansen on why we save IBRS state for NMI.

The normal interrupt code uses the 'error_entry' path which uses the
Code Segment (CS) of the instruction that was interrupted to tell
whether it interrupted the kernel or userspace and thus has to switch
IBRS, or leave it alone.

The NMI code is different. It uses 'paranoid_entry' because it can
interrupt the kernel while it is running with a userspace IBRS (and %GS
and CR3) value, but has a kernel CS. If we used the same approach as
the normal interrupt code, we might do the following;

SYSENTER_entry
<-------------- NMI HERE
IBRS=1
do_something()
IBRS=0
SYSRET

The NMI code might notice that we are running in the kernel and decide
that it is OK to skip the IBRS=1. This would leave it running
unprotected with IBRS=0, which is bad.

However, if we unconditionally set IBRS=1, in the NMI, we might get the
following case:

SYSENTER_entry
IBRS=1
do_something()
IBRS=0
<-------------- NMI HERE (set IBRS=1)
SYSRET

and we would return to userspace with IBRS=1. Userspace would run
slowly until we entered and exited the kernel again.

Instead of those two approaches, we chose a third one where we simply
save the IBRS value in a scratch register (%r13) and then restore that
value, verbatim.

Co-developed-by: Andrea Arcangeli <aarcange@xxxxxxxxxx>
Signed-off-by: Andrea Arcangeli <aarcange@xxxxxxxxxx>
Signed-off-by: Tim Chen <tim.c.chen@xxxxxxxxxxxxxxx>
---
arch/x86/entry/entry_64.S | 23 +++++++++++++++++++++++
arch/x86/entry/entry_64_compat.S | 14 +++++++++++++-
2 files changed, 36 insertions(+), 1 deletion(-)

diff --git a/arch/x86/entry/entry_64.S b/arch/x86/entry/entry_64.S
index 58dbf7a..0da9d16 100644
--- a/arch/x86/entry/entry_64.S
+++ b/arch/x86/entry/entry_64.S
@@ -171,6 +171,8 @@ ENTRY(entry_SYSCALL_64_trampoline)

/* Load the top of the task stack into RSP */
movq CPU_ENTRY_AREA_tss + TSS_sp1 + CPU_ENTRY_AREA, %rsp
+ /* Stack is usable, use the non-clobbering IBRS enable: */
+ ENABLE_IBRS

/* Start building the simulated IRET frame. */
pushq $__USER_DS /* pt_regs->ss */
@@ -214,6 +216,8 @@ ENTRY(entry_SYSCALL_64)
*/
movq %rsp, PER_CPU_VAR(rsp_scratch)
movq PER_CPU_VAR(cpu_current_top_of_stack), %rsp
+ /* Stack is usable, use the non-clobbering IBRS enable: */
+ ENABLE_IBRS

TRACE_IRQS_OFF

@@ -413,6 +417,7 @@ syscall_return_via_sysret:
* We are on the trampoline stack. All regs except RDI are live.
* We can do future final exit work right here.
*/
+ DISABLE_IBRS
SWITCH_TO_USER_CR3_STACK scratch_reg=%rdi

popq %rdi
@@ -768,6 +773,7 @@ GLOBAL(swapgs_restore_regs_and_return_to_usermode)
* We can do future final exit work right here.
*/

+ DISABLE_IBRS
SWITCH_TO_USER_CR3_STACK scratch_reg=%rdi

/* Restore RDI. */
@@ -855,6 +861,14 @@ native_irq_return_ldt:
SWAPGS /* to kernel GS */
SWITCH_TO_KERNEL_CR3 scratch_reg=%rdi /* to kernel CR3 */

+ /*
+ * Normally we enable IBRS when we switch to kernel's CR3.
+ * But we are going to switch back to user CR3 immediately
+ * in this routine after fixing ESPFIX stack. There is
+ * no vulnerable code branching for IBRS to protect.
+ * We don't toggle IBRS to avoid the cost of two MSR writes.
+ */
+
movq PER_CPU_VAR(espfix_waddr), %rdi
movq %rax, (0*8)(%rdi) /* user RAX */
movq (1*8)(%rsp), %rax /* user RIP */
@@ -988,6 +1002,8 @@ ENTRY(switch_to_thread_stack)
SWITCH_TO_KERNEL_CR3 scratch_reg=%rdi
movq %rsp, %rdi
movq PER_CPU_VAR(cpu_current_top_of_stack), %rsp
+ /* Stack is usable, use the non-clobbering IBRS enable: */
+ ENABLE_IBRS
UNWIND_HINT sp_offset=16 sp_reg=ORC_REG_DI

pushq 7*8(%rdi) /* regs->ss */
@@ -1288,6 +1304,7 @@ ENTRY(paranoid_entry)

1:
SAVE_AND_SWITCH_TO_KERNEL_CR3 scratch_reg=%rax save_reg=%r14
+ ENABLE_IBRS_SAVE_AND_CLOBBER save_reg=%r13d

ret
END(paranoid_entry)
@@ -1311,6 +1328,7 @@ ENTRY(paranoid_exit)
testl %ebx, %ebx /* swapgs needed? */
jnz .Lparanoid_exit_no_swapgs
TRACE_IRQS_IRETQ
+ RESTORE_IBRS_CLOBBER save_reg=%r13d
RESTORE_CR3 scratch_reg=%rbx save_reg=%r14
SWAPGS_UNSAFE_STACK
jmp .Lparanoid_exit_restore
@@ -1341,6 +1359,7 @@ ENTRY(error_entry)
SWAPGS
/* We have user CR3. Change to kernel CR3. */
SWITCH_TO_KERNEL_CR3 scratch_reg=%rax
+ ENABLE_IBRS_CLOBBER

.Lerror_entry_from_usermode_after_swapgs:
/* Put us onto the real thread stack. */
@@ -1388,6 +1407,7 @@ ENTRY(error_entry)
*/
SWAPGS
SWITCH_TO_KERNEL_CR3 scratch_reg=%rax
+ ENABLE_IBRS_CLOBBER
jmp .Lerror_entry_done

.Lbstep_iret:
@@ -1402,6 +1422,7 @@ ENTRY(error_entry)
*/
SWAPGS
SWITCH_TO_KERNEL_CR3 scratch_reg=%rax
+ ENABLE_IBRS

/*
* Pretend that the exception came from user mode: set up pt_regs
@@ -1503,6 +1524,7 @@ ENTRY(nmi)
SWITCH_TO_KERNEL_CR3 scratch_reg=%rdx
movq %rsp, %rdx
movq PER_CPU_VAR(cpu_current_top_of_stack), %rsp
+ ENABLE_IBRS
UNWIND_HINT_IRET_REGS base=%rdx offset=8
pushq 5*8(%rdx) /* pt_regs->ss */
pushq 4*8(%rdx) /* pt_regs->rsp */
@@ -1753,6 +1775,7 @@ end_repeat_nmi:
movq $-1, %rsi
call do_nmi

+ RESTORE_IBRS_CLOBBER save_reg=%r13d
RESTORE_CR3 scratch_reg=%r15 save_reg=%r14

testl %ebx, %ebx /* swapgs needed? */
diff --git a/arch/x86/entry/entry_64_compat.S b/arch/x86/entry/entry_64_compat.S
index 98d5358..8aef23c 100644
--- a/arch/x86/entry/entry_64_compat.S
+++ b/arch/x86/entry/entry_64_compat.S
@@ -54,6 +54,7 @@ ENTRY(entry_SYSENTER_compat)
SWITCH_TO_KERNEL_CR3 scratch_reg=%rsp

movq PER_CPU_VAR(cpu_current_top_of_stack), %rsp
+ ENABLE_IBRS

/*
* User tracing code (ptrace or signal handlers) might assume that
@@ -225,7 +226,12 @@ GLOBAL(entry_SYSCALL_compat_after_hwframe)
pushq $0 /* pt_regs->r15 = 0 */

/*
- * User mode is traced as though IRQs are on, and SYSENTER
+ * We just saved %rdi so it is safe to clobber. It is not
+ * preserved during the C calls inside TRACE_IRQS_OFF anyway.
+ */
+ ENABLE_IBRS_CLOBBER /* clobbers %rax, %rcx, %rdx */
+
+ /* User mode is traced as though IRQs are on, and SYSENTER
* turned them off.
*/
TRACE_IRQS_OFF
@@ -239,6 +245,12 @@ GLOBAL(entry_SYSCALL_compat_after_hwframe)
/* Opportunistic SYSRET */
sysret32_from_system_call:
TRACE_IRQS_ON /* User mode traces as IRQs on. */
+ /*
+ * Clobber of %rax, %rcx, %rdx is OK before register restoring.
+ * This is safe to do here because we have no indirect branches
+ * between here and the return to userspace (sysretl).
+ */
+ DISABLE_IBRS_CLOBBER
movq RBX(%rsp), %rbx /* pt_regs->rbx */
movq RBP(%rsp), %rbp /* pt_regs->rbp */
movq EFLAGS(%rsp), %r11 /* pt_regs->flags (in r11) */
--
2.9.4

Next message: Tim Chen: "[PATCH v3 4/5] x86/ibrs: Create boot option for IBRS"
Previous message: Tim Chen: "[PATCH v3 2/5] x86/enter: Create macros to set/clear IBRS"
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Next in thread: Peter Zijlstra: "Re: [PATCH v3 3/5] x86/enter: Use IBRS on syscall and interrupts"
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