[CodeGen] Don't resolve the stack protector frame accesses until PEI

Currently, stack protector loads and stores are resolved during
LocalStackSlotAllocation (if the pass needs to run). When this is the
case, the base register assigned to the frame access is going to be one
of the vregs created during LocalStackSlotAllocation. This means that we
are keeping a pointer to the stack protector slot, and we're using this
pointer to load and store to it.

In case register pressure goes up, we may end up spilling this pointer
to the stack, which can be a security concern.

Instead, leave it to PEI to resolve the frame accesses. In order to do
that, we make all stack protector accesses go through frame index
operands, then PEI will resolve this using an offset from sp/fp/bp.

Differential Revision: https://reviews.llvm.org/D64759

llvm-svn: 367068

llvm/lib/CodeGen/LocalStackSlotAllocation.cpp

@@ -351,6 +351,14 @@ bool LocalStackSlotPass::insertFrameReferenceRegisters(MachineFunction &Fn) {
     assert(MFI.isObjectPreAllocated(FrameIdx) &&
            "Only pre-allocated locals expected!");
 
+    // We need to keep the references to the stack protector slot through frame
+    // index operands so that it gets resolved by PEI rather than this pass.
+    // This avoids accesses to the stack protector though virtual base
+    // registers, and forces PEI to address it using fp/sp/bp.
+    if (MFI.hasStackProtectorIndex() &&
+        FrameIdx == MFI.getStackProtectorIndex())
+      continue;
+
     LLVM_DEBUG(dbgs() << "Considering: " << MI);
 
     unsigned idx = 0;

llvm/test/CodeGen/AArch64/stack-guard-reassign.ll

@@ -3,7 +3,6 @@
 ; Verify that the offset assigned to the stack protector is at the top of the
 ; frame, covering the locals.
 ; CHECK-LABEL: fn:
-; CHECK:      sub x8, x29, #24
-; CHECK-NEXT: adrp x9, __stack_chk_guard
-; CHECK-NEXT: ldr x9, [x9, :lo12:__stack_chk_guard]
-; CHECK-NEXT: str x9, [x8]
+; CHECK:      adrp x8, __stack_chk_guard
+; CHECK-NEXT: ldr x8, [x8, :lo12:__stack_chk_guard]
+; CHECK-NEXT: stur x8, [x29, #-24]

llvm/test/CodeGen/ARM/stack-guard-reassign.ll

@@ -5,10 +5,9 @@
 ; CHECK-LABEL: fn:
 ; CHECK:      sub sp, sp, #32
 ; CHECK-NEXT: sub sp, sp, #65536
+; CHECK-NEXT: ldr r1, .LCPI0_0
+; CHECK-NEXT: ldr r2, [r1]
 ; CHECK-NEXT: add lr, sp, #65536
-; CHECK-NEXT: add r1, lr, #28
-; CHECK-NEXT: ldr r2, .LCPI0_0
-; CHECK-NEXT: ldr r3, [r2]
-; CHECK-NEXT: str r3, [r1]
+; CHECK-NEXT: str r2, [lr, #28]
 ; CHECK: .LCPI0_0:
 ; CHECK-NEXT: .long __stack_chk_guard

llvm/test/CodeGen/PowerPC/stack-guard-reassign.ll

@@ -9,9 +9,8 @@
 ; CHECK-NEXT: ori 0, 0, 65488
 ; CHECK-NEXT: stwux 1, 1, 0
 ; CHECK-NEXT: subf 0, 0, 1
-; CHECK-NEXT: lis 4, 1
-; CHECK-NEXT: ori 4, 4, 44
-; CHECK-NEXT: add 4, 1, 4
-; CHECK-NEXT: lis 5, __stack_chk_guard@ha
-; CHECK-NEXT: lwz 6, __stack_chk_guard@l(5)
-; CHECK-NEXT: stw 6, 0(4)
+; CHECK-NEXT: lis 4, __stack_chk_guard@ha
+; CHECK-NEXT: lwz 5, __stack_chk_guard@l(4)
+; CHECK-NEXT: lis 6, 1
+; CHECK-NEXT: ori 6, 6, 44
+; CHECK-NEXT: stwx 5, 1, 6

llvm/test/CodeGen/Thumb/stack_guard_remat.ll

@@ -2,25 +2,40 @@
 ; RUN: llc < %s -mtriple=thumb-apple-darwin -relocation-model=static -no-integrated-as | FileCheck %s -check-prefix=NO-PIC  -check-prefix=STATIC
 ; RUN: llc < %s -mtriple=thumb-apple-darwin -relocation-model=dynamic-no-pic -no-integrated-as | FileCheck %s  -check-prefix=NO-PIC -check-prefix=DYNAMIC-NO-PIC
 
-;PIC:   foo2
-;PIC:   ldr [[R0:r[0-9]+]], [[LABEL0:LCPI[0-9_]+]]
-;PIC: [[LABEL1:LPC[0-9_]+]]:
-;PIC:   add [[R0]], pc
-;PIC:   ldr [[R1:r[0-9]+]], {{\[}}[[R0]]{{\]}}
-;PIC:   ldr [[R1:r[0-9]+]], {{\[}}[[R1]]{{\]}}
+;PIC:        foo2
+;PIC:        ldr [[SAVED_GUARD:r[0-9]+]], [[GUARD_STACK_OFFSET:LCPI[0-9_]+]]
+;PIC-NEXT:   add [[SAVED_GUARD]], sp
+;PIC-NEXT:   ldr [[SAVED_GUARD]], {{\[}}[[SAVED_GUARD]]{{\]}}
+;PIC-NEXT:   ldr [[ORIGINAL_GUARD:r[0-9]+]], [[ORIGINAL_GUARD_LABEL:LCPI[0-9_]+]]
+;PIC-NEXT: [[LABEL1:LPC[0-9_]+]]:
+;PIC-NEXT:   add [[ORIGINAL_GUARD]], pc
+;PIC-NEXT:   ldr [[ORIGINAL_GUARD]], {{\[}}[[ORIGINAL_GUARD]]{{\]}}
+;PIC-NEXT:   ldr [[ORIGINAL_GUARD]], {{\[}}[[ORIGINAL_GUARD]]{{\]}}
+;PIC-NEXT:   subs {{r[0-9]+}}, [[ORIGINAL_GUARD]], [[SAVED_GUARD]]
 
-;PIC:      [[LABEL0]]:
+;PIC:      [[GUARD_STACK_OFFSET]]:
+;PIC-NEXT:   .long 1028
+;PIC:      [[ORIGINAL_GUARD_LABEL]]:
 ;PIC-NEXT:   .long L___stack_chk_guard$non_lazy_ptr-([[LABEL1]]+4)
 
 ;NO-PIC:   foo2
-;NO-PIC:   ldr [[R0:r[0-9]+]], [[LABEL0:LCPI[0-9_]+]]
+;NO-PIC:                ldr [[SAVED_GUARD:r[0-9]+]], [[GUARD_STACK_OFFSET:LCPI[0-9_]+]]
+;NO-PIC-NEXT:           add [[SAVED_GUARD]], sp
+;NO-PIC-NEXT:           ldr [[SAVED_GUARD]], {{\[}}[[SAVED_GUARD]]{{\]}}
+;NO-PIC-NEXT:           ldr [[ORIGINAL_GUARD:r[0-9]+]], [[ORIGINAL_GUARD_LABEL:LCPI[0-9_]+]]
 ;NO-PIC-NOT: LPC
-;NO-PIC:   ldr {{r[0-9]+}}, {{\[}}[[R0]]{{\]}}
+;NO-PIC-NEXT:           ldr [[ORIGINAL_GUARD]], {{\[}}[[ORIGINAL_GUARD]]{{\]}}
+;DYNAMIC-NO-PIC-NEXT:   ldr [[ORIGINAL_GUARD]], {{\[}}[[ORIGINAL_GUARD]]{{\]}}
+;NO-PIC-NEXT:           subs {{r[0-9]+}}, [[ORIGINAL_GUARD]], [[SAVED_GUARD]]
 
-;STATIC:      [[LABEL0]]:
+;STATIC:      [[GUARD_STACK_OFFSET]]:
+;STATIC-NEXT:   .long 1028
+;STATIC:      [[ORIGINAL_GUARD_LABEL]]:
 ;STATIC-NEXT:   .long ___stack_chk_guard
 
-;DYNAMIC-NO-PIC:      [[LABEL0]]:
+;DYNAMIC-NO-PIC:      [[GUARD_STACK_OFFSET]]:
+;DYNAMIC-NO-PIC-NEXT:   .long 1028
+;DYNAMIC-NO-PIC:      [[ORIGINAL_GUARD_LABEL]]:
 ;DYNAMIC-NO-PIC-NEXT:   .long L___stack_chk_guard$non_lazy_ptr
 
 ; Function Attrs: nounwind ssp