Perpcu uses a clever design where the .percu ELF section has a virtual
address of zero and the relocation code avoid relocating specific
symbols. It makes the code simple and easily adaptable with or without
SMP support.

This design is incompatible with PIE because generated code always try to
access the zero virtual address relative to the default mapping address.
It becomes impossible when KASLR is configured to go below -2G. This
patch solves this problem by removing the zero mapping and adapting the GS
base to be relative to the expected address. These changes are done only
when PIE is enabled. The original implementation is kept as-is
by default.

The assembly and PER_CPU macros are changed to use relative references
when PIE is enabled.

The KALLSYMS_ABSOLUTE_PERCPU configuration is disabled with PIE given
percpu symbols are not absolute in this case.

Position Independent Executable (PIE) support will allow to extended the
KASLR randomization range below the -2G memory limit.

Signed-off-by: Thomas Garnier <thgar...@google.com>
---
 arch/x86/entry/calling.h       |  2 +-
 arch/x86/entry/entry_64.S      |  4 ++--
 arch/x86/include/asm/percpu.h  | 25 +++++++++++++++++++------
 arch/x86/kernel/cpu/common.c   |  4 +++-
 arch/x86/kernel/head_64.S      |  4 ++++
 arch/x86/kernel/setup_percpu.c |  2 +-
 arch/x86/kernel/vmlinux.lds.S  | 13 +++++++++++--
 arch/x86/lib/cmpxchg16b_emu.S  |  8 ++++----
 arch/x86/xen/xen-asm.S         | 12 ++++++------
 init/Kconfig                   |  2 +-
 10 files changed, 52 insertions(+), 24 deletions(-)

diff --git a/arch/x86/entry/calling.h b/arch/x86/entry/calling.h
index be63330c5511..7b48d97111b5 100644
--- a/arch/x86/entry/calling.h
+++ b/arch/x86/entry/calling.h
@@ -216,7 +216,7 @@ For 32-bit we have the following conventions - kernel is 
built with
 .endm
 
 #define THIS_CPU_user_pcid_flush_mask   \
-       PER_CPU_VAR(cpu_tlbstate) + TLB_STATE_user_pcid_flush_mask
+       PER_CPU_VAR(cpu_tlbstate + TLB_STATE_user_pcid_flush_mask)
 
 .macro SWITCH_TO_USER_CR3_NOSTACK scratch_reg:req scratch_reg2:req
        ALTERNATIVE "jmp .Lend_\@", "", X86_FEATURE_PTI
diff --git a/arch/x86/entry/entry_64.S b/arch/x86/entry/entry_64.S
index c53123468364..d34794368e20 100644
--- a/arch/x86/entry/entry_64.S
+++ b/arch/x86/entry/entry_64.S
@@ -358,7 +358,7 @@ ENTRY(__switch_to_asm)
 
 #ifdef CONFIG_CC_STACKPROTECTOR
        movq    TASK_stack_canary(%rsi), %rbx
-       movq    %rbx, PER_CPU_VAR(irq_stack_union)+stack_canary_offset
+       movq    %rbx, PER_CPU_VAR(irq_stack_union + stack_canary_offset)
 #endif
 
 #ifdef CONFIG_RETPOLINE
@@ -896,7 +896,7 @@ apicinterrupt IRQ_WORK_VECTOR                       
irq_work_interrupt              smp_irq_work_interrupt
 /*
  * Exception entry points.
  */
-#define CPU_TSS_IST(x) PER_CPU_VAR(cpu_tss_rw) + (TSS_ist + ((x) - 1) * 8)
+#define CPU_TSS_IST(x) PER_CPU_VAR(cpu_tss_rw + (TSS_ist + ((x) - 1) * 8))
 
 .macro idtentry sym do_sym has_error_code:req paranoid=0 shift_ist=-1
 ENTRY(\sym)
diff --git a/arch/x86/include/asm/percpu.h b/arch/x86/include/asm/percpu.h
index a06b07399d17..7d1271b536ea 100644
--- a/arch/x86/include/asm/percpu.h
+++ b/arch/x86/include/asm/percpu.h
@@ -5,9 +5,11 @@
 #ifdef CONFIG_X86_64
 #define __percpu_seg           gs
 #define __percpu_mov_op                movq
+#define __percpu_rel           (%rip)
 #else
 #define __percpu_seg           fs
 #define __percpu_mov_op                movl
+#define __percpu_rel
 #endif
 
 #ifdef __ASSEMBLY__
@@ -28,10 +30,14 @@
 #define PER_CPU(var, reg)                                              \
        __percpu_mov_op %__percpu_seg:this_cpu_off, reg;                \
        lea var(reg), reg
-#define PER_CPU_VAR(var)       %__percpu_seg:var
+/* Compatible with Position Independent Code */
+#define PER_CPU_VAR(var)               %__percpu_seg:(var)##__percpu_rel
+/* Rare absolute reference */
+#define PER_CPU_VAR_ABS(var)           %__percpu_seg:var
 #else /* ! SMP */
 #define PER_CPU(var, reg)      __percpu_mov_op $var, reg
-#define PER_CPU_VAR(var)       var
+#define PER_CPU_VAR(var)       (var)##__percpu_rel
+#define PER_CPU_VAR_ABS(var)   var
 #endif /* SMP */
 
 #ifdef CONFIG_X86_64_SMP
@@ -209,27 +215,34 @@ do {                                                      
                \
        pfo_ret__;                                      \
 })
 
+/* Position Independent code uses relative addresses only */
+#ifdef CONFIG_X86_PIE
+#define __percpu_stable_arg __percpu_arg(a1)
+#else
+#define __percpu_stable_arg __percpu_arg(P1)
+#endif
+
 #define percpu_stable_op(op, var)                      \
 ({                                                     \
        typeof(var) pfo_ret__;                          \
        switch (sizeof(var)) {                          \
        case 1:                                         \
-               asm(op "b "__percpu_arg(P1)",%0"        \
+               asm(op "b "__percpu_stable_arg ",%0"    \
                    : "=q" (pfo_ret__)                  \
                    : "p" (&(var)));                    \
                break;                                  \
        case 2:                                         \
-               asm(op "w "__percpu_arg(P1)",%0"        \
+               asm(op "w "__percpu_stable_arg ",%0"    \
                    : "=r" (pfo_ret__)                  \
                    : "p" (&(var)));                    \
                break;                                  \
        case 4:                                         \
-               asm(op "l "__percpu_arg(P1)",%0"        \
+               asm(op "l "__percpu_stable_arg ",%0"    \
                    : "=r" (pfo_ret__)                  \
                    : "p" (&(var)));                    \
                break;                                  \
        case 8:                                         \
-               asm(op "q "__percpu_arg(P1)",%0"        \
+               asm(op "q "__percpu_stable_arg ",%0"    \
                    : "=r" (pfo_ret__)                  \
                    : "p" (&(var)));                    \
                break;                                  \
diff --git a/arch/x86/kernel/cpu/common.c b/arch/x86/kernel/cpu/common.c
index 348cf4821240..0e9a87a34f76 100644
--- a/arch/x86/kernel/cpu/common.c
+++ b/arch/x86/kernel/cpu/common.c
@@ -487,7 +487,9 @@ void load_percpu_segment(int cpu)
        loadsegment(fs, __KERNEL_PERCPU);
 #else
        __loadsegment_simple(gs, 0);
-       wrmsrl(MSR_GS_BASE, (unsigned long)per_cpu(irq_stack_union.gs_base, 
cpu));
+       wrmsrl(MSR_GS_BASE,
+              (unsigned long)per_cpu(irq_stack_union.gs_base, cpu) -
+              (unsigned long)__per_cpu_start);
 #endif
        load_stack_canary_segment();
 }
diff --git a/arch/x86/kernel/head_64.S b/arch/x86/kernel/head_64.S
index 48652f3ec46a..87624e1fe22f 100644
--- a/arch/x86/kernel/head_64.S
+++ b/arch/x86/kernel/head_64.S
@@ -266,7 +266,11 @@ ENDPROC(start_cpu0)
        GLOBAL(initial_code)
        .quad   x86_64_start_kernel
        GLOBAL(initial_gs)
+#ifdef CONFIG_X86_PIE
+       .quad   0
+#else
        .quad   INIT_PER_CPU_VAR(irq_stack_union)
+#endif
        GLOBAL(initial_stack)
        /*
         * The SIZEOF_PTREGS gap is a convention which helps the in-kernel
diff --git a/arch/x86/kernel/setup_percpu.c b/arch/x86/kernel/setup_percpu.c
index ea554f812ee1..f7655979f19a 100644
--- a/arch/x86/kernel/setup_percpu.c
+++ b/arch/x86/kernel/setup_percpu.c
@@ -26,7 +26,7 @@
 DEFINE_PER_CPU_READ_MOSTLY(int, cpu_number);
 EXPORT_PER_CPU_SYMBOL(cpu_number);
 
-#ifdef CONFIG_X86_64
+#if defined(CONFIG_X86_64) && !defined(CONFIG_X86_PIE)
 #define BOOT_PERCPU_OFFSET ((unsigned long)__per_cpu_load)
 #else
 #define BOOT_PERCPU_OFFSET 0
diff --git a/arch/x86/kernel/vmlinux.lds.S b/arch/x86/kernel/vmlinux.lds.S
index 1c43a2e839fa..4a5957900dcc 100644
--- a/arch/x86/kernel/vmlinux.lds.S
+++ b/arch/x86/kernel/vmlinux.lds.S
@@ -209,9 +209,14 @@ SECTIONS
        /*
         * percpu offsets are zero-based on SMP.  PERCPU_VADDR() changes the
         * output PHDR, so the next output section - .init.text - should
-        * start another segment - init.
+        * start another segment - init. For Position Independent Code, the
+        * per-cpu section cannot be zero-based because everything is relative.
         */
+#ifdef CONFIG_X86_PIE
+       PERCPU_SECTION(INTERNODE_CACHE_BYTES)
+#else
        PERCPU_VADDR(INTERNODE_CACHE_BYTES, 0, :percpu)
+#endif
        ASSERT(SIZEOF(.data..percpu) < CONFIG_PHYSICAL_START,
               "per-CPU data too large - increase CONFIG_PHYSICAL_START")
 #endif
@@ -387,7 +392,11 @@ SECTIONS
  * Per-cpu symbols which need to be offset from __per_cpu_load
  * for the boot processor.
  */
+#ifdef CONFIG_X86_PIE
+#define INIT_PER_CPU(x) init_per_cpu__##x = x
+#else
 #define INIT_PER_CPU(x) init_per_cpu__##x = x + __per_cpu_load
+#endif
 INIT_PER_CPU(gdt_page);
 INIT_PER_CPU(irq_stack_union);
 
@@ -397,7 +406,7 @@ INIT_PER_CPU(irq_stack_union);
 . = ASSERT((_end - _text <= KERNEL_IMAGE_SIZE),
           "kernel image bigger than KERNEL_IMAGE_SIZE");
 
-#ifdef CONFIG_SMP
+#if defined(CONFIG_SMP) && !defined(CONFIG_X86_PIE)
 . = ASSERT((irq_stack_union == 0),
            "irq_stack_union is not at start of per-cpu area");
 #endif
diff --git a/arch/x86/lib/cmpxchg16b_emu.S b/arch/x86/lib/cmpxchg16b_emu.S
index 9b330242e740..254950604ae4 100644
--- a/arch/x86/lib/cmpxchg16b_emu.S
+++ b/arch/x86/lib/cmpxchg16b_emu.S
@@ -33,13 +33,13 @@ ENTRY(this_cpu_cmpxchg16b_emu)
        pushfq
        cli
 
-       cmpq PER_CPU_VAR((%rsi)), %rax
+       cmpq PER_CPU_VAR_ABS((%rsi)), %rax
        jne .Lnot_same
-       cmpq PER_CPU_VAR(8(%rsi)), %rdx
+       cmpq PER_CPU_VAR_ABS(8(%rsi)), %rdx
        jne .Lnot_same
 
-       movq %rbx, PER_CPU_VAR((%rsi))
-       movq %rcx, PER_CPU_VAR(8(%rsi))
+       movq %rbx, PER_CPU_VAR_ABS((%rsi))
+       movq %rcx, PER_CPU_VAR_ABS(8(%rsi))
 
        popfq
        mov $1, %al
diff --git a/arch/x86/xen/xen-asm.S b/arch/x86/xen/xen-asm.S
index 8019edd0125c..a5d73d3218be 100644
--- a/arch/x86/xen/xen-asm.S
+++ b/arch/x86/xen/xen-asm.S
@@ -21,7 +21,7 @@
 ENTRY(xen_irq_enable_direct)
        FRAME_BEGIN
        /* Unmask events */
-       movb $0, PER_CPU_VAR(xen_vcpu_info) + XEN_vcpu_info_mask
+       movb $0, PER_CPU_VAR(xen_vcpu_info + XEN_vcpu_info_mask)
 
        /*
         * Preempt here doesn't matter because that will deal with any
@@ -30,7 +30,7 @@ ENTRY(xen_irq_enable_direct)
         */
 
        /* Test for pending */
-       testb $0xff, PER_CPU_VAR(xen_vcpu_info) + XEN_vcpu_info_pending
+       testb $0xff, PER_CPU_VAR(xen_vcpu_info + XEN_vcpu_info_pending)
        jz 1f
 
        call check_events
@@ -45,7 +45,7 @@ ENTRY(xen_irq_enable_direct)
  * non-zero.
  */
 ENTRY(xen_irq_disable_direct)
-       movb $1, PER_CPU_VAR(xen_vcpu_info) + XEN_vcpu_info_mask
+       movb $1, PER_CPU_VAR(xen_vcpu_info + XEN_vcpu_info_mask)
        ret
 ENDPROC(xen_irq_disable_direct)
 
@@ -59,7 +59,7 @@ ENDPROC(xen_irq_disable_direct)
  * x86 use opposite senses (mask vs enable).
  */
 ENTRY(xen_save_fl_direct)
-       testb $0xff, PER_CPU_VAR(xen_vcpu_info) + XEN_vcpu_info_mask
+       testb $0xff, PER_CPU_VAR(xen_vcpu_info + XEN_vcpu_info_mask)
        setz %ah
        addb %ah, %ah
        ret
@@ -80,7 +80,7 @@ ENTRY(xen_restore_fl_direct)
 #else
        testb $X86_EFLAGS_IF>>8, %ah
 #endif
-       setz PER_CPU_VAR(xen_vcpu_info) + XEN_vcpu_info_mask
+       setz PER_CPU_VAR(xen_vcpu_info + XEN_vcpu_info_mask)
        /*
         * Preempt here doesn't matter because that will deal with any
         * pending interrupts.  The pending check may end up being run
@@ -88,7 +88,7 @@ ENTRY(xen_restore_fl_direct)
         */
 
        /* check for unmasked and pending */
-       cmpw $0x0001, PER_CPU_VAR(xen_vcpu_info) + XEN_vcpu_info_pending
+       cmpw $0x0001, PER_CPU_VAR(xen_vcpu_info + XEN_vcpu_info_pending)
        jnz 1f
        call check_events
 1:
diff --git a/init/Kconfig b/init/Kconfig
index 221cac95044f..acc9087546ac 100644
--- a/init/Kconfig
+++ b/init/Kconfig
@@ -1365,7 +1365,7 @@ config KALLSYMS_ALL
 config KALLSYMS_ABSOLUTE_PERCPU
        bool
        depends on KALLSYMS
-       default X86_64 && SMP
+       default X86_64 && SMP && !X86_PIE
 
 config KALLSYMS_BASE_RELATIVE
        bool
-- 
2.16.2.660.g709887971b-goog


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