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SubjectRe: [PATCH v3] x86/asm: Replace __force_order with memory clobber
*thread ping*

Can an x86 maintainer please take this for -next? Getting this landed
for v5.10 would be very helpful! :)

-Kees

On Wed, Sep 02, 2020 at 07:21:52PM -0400, Arvind Sankar wrote:
> The CRn accessor functions use __force_order as a dummy operand to
> prevent the compiler from reordering CRn reads/writes with respect to
> each other.
>
> The fact that the asm is volatile should be enough to prevent this:
> volatile asm statements should be executed in program order. However GCC
> 4.9.x and 5.x have a bug that might result in reordering. This was fixed
> in 8.1, 7.3 and 6.5. Versions prior to these, including 5.x and 4.9.x,
> may reorder volatile asm statements with respect to each other.
>
> There are some issues with __force_order as implemented:
> - It is used only as an input operand for the write functions, and hence
> doesn't do anything additional to prevent reordering writes.
> - It allows memory accesses to be cached/reordered across write
> functions, but CRn writes affect the semantics of memory accesses, so
> this could be dangerous.
> - __force_order is not actually defined in the kernel proper, but the
> LLVM toolchain can in some cases require a definition: LLVM (as well
> as GCC 4.9) requires it for PIE code, which is why the compressed
> kernel has a definition, but also the clang integrated assembler may
> consider the address of __force_order to be significant, resulting in
> a reference that requires a definition.
>
> Fix this by:
> - Using a memory clobber for the write functions to additionally prevent
> caching/reordering memory accesses across CRn writes.
> - Using a dummy input operand with an arbitrary constant address for the
> read functions, instead of a global variable. This will prevent reads
> from being reordered across writes, while allowing memory loads to be
> cached/reordered across CRn reads, which should be safe.
>
> Tested-by: Nathan Chancellor <natechancellor@gmail.com>
> Tested-by: Sedat Dilek <sedat.dilek@gmail.com>
> Signed-off-by: Arvind Sankar <nivedita@alum.mit.edu>
> Link: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=82602
> Link: https://lore.kernel.org/lkml/20200527135329.1172644-1-arnd@arndb.de/
> ---
> Changes from v2:
> - Clarify commit log and source comment some more
> Changes from v1:
> - Add lore link to email thread and mention state of 5.x/4.9.x in commit log
>
> arch/x86/boot/compressed/pgtable_64.c | 9 ---------
> arch/x86/include/asm/special_insns.h | 28 ++++++++++++++-------------
> arch/x86/kernel/cpu/common.c | 4 ++--
> 3 files changed, 17 insertions(+), 24 deletions(-)
>
> diff --git a/arch/x86/boot/compressed/pgtable_64.c b/arch/x86/boot/compressed/pgtable_64.c
> index c8862696a47b..7d0394f4ebf9 100644
> --- a/arch/x86/boot/compressed/pgtable_64.c
> +++ b/arch/x86/boot/compressed/pgtable_64.c
> @@ -5,15 +5,6 @@
> #include "pgtable.h"
> #include "../string.h"
>
> -/*
> - * __force_order is used by special_insns.h asm code to force instruction
> - * serialization.
> - *
> - * It is not referenced from the code, but GCC < 5 with -fPIE would fail
> - * due to an undefined symbol. Define it to make these ancient GCCs work.
> - */
> -unsigned long __force_order;
> -
> #define BIOS_START_MIN 0x20000U /* 128K, less than this is insane */
> #define BIOS_START_MAX 0x9f000U /* 640K, absolute maximum */
>
> diff --git a/arch/x86/include/asm/special_insns.h b/arch/x86/include/asm/special_insns.h
> index 59a3e13204c3..d6e3bb9363d2 100644
> --- a/arch/x86/include/asm/special_insns.h
> +++ b/arch/x86/include/asm/special_insns.h
> @@ -11,45 +11,47 @@
> #include <linux/jump_label.h>
>
> /*
> - * Volatile isn't enough to prevent the compiler from reordering the
> - * read/write functions for the control registers and messing everything up.
> - * A memory clobber would solve the problem, but would prevent reordering of
> - * all loads stores around it, which can hurt performance. Solution is to
> - * use a variable and mimic reads and writes to it to enforce serialization
> + * The compiler should not reorder volatile asm statements with respect to each
> + * other: they should execute in program order. However GCC 4.9.x and 5.x have
> + * a bug (which was fixed in 8.1, 7.3 and 6.5) where they might reorder
> + * volatile asm. The write functions are not affected since they have memory
> + * clobbers preventing reordering. To prevent reads from being reordered with
> + * respect to writes, use a dummy memory operand.
> */
> -extern unsigned long __force_order;
> +
> +#define __FORCE_ORDER "m"(*(unsigned int *)0x1000UL)
>
> void native_write_cr0(unsigned long val);
>
> static inline unsigned long native_read_cr0(void)
> {
> unsigned long val;
> - asm volatile("mov %%cr0,%0\n\t" : "=r" (val), "=m" (__force_order));
> + asm volatile("mov %%cr0,%0\n\t" : "=r" (val) : __FORCE_ORDER);
> return val;
> }
>
> static __always_inline unsigned long native_read_cr2(void)
> {
> unsigned long val;
> - asm volatile("mov %%cr2,%0\n\t" : "=r" (val), "=m" (__force_order));
> + asm volatile("mov %%cr2,%0\n\t" : "=r" (val) : __FORCE_ORDER);
> return val;
> }
>
> static __always_inline void native_write_cr2(unsigned long val)
> {
> - asm volatile("mov %0,%%cr2": : "r" (val), "m" (__force_order));
> + asm volatile("mov %0,%%cr2": : "r" (val) : "memory");
> }
>
> static inline unsigned long __native_read_cr3(void)
> {
> unsigned long val;
> - asm volatile("mov %%cr3,%0\n\t" : "=r" (val), "=m" (__force_order));
> + asm volatile("mov %%cr3,%0\n\t" : "=r" (val) : __FORCE_ORDER);
> return val;
> }
>
> static inline void native_write_cr3(unsigned long val)
> {
> - asm volatile("mov %0,%%cr3": : "r" (val), "m" (__force_order));
> + asm volatile("mov %0,%%cr3": : "r" (val) : "memory");
> }
>
> static inline unsigned long native_read_cr4(void)
> @@ -64,10 +66,10 @@ static inline unsigned long native_read_cr4(void)
> asm volatile("1: mov %%cr4, %0\n"
> "2:\n"
> _ASM_EXTABLE(1b, 2b)
> - : "=r" (val), "=m" (__force_order) : "0" (0));
> + : "=r" (val) : "0" (0), __FORCE_ORDER);
> #else
> /* CR4 always exists on x86_64. */
> - asm volatile("mov %%cr4,%0\n\t" : "=r" (val), "=m" (__force_order));
> + asm volatile("mov %%cr4,%0\n\t" : "=r" (val) : __FORCE_ORDER);
> #endif
> return val;
> }
> diff --git a/arch/x86/kernel/cpu/common.c b/arch/x86/kernel/cpu/common.c
> index c5d6f17d9b9d..178499f90366 100644
> --- a/arch/x86/kernel/cpu/common.c
> +++ b/arch/x86/kernel/cpu/common.c
> @@ -359,7 +359,7 @@ void native_write_cr0(unsigned long val)
> unsigned long bits_missing = 0;
>
> set_register:
> - asm volatile("mov %0,%%cr0": "+r" (val), "+m" (__force_order));
> + asm volatile("mov %0,%%cr0": "+r" (val) : : "memory");
>
> if (static_branch_likely(&cr_pinning)) {
> if (unlikely((val & X86_CR0_WP) != X86_CR0_WP)) {
> @@ -378,7 +378,7 @@ void native_write_cr4(unsigned long val)
> unsigned long bits_changed = 0;
>
> set_register:
> - asm volatile("mov %0,%%cr4": "+r" (val), "+m" (cr4_pinned_bits));
> + asm volatile("mov %0,%%cr4": "+r" (val) : : "memory");
>
> if (static_branch_likely(&cr_pinning)) {
> if (unlikely((val & cr4_pinned_mask) != cr4_pinned_bits)) {
> --
> 2.26.2
>

--
Kees Cook

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