Re: [RFC PATCH 05/28] x86: Define the stack protector guard symbol explicitly

[Uros Bizjak]

On Wed, Sep 25, 2024 at 5:02 PM Ard Biesheuvel <ardb+git@xxxxxxxxxx> wrote:
>
> From: Ard Biesheuvel <ardb@xxxxxxxxxx>
>
> Specify the guard symbol for the stack cookie explicitly, rather than
> positioning it exactly 40 bytes into the per-CPU area. Doing so removes
> the need for the per-CPU region to be absolute rather than relative to
> the placement of the per-CPU template region in the kernel image, and
> this allows the special handling for absolute per-CPU symbols to be
> removed entirely.
>
> This is a worthwhile cleanup in itself, but it is also a prerequisite
> for PIE codegen and PIE linking, which can replace our bespoke and
> rather clunky runtime relocation handling.

I would like to point out a series that converted the stack protector
guard symbol to a normal percpu variable [1], so there was no need to
assume anything about the location of the guard symbol.

[1] "[PATCH v4 00/16] x86-64: Stack protector and percpu improvements"
https://lore.kernel.org/lkml/20240322165233.71698-1-brgerst@xxxxxxxxx/

Uros.

>
> Signed-off-by: Ard Biesheuvel <ardb@xxxxxxxxxx>
> ---
>  arch/x86/Makefile                     |  4 ++++
>  arch/x86/include/asm/init.h           |  2 +-
>  arch/x86/include/asm/processor.h      | 11 +++--------
>  arch/x86/include/asm/stackprotector.h |  4 ----
>  tools/perf/util/annotate.c            |  4 ++--
>  5 files changed, 10 insertions(+), 15 deletions(-)
>
> diff --git a/arch/x86/Makefile b/arch/x86/Makefile
> index 6b3fe6e2aadd..b78b7623a4a9 100644
> --- a/arch/x86/Makefile
> +++ b/arch/x86/Makefile
> @@ -193,6 +193,10 @@ else
>          KBUILD_RUSTFLAGS += -Cno-redzone=y
>          KBUILD_RUSTFLAGS += -Ccode-model=kernel
>
> +        ifeq ($(CONFIG_STACKPROTECTOR),y)
> +                KBUILD_CFLAGS += -mstack-protector-guard-symbol=fixed_percpu_data
> +        endif
> +
>          # Don't emit relaxable GOTPCREL relocations
>          KBUILD_AFLAGS_KERNEL += -Wa,-mrelax-relocations=no
>          KBUILD_CFLAGS_KERNEL += -Wa,-mrelax-relocations=no
> diff --git a/arch/x86/include/asm/init.h b/arch/x86/include/asm/init.h
> index 14d72727d7ee..3ed0e8ec973f 100644
> --- a/arch/x86/include/asm/init.h
> +++ b/arch/x86/include/asm/init.h
> @@ -2,7 +2,7 @@
>  #ifndef _ASM_X86_INIT_H
>  #define _ASM_X86_INIT_H
>
> -#define __head __section(".head.text")
> +#define __head __section(".head.text") __no_stack_protector
>
>  struct x86_mapping_info {
>         void *(*alloc_pgt_page)(void *); /* allocate buf for page table */
> diff --git a/arch/x86/include/asm/processor.h b/arch/x86/include/asm/processor.h
> index 4a686f0e5dbf..56bc36116814 100644
> --- a/arch/x86/include/asm/processor.h
> +++ b/arch/x86/include/asm/processor.h
> @@ -402,14 +402,9 @@ struct irq_stack {
>  #ifdef CONFIG_X86_64
>  struct fixed_percpu_data {
>         /*
> -        * GCC hardcodes the stack canary as %gs:40.  Since the
> -        * irq_stack is the object at %gs:0, we reserve the bottom
> -        * 48 bytes of the irq stack for the canary.
> -        *
> -        * Once we are willing to require -mstack-protector-guard-symbol=
> -        * support for x86_64 stackprotector, we can get rid of this.
> +        * Since the irq_stack is the object at %gs:0, the bottom 8 bytes of
> +        * the irq stack are reserved for the canary.
>          */
> -       char            gs_base[40];
>         unsigned long   stack_canary;
>  };
>
> @@ -418,7 +413,7 @@ DECLARE_INIT_PER_CPU(fixed_percpu_data);
>
>  static inline unsigned long cpu_kernelmode_gs_base(int cpu)
>  {
> -       return (unsigned long)per_cpu(fixed_percpu_data.gs_base, cpu);
> +       return (unsigned long)&per_cpu(fixed_percpu_data, cpu);
>  }
>
>  extern asmlinkage void entry_SYSCALL32_ignore(void);
> diff --git a/arch/x86/include/asm/stackprotector.h b/arch/x86/include/asm/stackprotector.h
> index 00473a650f51..d1dcd22a0a4c 100644
> --- a/arch/x86/include/asm/stackprotector.h
> +++ b/arch/x86/include/asm/stackprotector.h
> @@ -51,10 +51,6 @@ static __always_inline void boot_init_stack_canary(void)
>  {
>         unsigned long canary = get_random_canary();
>
> -#ifdef CONFIG_X86_64
> -       BUILD_BUG_ON(offsetof(struct fixed_percpu_data, stack_canary) != 40);
> -#endif
> -
>         current->stack_canary = canary;
>  #ifdef CONFIG_X86_64
>         this_cpu_write(fixed_percpu_data.stack_canary, canary);
> diff --git a/tools/perf/util/annotate.c b/tools/perf/util/annotate.c
> index 37ce43c4eb8f..7ecfedf5edb9 100644
> --- a/tools/perf/util/annotate.c
> +++ b/tools/perf/util/annotate.c
> @@ -2485,10 +2485,10 @@ static bool is_stack_operation(struct arch *arch, struct disasm_line *dl)
>
>  static bool is_stack_canary(struct arch *arch, struct annotated_op_loc *loc)
>  {
> -       /* On x86_64, %gs:40 is used for stack canary */
> +       /* On x86_64, %gs:0 is used for stack canary */
>         if (arch__is(arch, "x86")) {
>                 if (loc->segment == INSN_SEG_X86_GS && loc->imm &&
> -                   loc->offset == 40)
> +                   loc->offset == 0)
>                         return true;
>         }
>
> --
> 2.46.0.792.g87dc391469-goog
>