Re: [PATCH v5 01/16] x86/stackprotector: Work around strict Clang TLS symbol requirements

[Nathan Chancellor]

On Tue, Nov 05, 2024 at 10:57:46AM -0500, Brian Gerst wrote:
> From: Ard Biesheuvel <ardb@xxxxxxxxxx>
> 
> GCC and Clang both implement stack protector support based on Thread
> Local Storage (TLS) variables, and this is used in the kernel to
> implement per-task stack cookies, by copying a task's stack cookie into
> a per-CPU variable every time it is scheduled in.
> 
> Both now also implement -mstack-protector-guard-symbol=, which permits
> the TLS variable to be specified directly. This is useful because it
> will allow us to move away from using a fixed offset of 40 bytes into
> the per-CPU area on x86_64, which requires a lot of special handling in
> the per-CPU code and the runtime relocation code.
> 
> However, while GCC is rather lax in its implementation of this command
> line option, Clang actually requires that the provided symbol name
> refers to a TLS variable (i.e., one declared with __thread), although it
> also permits the variable to be undeclared entirely, in which case it
> will use an implicit declaration of the right type.
> 
> The upshot of this is that Clang will emit the correct references to the
> stack cookie variable in most cases, e.g.,
> 
>    10d:       64 a1 00 00 00 00       mov    %fs:0x0,%eax
>                       10f: R_386_32   __stack_chk_guard
> 
> However, if a non-TLS definition of the symbol in question is visible in
> the same compilation unit (which amounts to the whole of vmlinux if LTO
> is enabled), it will drop the per-CPU prefix and emit a load from a
> bogus address.
> 
> Work around this by using a symbol name that never occurs in C code, and
> emit it as an alias in the linker script.
> 
> Fixes: 3fb0fdb3bbe7 ("x86/stackprotector/32: Make the canary into a regular percpu variable")
> Cc: <stable@xxxxxxxxxxxxxxx>
> Cc: Fangrui Song <i@xxxxxxxxxx>
> Cc: Uros Bizjak <ubizjak@xxxxxxxxx>
> Cc: Nathan Chancellor <nathan@xxxxxxxxxx>
> Cc: Andy Lutomirski <luto@xxxxxxxxxx>
> Link: https://github.com/ClangBuiltLinux/linux/issues/1854
> Signed-off-by: Ard Biesheuvel <ardb@xxxxxxxxxx>
> Signed-off-by: Brian Gerst <brgerst@xxxxxxxxx>

>From https://lore.kernel.org/20241016021045.GA1000009@thelio-3990X/:

Reviewed-by: Nathan Chancellor <nathan@xxxxxxxxxx>
Tested-by: Nathan Chancellor <nathan@xxxxxxxxxx>

> ---
>  arch/x86/Makefile                     |  5 +++--
>  arch/x86/entry/entry.S                | 16 ++++++++++++++++
>  arch/x86/include/asm/asm-prototypes.h |  3 +++
>  arch/x86/kernel/cpu/common.c          |  2 ++
>  arch/x86/kernel/vmlinux.lds.S         |  3 +++
>  5 files changed, 27 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/x86/Makefile b/arch/x86/Makefile
> index cd75e78a06c1..5b773b34768d 100644
> --- a/arch/x86/Makefile
> +++ b/arch/x86/Makefile
> @@ -142,9 +142,10 @@ ifeq ($(CONFIG_X86_32),y)
>  
>      ifeq ($(CONFIG_STACKPROTECTOR),y)
>          ifeq ($(CONFIG_SMP),y)
> -			KBUILD_CFLAGS += -mstack-protector-guard-reg=fs -mstack-protector-guard-symbol=__stack_chk_guard
> +            KBUILD_CFLAGS += -mstack-protector-guard-reg=fs \
> +                             -mstack-protector-guard-symbol=__ref_stack_chk_guard
>          else
> -			KBUILD_CFLAGS += -mstack-protector-guard=global
> +            KBUILD_CFLAGS += -mstack-protector-guard=global
>          endif
>      endif
>  else
> diff --git a/arch/x86/entry/entry.S b/arch/x86/entry/entry.S
> index 324686bca368..b7ea3e8e9ecc 100644
> --- a/arch/x86/entry/entry.S
> +++ b/arch/x86/entry/entry.S
> @@ -51,3 +51,19 @@ EXPORT_SYMBOL_GPL(mds_verw_sel);
>  .popsection
>  
>  THUNK warn_thunk_thunk, __warn_thunk
> +
> +#ifndef CONFIG_X86_64
> +/*
> + * Clang's implementation of TLS stack cookies requires the variable in
> + * question to be a TLS variable. If the variable happens to be defined as an
> + * ordinary variable with external linkage in the same compilation unit (which
> + * amounts to the whole of vmlinux with LTO enabled), Clang will drop the
> + * segment register prefix from the references, resulting in broken code. Work
> + * around this by avoiding the symbol used in -mstack-protector-guard-symbol=
> + * entirely in the C code, and use an alias emitted by the linker script
> + * instead.
> + */
> +#ifdef CONFIG_STACKPROTECTOR
> +EXPORT_SYMBOL(__ref_stack_chk_guard);
> +#endif
> +#endif
> diff --git a/arch/x86/include/asm/asm-prototypes.h b/arch/x86/include/asm/asm-prototypes.h
> index 25466c4d2134..3674006e3974 100644
> --- a/arch/x86/include/asm/asm-prototypes.h
> +++ b/arch/x86/include/asm/asm-prototypes.h
> @@ -20,3 +20,6 @@
>  extern void cmpxchg8b_emu(void);
>  #endif
>  
> +#if defined(__GENKSYMS__) && defined(CONFIG_STACKPROTECTOR)
> +extern unsigned long __ref_stack_chk_guard;
> +#endif
> diff --git a/arch/x86/kernel/cpu/common.c b/arch/x86/kernel/cpu/common.c
> index 8f41ab219cf1..9d42bd15e06c 100644
> --- a/arch/x86/kernel/cpu/common.c
> +++ b/arch/x86/kernel/cpu/common.c
> @@ -2091,8 +2091,10 @@ void syscall_init(void)
>  
>  #ifdef CONFIG_STACKPROTECTOR
>  DEFINE_PER_CPU(unsigned long, __stack_chk_guard);
> +#ifndef CONFIG_SMP
>  EXPORT_PER_CPU_SYMBOL(__stack_chk_guard);
>  #endif
> +#endif
>  
>  #endif	/* CONFIG_X86_64 */
>  
> diff --git a/arch/x86/kernel/vmlinux.lds.S b/arch/x86/kernel/vmlinux.lds.S
> index 410546bacc0f..d61c3584f3e6 100644
> --- a/arch/x86/kernel/vmlinux.lds.S
> +++ b/arch/x86/kernel/vmlinux.lds.S
> @@ -468,6 +468,9 @@ SECTIONS
>  . = ASSERT((_end - LOAD_OFFSET <= KERNEL_IMAGE_SIZE),
>  	   "kernel image bigger than KERNEL_IMAGE_SIZE");
>  
> +/* needed for Clang - see arch/x86/entry/entry.S */
> +PROVIDE(__ref_stack_chk_guard = __stack_chk_guard);
> +
>  #ifdef CONFIG_X86_64
>  /*
>   * Per-cpu symbols which need to be offset from __per_cpu_load
> -- 
> 2.47.0
>