On Thu, 12 Dec 2024 at 16:54, Greg Kroah-Hartman
<gregkh@xxxxxxxxxxxxxxxxxxx> wrote:
>
> 6.1-stable review patch. If anyone has any objections, please let me know.
>
> ------------------
>
> From: Ard Biesheuvel <ardb@xxxxxxxxxx>
>
> [ Upstream commit 577c134d311b9b94598d7a0c86be1f431f823003 ]
>
There is a follow-up patch that addresses a build issue with this
patch and older GCC versions. Given that this fix is for 32-bit only,
whereas the build issue affects 64-bit too, let's hold off on this one
until we can apply them as a pair.
> 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 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")
> Signed-off-by: Ard Biesheuvel <ardb@xxxxxxxxxx>
> Signed-off-by: Brian Gerst <brgerst@xxxxxxxxx>
> Signed-off-by: Borislav Petkov (AMD) <bp@xxxxxxxxx>
> Reviewed-by: Nathan Chancellor <nathan@xxxxxxxxxx>
> Tested-by: Nathan Chancellor <nathan@xxxxxxxxxx>
> Cc: stable@xxxxxxxxxxxxxxx
> Link: https://github.com/ClangBuiltLinux/linux/issues/1854
> Link: https://lore.kernel.org/r/20241105155801.1779119-2-brgerst@xxxxxxxxx
> Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>
> ---
> arch/x86/Makefile | 3 ++-
> arch/x86/entry/entry.S | 15 +++++++++++++++
> 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, 25 insertions(+), 1 deletion(-)
>
> diff --git a/arch/x86/Makefile b/arch/x86/Makefile
> index 3419ffa2a3507..a88eede6e7db4 100644
> --- a/arch/x86/Makefile
> +++ b/arch/x86/Makefile
> @@ -113,7 +113,8 @@ 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
> endif
> diff --git a/arch/x86/entry/entry.S b/arch/x86/entry/entry.S
> index f4419afc7147d..23f9efbe9d705 100644
> --- a/arch/x86/entry/entry.S
> +++ b/arch/x86/entry/entry.S
> @@ -48,3 +48,18 @@ EXPORT_SYMBOL_GPL(mds_verw_sel);
>
> .popsection
>
> +#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 5cdccea455544..390b13db24b81 100644
> --- a/arch/x86/include/asm/asm-prototypes.h
> +++ b/arch/x86/include/asm/asm-prototypes.h
> @@ -18,3 +18,6 @@
> 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 7f922a359ccc5..b4e999048e9a4 100644
> --- a/arch/x86/kernel/cpu/common.c
> +++ b/arch/x86/kernel/cpu/common.c
> @@ -2158,8 +2158,10 @@ EXPORT_PER_CPU_SYMBOL(cpu_current_top_of_stack);
>
> #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 78ccb5ec3c0e7..c1e776ed71b06 100644
> --- a/arch/x86/kernel/vmlinux.lds.S
> +++ b/arch/x86/kernel/vmlinux.lds.S
> @@ -486,6 +486,9 @@ SECTIONS
> ASSERT(SIZEOF(.rela.dyn) == 0, "Unexpected run-time relocations (.rela) detected!")
> }
>
> +/* needed for Clang - see arch/x86/entry/entry.S */
> +PROVIDE(__ref_stack_chk_guard = __stack_chk_guard);
> +
> /*
> * The ASSERT() sink to . is intentional, for binutils 2.14 compatibility:
> */
> --
> 2.43.0
>
>
>
