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

Ard Biesheuvel
2024-10-02T11:04:02Z

On Wed, 2 Oct 2024 at 11:25, Ard Biesheuvel <ardb+git@google.com> wrote:
>
> From: Ard Biesheuvel <ardb@kernel.org>
>
> 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@vger.kernel.org>
> Cc: Fangrui Song <i@maskray.me>
> Cc: Brian Gerst <brgerst@gmail.com>
> Cc: Uros Bizjak <ubizjak@gmail.com>
> Cc: Nathan Chancellor <nathan@kernel.org>
> Cc: Andy Lutomirski <luto@kernel.org>
> Link: https://github.com/ClangBuiltLinux/linux/issues/1854
> Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
> ---
> arch/x86/Makefile | 5 +++--
> arch/x86/entry/entry.S | 16 ++++++++++++++++
> arch/x86/kernel/cpu/common.c | 2 ++
> arch/x86/kernel/vmlinux.lds.S | 3 +++
> 4 files changed, 24 insertions(+), 2 deletions(-)
>

This needs the hunk below applied on top for CONFIG_MODVERSIONS:

--- 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

+#ifdef CONFIG_STACKPROTECTOR
+extern unsigned long __ref_stack_chk_guard;
+#endif