x86/stackprotector: Work around strict Clang TLS symbol requirements

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@kernel.org>
Signed-off-by: Brian Gerst <brgerst@gmail.com>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Reviewed-by: Nathan Chancellor <nathan@kernel.org>
Tested-by: Nathan Chancellor <nathan@kernel.org>
Cc: stable@vger.kernel.org
Link: https://github.com/ClangBuiltLinux/linux/issues/1854
Link: https://lore.kernel.org/r/20241105155801.1779119-2-brgerst@gmail.com

diff --git a/arch/x86/Makefile b/arch/x86/Makefile
index cd75e78a06c1090c9c8fd51437a0a2105d05feb5..5b773b34768d1282ab4b6e0cb56f2b1a851cc251 100644 (file)
--- 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 324686bca36813a16168fda28bf0875b23457efc..b7ea3e8e9eccd540a2ff55eda7a05266a48a4f49 100644 (file)
--- 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 25466c4d213481bed7970ee135e3a5697b3f1b77..3674006e39744ee028df3dc051865733c16ba004 100644 (file)
--- 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 a5f221ea5688851020e305fbbe32005b80a5e90a..f43bb974fc66d7b21ad52c64da22694ccc274187 100644 (file)
--- a/arch/x86/kernel/cpu/common.c
+++ b/arch/x86/kernel/cpu/common.c
@@ -2089,8 +2089,10 @@ 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 b8c5741d2fb48017813060c977f4c31356168d5d..feb8102a9ca78cd4e2224a9ce710b17f68f3b835 100644 (file)
--- a/arch/x86/kernel/vmlinux.lds.S
+++ b/arch/x86/kernel/vmlinux.lds.S
@@ -491,6 +491,9 @@ 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