[PATCH 6.1.y] start_kernel: Add __no_stack_protector function attribute

[Artem Dinaburg <artem@trailofbits.com>]

From: "ndesaulniers@google.com" <ndesaulniers@google.com>

[ Upstream commit 514ca14ed5444b911de59ed3381dfd195d99fe4b ]

Back during the discussion of
commit a9a3ed1eff36 ("x86: Fix early boot crash on gcc-10, third try")
we discussed the need for a function attribute to control the omission
of stack protectors on a per-function basis; at the time Clang had
support for no_stack_protector but GCC did not. This was fixed in
gcc-11. Now that the function attribute is available, let's start using
it.

Callers of boot_init_stack_canary need to use this function attribute
unless they're compiled with -fno-stack-protector, otherwise the canary
stored in the stack slot of the caller will differ upon the call to
boot_init_stack_canary. This will lead to a call to __stack_chk_fail()
then panic.

Link: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94722
Link: https://lore.kernel.org/all/20200316130414.GC12561@hirez.programming.kicks-ass.net/
Tested-by: Nathan Chancellor <nathan@kernel.org>
Acked-by: Michael Ellerman <mpe@ellerman.id.au> (powerpc)
Acked-by: Miguel Ojeda <ojeda@kernel.org>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Nick Desaulniers <ndesaulniers@google.com>
Link: https://lore.kernel.org/r/20230412-no_stackp-v2-1-116f9fe4bbe7@google.com
Signed-off-by: Josh Poimboeuf <jpoimboe@kernel.org>

Signed-off-by: ndesaulniers@google.com <ndesaulniers@google.com>

[ Backport to 6.1.y: retain this tree's start_kernel() signature, which
  predates its upstream __noreturn annotation, and add
  __no_stack_protector. ]
Assisted-by: LLM
Signed-off-by: Artem Dinaburg <artem@trailofbits.com>
---
Hi Greg, Sasha, and kernel and powerpc maintainers,

I am continuing CVE backports still missing from 6.1.y.
This fix is inherited by v6.6 and every later mainline release, but 6.1.y
still has the affected code. The target-specific adjustment is described
in the bracketed note above.

Could you please queue it for 6.1.y?

Thanks,
Artem Dinaburg

CVE: CVE-2023-53491

Build validation: The touched powerpc allmodconfig objects built
with CONFIG_WERROR=y, and the patched functions had no stack_chk_fail
reference.

AI assistance: An LLM helped find, adapt, and validate this
backport; I reviewed the patch and test output.

 arch/powerpc/kernel/smp.c           |  1 +
 include/linux/compiler_attributes.h | 12 ++++++++++++
 init/main.c                         |  3 ++-
 3 files changed, 15 insertions(+), 1 deletion(-)

diff --git a/arch/powerpc/kernel/smp.c b/arch/powerpc/kernel/smp.c
index 980f2bc832d98d..fa17fdcc55a3de 100644
--- a/arch/powerpc/kernel/smp.c
+++ b/arch/powerpc/kernel/smp.c
@@ -1608,6 +1608,7 @@ static void add_cpu_to_masks(int cpu)
 }
 
 /* Activate a secondary processor. */
+__no_stack_protector
 void start_secondary(void *unused)
 {
 	unsigned int cpu = raw_smp_processor_id();
diff --git a/include/linux/compiler_attributes.h b/include/linux/compiler_attributes.h
index ab1fe28162b5e8..b9778e13b0a085 100644
--- a/include/linux/compiler_attributes.h
+++ b/include/linux/compiler_attributes.h
@@ -277,6 +277,18 @@
  */
 #define __noreturn                      __attribute__((__noreturn__))
 
+/*
+ * Optional: only supported since GCC >= 11.1, clang >= 7.0.
+ *
+ *   gcc: https://gcc.gnu.org/onlinedocs/gcc/Common-Function-Attributes.html#index-no_005fstack_005fprotector-function-attribute
+ *   clang: https://clang.llvm.org/docs/AttributeReference.html#no-stack-protector-safebuffers
+ */
+#if __has_attribute(__no_stack_protector__)
+# define __no_stack_protector		__attribute__((__no_stack_protector__))
+#else
+# define __no_stack_protector
+#endif
+
 /*
  * Optional: not supported by gcc.
  * Optional: not supported by icc.
diff --git a/init/main.c b/init/main.c
index 50b84f1f9ff38a..6acfd5a78bd7a5 100644
--- a/init/main.c
+++ b/init/main.c
@@ -908,7 +908,8 @@ static void __init print_unknown_bootoptions(void)
 	memblock_free(unknown_options, len);
 }
 
-asmlinkage __visible void __init __no_sanitize_address start_kernel(void)
+asmlinkage __visible __init __no_sanitize_address __no_stack_protector
+void start_kernel(void)
 {
 	char *command_line;
 	char *after_dashes;
-- 
2.39.5