arm64: Revert "scs: Work around full LTO issue with dynamic SCS"

This reverts commit 8c5a19cb17a71e ("arm64: scs: Work around full LTO
issue with dynamic SCS"), which did not quite fix the issue as intended.
Apparently, -fno-unwind-tables is ignored for the final full LTO link
when it is set on any of the objects, resulting in an early boot crash
due to the SCS patching code patching itself, and attempting to pop the
return address from the shadow stack while the associated push was still
a PACIASP instruction when it executed.

Reported-by: Sami Tolvanen <samitolvanen@google.com>
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Reviewed-by: Kees Cook <keescook@chromium.org>
Reviewed-by: Sami Tolvanen <samitolvanen@google.com>
Tested-by: Sami Tolvanen <samitolvanen@google.com>
Link: https://lore.kernel.org/r/20240123133052.1417449-5-ardb+git@google.com
Signed-off-by: Will Deacon <will@kernel.org>

diff --git a/arch/arm64/kernel/Makefile b/arch/arm64/kernel/Makefile
index e5d03a7039b4bf9cce893b1ea39712eef3e2f4ad..d95b3d6b471a7d63957c47151fd6cb404ca0f4c7 100644 (file)
--- a/arch/arm64/kernel/Makefile
+++ b/arch/arm64/kernel/Makefile
@@ -73,13 +73,7 @@ obj-$(CONFIG_ARM64_MTE)                      += mte.o
 obj-y                                  += vdso-wrap.o
 obj-$(CONFIG_COMPAT_VDSO)              += vdso32-wrap.o
 obj-$(CONFIG_UNWIND_PATCH_PAC_INTO_SCS)        += patch-scs.o
-
-# We need to prevent the SCS patching code from patching itself. Using
-# -mbranch-protection=none here to avoid the patchable PAC opcodes from being
-# generated triggers an issue with full LTO on Clang, which stops emitting PAC
-# instructions altogether. So instead, omit the unwind tables used by the
-# patching code, so it will not be able to locate its own PAC instructions.
-CFLAGS_patch-scs.o                     += -fno-asynchronous-unwind-tables -fno-unwind-tables
+CFLAGS_patch-scs.o                     += -mbranch-protection=none
 
 # Force dependency (vdso*-wrap.S includes vdso.so through incbin)
 $(obj)/vdso-wrap.o: $(obj)/vdso/vdso.so