[PATCH] kasan: Disable Software Tag-Based KASAN with GCC

[Will Deacon]

Syzbot reports a KASAN failure early during boot on arm64 when building
with GCC 12.2.0 and using the Software Tag-Based KASAN mode:

  | BUG: KASAN: invalid-access in smp_build_mpidr_hash arch/arm64/kernel/setup.c:133 [inline]
  | BUG: KASAN: invalid-access in setup_arch+0x984/0xd60 arch/arm64/kernel/setup.c:356
  | Write of size 4 at addr 03ff800086867e00 by task swapper/0
  | Pointer tag: [03], memory tag: [fe]

Initial triage indicates that the report is a false positive and a
thorough investigation of the crash by Mark Rutland revealed the root
cause to be a bug in GCC:

  > When GCC is passed `-fsanitize=hwaddress` or
  > `-fsanitize=kernel-hwaddress` it ignores
  > `__attribute__((no_sanitize_address))`, and instruments functions
  > we require are not instrumented.
  >
  > [...]
  >
  > All versions [of GCC] I tried were broken, from 11.3.0 to 14.2.0
  > inclusive.
  >
  > I think we have to disable KASAN_SW_TAGS with GCC until this is
  > fixed

Disable Software Tag-Based KASAN when building with GCC by making
CC_HAS_KASAN_SW_TAGS depend on !CC_IS_GCC.

Cc: Andrey Konovalov <andreyknvl@xxxxxxxxx>
Suggested-by: Mark Rutland <mark.rutland@xxxxxxx>
Reported-by: syzbot+908886656a02769af987@xxxxxxxxxxxxxxxxxxxxxxxxx
Link: https://lore.kernel.org/r/000000000000f362e80620e27859@xxxxxxxxxx
Link: https://lore.kernel.org/r/ZvFGwKfoC4yVjN_X@J2N7QTR9R3
Link: https://bugzilla.kernel.org/show_bug.cgi?id=218854
Signed-off-by: Will Deacon <will@xxxxxxxxxx>
---
 lib/Kconfig.kasan | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

While sweeping up pending fixes and open bug reports, I noticed this one
had slipped through the cracks...

diff --git a/lib/Kconfig.kasan b/lib/Kconfig.kasan
index 98016e137b7f..233ab2096924 100644
--- a/lib/Kconfig.kasan
+++ b/lib/Kconfig.kasan
@@ -22,8 +22,11 @@ config ARCH_DISABLE_KASAN_INLINE
 config CC_HAS_KASAN_GENERIC
 	def_bool $(cc-option, -fsanitize=kernel-address)
 
+# GCC appears to ignore no_sanitize_address when -fsanitize=kernel-hwaddress
+# is passed. See https://bugzilla.kernel.org/show_bug.cgi?id=218854 (and
+# the linked LKML thread) for more details.
 config CC_HAS_KASAN_SW_TAGS
-	def_bool $(cc-option, -fsanitize=kernel-hwaddress)
+	def_bool !CC_IS_GCC && $(cc-option, -fsanitize=kernel-hwaddress)
 
 # This option is only required for software KASAN modes.
 # Old GCC versions do not have proper support for no_sanitize_address.
@@ -98,7 +101,7 @@ config KASAN_SW_TAGS
 	help
 	  Enables Software Tag-Based KASAN.
 
-	  Requires GCC 11+ or Clang.
+	  Requires Clang.
 
 	  Supported only on arm64 CPUs and relies on Top Byte Ignore.
 
-- 
2.47.0.rc1.288.g06298d1525-goog