mm: fix VM_UFFD_MINOR == VM_SHADOW_STACK on USERFAULTFD=y && ARM64_GCS=y

On configs with CONFIG_ARM64_GCS=y, VM_SHADOW_STACK is bit 38.  On configs
with CONFIG_HAVE_ARCH_USERFAULTFD_MINOR=y (selected by CONFIG_ARM64 when
CONFIG_USERFAULTFD=y), VM_UFFD_MINOR is _also_ bit 38.

This bit being shared by two different VMA flags could lead to all sorts
of unintended behaviors.  Presumably, a process could maybe call into
userfaultfd in a way that disables the shadow stack vma flag.  I can't
think of any attack where this would help (presumably, if an attacker
tries to disable shadow stacks, they are trying to hijack control flow so
can't arbitrarily call into userfaultfd yet anyway) but this still feels
somewhat scary.

Link: https://lkml.kernel.org/r/20250507131000.1204175-2-revest@chromium.org
Fixes: ae80e1629aea ("mm: Define VM_SHADOW_STACK for arm64 when we support GCS")
Signed-off-by: Florent Revest <revest@chromium.org>
Reviewed-by: Mark Brown <broonie@kernel.org>
Cc: Borislav Betkov <bp@alien8.de>
Cc: Brendan Jackman <jackmanb@google.com>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Florent Revest <revest@chromium.org>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Thiago Jung Bauermann <thiago.bauermann@linaro.org>
Cc: Thomas Gleinxer <tglx@linutronix.de>
Cc: Will Deacon <will@kernel.org>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

diff --git a/include/linux/mm.h b/include/linux/mm.h
index bf55206935c467f7508e863332063bb15f904a24..fdda6b16263b35e6a996087b44105bdcb930a1a2 100644 (file)
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -385,7 +385,7 @@ extern unsigned int kobjsize(const void *objp);
 #endif
 
 #ifdef CONFIG_HAVE_ARCH_USERFAULTFD_MINOR
-# define VM_UFFD_MINOR_BIT     38
+# define VM_UFFD_MINOR_BIT     41
 # define VM_UFFD_MINOR         BIT(VM_UFFD_MINOR_BIT)  /* UFFD minor faults */
 #else /* !CONFIG_HAVE_ARCH_USERFAULTFD_MINOR */
 # define VM_UFFD_MINOR         VM_NONE