The patch titled Subject: mm: fix VM_UFFD_MINOR == VM_SHADOW_STACK on USERFAULTFD=y && ARM64_GCS=y has been added to the -mm mm-hotfixes-unstable branch. Its filename is mm-fix-vm_uffd_minor-==-vm_shadow_stack-on-userfaultfd=y-arm64_gcs=y.patch

This patch will shortly appear at https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches...

This patch will later appear in the mm-hotfixes-unstable branch at git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm

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------------------------------------------------------ From: Florent Revest revest@chromium.org Subject: mm: fix VM_UFFD_MINOR == VM_SHADOW_STACK on USERFAULTFD=y && ARM64_GCS=y Date: Wed, 7 May 2025 15:09:57 +0200

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 ---

include/linux/mm.h | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-)

--- a/include/linux/mm.h~mm-fix-vm_uffd_minor-==-vm_shadow_stack-on-userfaultfd=y-arm64_gcs=y +++ a/include/linux/mm.h @@ -385,7 +385,7 @@ extern unsigned int kobjsize(const void #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 _

Patches currently in -mm which might be from revest@chromium.org are

mm-fix-vm_uffd_minor-==-vm_shadow_stack-on-userfaultfd=y-arm64_gcs=y.patch