Re: [RFC PATCH v1 08/28] mm: Define VM_SHADOW_STACK for RISC-V

[Deepak Gupta <debug@rivosinc.com>]

On Thu, Jan 25, 2024 at 09:17:01AM +0100, David Hildenbrand wrote:
>On 25.01.24 07:21, debug@rivosinc.com wrote:
>>From: Deepak Gupta <debug@rivosinc.com>
>>
>>VM_SHADOW_STACK is defined by x86 as vm flag to mark a shadow stack vma.
>>
>>x86 uses VM_HIGH_ARCH_5 bit but that limits shadow stack vma to 64bit only.
>>arm64 follows same path
>>https://lore.kernel.org/lkml/20231009-arm64-gcs-v6-12-78e55deaa4dd@kernel.org/#r
>>
>>On RISC-V, write-only page table encodings are shadow stack pages. This patch
>>re-defines VM_WRITE only to be VM_SHADOW_STACK.
>>
>>Next set of patches will set guard rail that no other mm flow can set VM_WRITE
>>only in vma except when specifically creating shadow stack.
>>
>>Signed-off-by: Deepak Gupta <debug@rivosinc.com>
>>---
>>  include/linux/mm.h | 14 +++++++++++++-
>>  1 file changed, 13 insertions(+), 1 deletion(-)
>>
>>diff --git a/include/linux/mm.h b/include/linux/mm.h
>>index 418d26608ece..dfe0e8118669 100644
>>--- a/include/linux/mm.h
>>+++ b/include/linux/mm.h
>>@@ -352,7 +352,19 @@ extern unsigned int kobjsize(const void *objp);
>>   * for more details on the guard size.
>>   */
>>  # define VM_SHADOW_STACK	VM_HIGH_ARCH_5
>>-#else
>>+#endif
>>+
>>+#ifdef CONFIG_RISCV_USER_CFI
>>+/*
>>+ * On RISC-V pte encodings for shadow stack is R=0, W=1, X=0 and thus RISCV
>>+ * choosing to use similar mechanism on vm_flags where VM_WRITE only means
>>+ * VM_SHADOW_STACK. RISCV as well doesn't support VM_SHADOW_STACK to be set
>>+ * with VM_SHARED.
>>+ */
>>+#define VM_SHADOW_STACK	VM_WRITE
>>+#endif
>>+
>>+#ifndef VM_SHADOW_STACK
>>  # define VM_SHADOW_STACK	VM_NONE
>>  #endif
>
>That just screams for trouble. Can we find a less hacky way, please?
>
>Maybe just start with 64bit support only and do it like the other 
>archs. No need to be special.
>
>When wanting to support 32bit, we'll just finally clean up this high 
>flag mess and allow for more vm flags on 32bit as well.

Noted. I wanted to give a chance to anyone who cares about 32bit on riscv.
Will fix it in next series.

>
>-- 
>Cheers,
>
>David / dhildenb
>