[PATCH v2 6/6] lkdtm: Fix CFI_BACKWARD on RISC-V

Sami Tolvanen
2023-08-15T20:34:57Z

On RISC-V, the return address is before the current frame pointer,
unlike on most other architectures. Use the correct offset on RISC-V
to fix the CFI_BACKWARD test.

Signed-off-by: Sami Tolvanen <samitolvanen@google.com>
---
drivers/misc/lkdtm/cfi.c | 13 +++++++++++--
1 file changed, 11 insertions(+), 2 deletions(-)

diff --git a/drivers/misc/lkdtm/cfi.c b/drivers/misc/lkdtm/cfi.c
index fc28714ae3a6..6a33889d0902 100644
--- a/drivers/misc/lkdtm/cfi.c
+++ b/drivers/misc/lkdtm/cfi.c
@@ -68,12 +68,20 @@ static void lkdtm_CFI_FORWARD_PROTO(void)
#define no_pac_addr(addr) \
((__force __typeof__(addr))((uintptr_t)(addr) | PAGE_OFFSET))

+#ifdef CONFIG_RISCV
+/* https://github.com/riscv-non-isa/riscv-elf-psabi-doc/blob/master/riscv-cc.adoc#frame-pointer-convention */
+#define FRAME_RA_OFFSET (-1)
+#else
+#define FRAME_RA_OFFSET 1
+#endif
+
/* The ultimate ROP gadget. */
static noinline __no_ret_protection
void set_return_addr_unchecked(unsigned long *expected, unsigned long *addr)
{
/* Use of volatile is to make sure final write isn't seen as a dead store. */
- unsigned long * volatile *ret_addr = (unsigned long **)__builtin_frame_address(0) + 1;
+ unsigned long * volatile *ret_addr =
+ (unsigned long **)__builtin_frame_address(0) + FRAME_RA_OFFSET;

/* Make sure we've found the right place on the stack before writing it. */
if (no_pac_addr(*ret_addr) == expected)
@@ -88,7 +96,8 @@ static noinline
void set_return_addr(unsigned long *expected, unsigned long *addr)
{
/* Use of volatile is to make sure final write isn't seen as a dead store. */
- unsigned long * volatile *ret_addr = (unsigned long **)__builtin_frame_address(0) + 1;
+ unsigned long * volatile *ret_addr =
+ (unsigned long **)__builtin_frame_address(0) + FRAME_RA_OFFSET;

/* Make sure we've found the right place on the stack before writing it. */
if (no_pac_addr(*ret_addr) == expected)
--
2.41.0.694.ge786442a9b-goog