On Thu, Sep 18, 2025 at 07:54:12AM -0500, Jeremy Linton wrote:

On 8/24/25 10:34 PM, Jeremy Linton wrote:

quoted

The arm64 probe simulation doesn't currently have logic in place
to deal with GCS and this results in core dumps if probes are inserted
at control flow locations. Fix-up bl, blr and ret to manipulate the
shadow stack as needed.

While we manipulate and validate the shadow stack correctly, the
hardware provides additional security by only allowing GCS operations
against pages which are marked to support GCS. For writing there is
gcssttr() which enforces this, but there isn't an equivalent for
reading. This means that uprobe users should be aware that probing on
control flow instructions which require reading the shadow stack (ex:
ret) offers lower security guarantees than what is achieved without
the uprobe active.

Signed-off-by: Jeremy Linton <redacted>
Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
---
  arch/arm64/kernel/probes/simulate-insn.c | 44 +++++++++++++++++++-----
  1 file changed, 35 insertions(+), 9 deletions(-)

diff --git a/arch/arm64/kernel/probes/simulate-insn.c b/arch/arm64/kernel/probes/simulate-insn.c
index 09a0b36122d0..97ed4db75417 100644
--- a/arch/arm64/kernel/probes/simulate-insn.c
+++ b/arch/arm64/kernel/probes/simulate-insn.c

@@ -13,6 +13,7 @@
  #include <asm/traps.h>
  #include "simulate-insn.h"
+#include "asm/gcs.h"
  #define bbl_displacement(insn)		\
  	sign_extend32(((insn) & 0x3ffffff) << 2, 27)

@@ -49,6 +50,21 @@ static inline u32 get_w_reg(struct pt_regs *regs, int reg)
  	return lower_32_bits(pt_regs_read_reg(regs, reg));
  }
+static inline int update_lr(struct pt_regs *regs, long addr)
+{
+	int err = 0;
+
+	if (user_mode(regs) && task_gcs_el0_enabled(current)) {
+		push_user_gcs(addr, &err);
+		if (err) {
+			force_sig(SIGSEGV);
+			return err;
+		}
+	}
+	procedure_link_pointer_set(regs, addr);
+	return err;
+}
+
  static bool __kprobes check_cbz(u32 opcode, struct pt_regs *regs)
  {
  	int xn = opcode & 0x1f;

@@ -107,9 +123,9 @@ simulate_b_bl(u32 opcode, long addr, struct pt_regs *regs)
  {
  	int disp = bbl_displacement(opcode);
-	/* Link register is x30 */
  	if (opcode & (1 << 31))
-		set_x_reg(regs, 30, addr + 4);
+		if (update_lr(regs, addr + 4))
+			return;
  	instruction_pointer_set(regs, addr + disp);
  }

@@ -129,21 +145,31 @@ void __kprobes
  simulate_br_blr(u32 opcode, long addr, struct pt_regs *regs)
  {
  	int xn = (opcode >> 5) & 0x1f;
+	int b_target = get_x_reg(regs, xn);

Ugh, I was staring at this set and realized that this type is wrong, it
should be 'long'. I will send a patch once I sanity check it on something
that isn't a model.

Thanks. I'm happy to add fixes on top of what I've already queued, so
please fire away.

Will