[PATCH v2 6/7] riscv: Ensure gp is NULL or points to valid data

[Sean Anderson]

This allows code to use a construct like `if (gd & gd->...) { ... }` when
accessing the global data pointer. Without this change, it was possible for
a very early trap to cause _exit_trap to read arbitrary memory. This could
cause a second trap, preventing show_regs from being printed.

XIP cannot use locks because flash is not writable. This leaves it
vulnerable to the same class of bugs regarding already-pending IPIs as
before this series. Fixing that would require finding another method of
synchronization, which is outside the scope of this series.

Fixes: 7c6ca03eae ("riscv: additional crash information")
Signed-off-by: Sean Anderson <seanga2 at gmail.com>
---

Changes in v2:
- Set gp early with XIP

 arch/riscv/cpu/start.S      | 26 +++++++++++++++++++++++---
 arch/riscv/lib/interrupts.c |  3 ++-
 2 files changed, 25 insertions(+), 4 deletions(-)

diff --git a/arch/riscv/cpu/start.S b/arch/riscv/cpu/start.S
index 66ca1c7020..a16af79fbe 100644
--- a/arch/riscv/cpu/start.S
+++ b/arch/riscv/cpu/start.S
@@ -47,6 +47,13 @@ _start:
 	mv	tp, a0
 	mv	s1, a1
 
+	/*
+	 * Set the global data pointer to a known value in case we get a very
+	 * early trap. The global data pointer will be set its actual value only
+	 * after it has been initialized.
+	 */
+	mv	gp, zero
+
 	la	t0, trap_entry
 	csrw	MODE_PREFIX(tvec), t0
 
@@ -85,10 +92,10 @@ call_board_init_f_0:
 	jal	board_init_f_alloc_reserve
 
 	/*
-	 * Set global data pointer here for all harts, uninitialized at this
-	 * point.
+	 * Save global data pointer for later. We don't set it here because it
+	 * is not initialized yet.
 	 */
-	mv	gp, a0
+	mv	s0, a0
 
 	/* setup stack */
 #if CONFIG_IS_ENABLED(SMP)
@@ -109,6 +116,12 @@ call_board_init_f_0:
 	amoswap.w s2, t1, 0(t0)
 	bnez	s2, wait_for_gd_init
 #else
+	/*
+	 * FIXME: gp is set before it is initialized. If an XIP U-Boot ever
+	 * encounters a pending IPI on boot it is liable to jump to whatever
+	 * memory happens to be in ipi_data.addr on boot.
+	 */
+	mv	gp, s0
 	bnez	tp, secondary_hart_loop
 #endif
 
@@ -133,6 +146,13 @@ wait_for_gd_init:
 1:	amoswap.w.aq t1, t1, 0(t0)
 	bnez	t1, 1b
 
+	/*
+	 * Set the global data pointer only when gd_t has been initialized.
+	 * This was already set by arch_setup_gd on the boot hart, but all other
+	 * harts' global data pointers gets set here.
+	 */
+	mv	gp, s0
+
 	/* register available harts in the available_harts mask */
 	li	t1, 1
 	sll	t1, t1, tp
diff --git a/arch/riscv/lib/interrupts.c b/arch/riscv/lib/interrupts.c
index cd47e64487..ad870e98d8 100644
--- a/arch/riscv/lib/interrupts.c
+++ b/arch/riscv/lib/interrupts.c
@@ -78,7 +78,8 @@ static void _exit_trap(ulong code, ulong epc, ulong tval, struct pt_regs *regs)
 
 	printf("EPC: " REG_FMT " RA: " REG_FMT " TVAL: " REG_FMT "\n",
 	       epc, regs->ra, tval);
-	if (gd->flags & GD_FLG_RELOC)
+	/* Print relocation adjustments, but only if gd is initialized */
+	if (gd && gd->flags & GD_FLG_RELOC)
 		printf("EPC: " REG_FMT " RA: " REG_FMT " reloc adjusted\n\n",
 		       epc - gd->reloc_off, regs->ra - gd->reloc_off);
 
-- 
2.28.0