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

Sean Anderson
2020-09-14T14:23:02Z

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@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