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

[Bin Meng]

On Tue, Sep 8, 2020 at 2:17 AM Sean Anderson <seanga2 at gmail.com> wrote:
>
> 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.
>
> Fixes: 7c6ca03eaed0035ca6676e9bc7f5f1dfcaae7e8f

nits: wrong fixes tag format

> Signed-off-by: Sean Anderson <seanga2 at gmail.com>
> ---
>
>  arch/riscv/cpu/start.S      | 20 +++++++++++++++++---
>  arch/riscv/lib/interrupts.c |  3 ++-
>  2 files changed, 19 insertions(+), 4 deletions(-)
>
> diff --git a/arch/riscv/cpu/start.S b/arch/riscv/cpu/start.S
> index ad18e746b6..59d3d7bbf4 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)
> @@ -135,6 +142,13 @@ wait_for_gd_init:
>         fence   r, rw
>         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

This is currently in the #ifndef CONFIG_XIP block. Should consider the
#else branch?

> +
>         /* 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);
>
> --

Regards,
Bin