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

On Tue, Sep 8, 2020 at 2:17 AM Sean Anderson seanga2@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@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