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

[Sean Anderson]

On 9/14/20 1:25 AM, Bin Meng wrote:
> 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?

gp is set by arch_setup_gd in board_init_f_init_reserve on the boot
hart. Setting it here is only necessary for secondary harts.

> 
>> +
>>         /* 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
>