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

[Sean Anderson]

On 9/14/20 9:03 AM, Sean Anderson wrote:
> 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.

Ok, I see what you mean. I guess I can set gp just before XIP jumps to
secondary_hart_loop. Doing it that way is still vulnerable to pending
IPIs, but not all cores should have that problem. Perhaps that
limitation should be documented somewhere? In any case, I don't have an
XIP core to test on (and there isn't a CI configuration either).

--Sean

>>
>>> +
>>>         /* 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
>>
>