[PATCH v10 14/16] riscv: Fix a kernel panic issue if $s2 is set to a specific value before entering Linux

[Guo Ren]

> Panic log:
> [    0.018707] Unable to handle kernel NULL pointer dereference at virtual address 0000000000000000
> [    0.023060] Oops [#1]
> [    0.023214] Modules linked in:
> [    0.023725] CPU: 0 PID: 0 Comm: swapper/0 Not tainted 5.14.0 #33
> [    0.023955] Hardware name: SiFive,FU800 (DT)
> [    0.024150] epc : __vstate_save+0x1c/0x48
> [    0.024654]  ra : arch_dup_task_struct+0x70/0x108
> [    0.024815] epc : ffffffff80005ad8 ra : ffffffff800035a8 sp : ffffffff81203d50
> [    0.025020]  gp : ffffffff812e8290 tp : ffffffff8120bdc0 t0 : 0000000000000000
> [    0.025216]  t1 : 0000000000000000 t2 : 0000000000000000 s0 : ffffffff81203d80
> [    0.025424]  s1 : ffffffff8120bdc0 a0 : ffffffff8120c820 a1 : 0000000000000000
> [    0.025659]  a2 : 0000000000001000 a3 : 0000000000000000 a4 : 0000000000000600
> [    0.025869]  a5 : ffffffff8120cdc0 a6 : ffffffe00160b400 a7 : ffffffff80a1fe60
> [    0.026069]  s2 : ffffffe0016b8000 s3 : ffffffff81204000 s4 : 0000000000004000
> [    0.026267]  s5 : 0000000000000000 s6 : ffffffe0016b8000 s7 : ffffffe0016b9000
> [    0.026475]  s8 : ffffffff81203ee0 s9 : 0000000000800300 s10: ffffffff812e9088
> [    0.026689]  s11: ffffffd004008000 t3 : 0000000000000000 t4 : 0000000000000100
> [    0.026900]  t5 : 0000000000000600 t6 : ffffffe00167bcc4
> [    0.027057] status: 8000000000000720 badaddr: 0000000000000000 cause: 000000000000000f
> [    0.027344] [<ffffffff80005ad8>] __vstate_save+0x1c/0x48
> [    0.027567] [<ffffffff8000abe8>] copy_process+0x266/0x11a0
> [    0.027739] [<ffffffff8000bc98>] kernel_clone+0x90/0x2aa
> [    0.027915] [<ffffffff8000c062>] kernel_thread+0x76/0x92
> [    0.028075] [<ffffffff8072e34c>] rest_init+0x26/0xfc
> [    0.028242] [<ffffffff80800638>] arch_call_rest_init+0x10/0x18
> [    0.028423] [<ffffffff80800c4a>] start_kernel+0x5ce/0x5fe
> [    0.029188] ---[ end trace 9a59af33f7ba3df4 ]---
> [    0.029479] Kernel panic - not syncing: Attempted to kill the idle task!
> [    0.029907] ---[ end Kernel panic - not syncing: Attempted to kill the idle task! ]---
>
> The NULL pointer accessing caused the kernel panic. There is a NULL
> pointer is because in vstate_save() function it will check
> (regs->status & SR_VS) == SR_VS_DIRTY and this is true, but it shouldn't
> be true because vector is not used here. Since vector is not used, datap
> won't be allocated so it is NULL. The reason why regs->status is set to
> a wrong value is because pt_regs->status is put in stack and it is polluted
> after setup_vm() called.
>
> In prologue of setup_vm(), we can observe it will save s2 to stack however
> s2 is meaningless here because the caller is assembly code and s2 is just
> some value from previous stage. The compiler will base on calling
> convention to save the register to stack. Then 0x80008638 in s2 is saved
> to stack. It might be any value. In this failure case it is 0x80008638 and
> it will accidentally cause SR_VS_DIRTY to call the vstate_save() function.
>
> (gdb) info addr setup_vm
> Symbol "setup_vm" is a function at address 0xffffffff80802c8a.
> (gdb) va2pa 0xffffffff80802c8a
> $64 = 0x80a02c8a
> (gdb) x/10i 0x80a02c8a
>     0x80a02c8a:  addi    sp,sp,-48
>     0x80a02c8c:  li      a3,-1
>     0x80a02c8e:  auipc   a5,0xff7fd
>     0x80a02c92:  addi    a5,a5,882
>     0x80a02c96:  sd      s0,32(sp)
>     0x80a02c98:  sd      s2,16(sp) <-- store to stack
>
> After returning from setup_vm()
> (gdb) x/20i  0x0000000080201138
>     0x80201138:  mv      a0,s1
>     0x8020113a:  auipc   ra,0x802
>     0x8020113e:  jalr    -1200(ra)    <-- jump to setup_vm()
>     0x80201142:  auipc   a0,0xa03
> (gdb) p/x $sp
> $70 = 0x81404000
> (gdb) p/x *(struct pt_regs*)($sp-0x120)
> $71 = {
>    epc = 0x0,
>    ra = 0x0,
>    sp = 0x0,
>    gp = 0x0,
>    tp = 0x0,
>    t0 = 0x0,
>    t1 = 0x0,
>    t2 = 0x0,
>    s0 = 0x0,
>    s1 = 0x0,
>    a0 = 0x0,
>    a1 = 0x0,
>    a2 = 0x0,
>    a3 = 0x81403f90,
>    a4 = 0x80c04000,
>    a5 = 0x1,
>    a6 = 0xffffffff81337000,
>    a7 = 0x81096700,
>    s2 = 0x81400000,
>    s3 = 0xffffffff81200000,
>    s4 = 0x81403fd0,
>    s5 = 0x80a02c6c,
>    s6 = 0x8000000000006800,
>    s7 = 0x0,
>    s8 = 0xfffffffffffffff3,
>    s9 = 0x80c01000,
>    s10 = 0x81096700,
>    s11 = 0x82200000,
>    t3 = 0x81404000,
>    t4 = 0x80a02dea,
>    t5 = 0x0,
>    t6 = 0x82200000,
>    status = 0x80008638, <- Wrong value in stack!!!
>    badaddr = 0x82200000,
>    cause = 0x0,
>    orig_a0 = 0x80201142
> }
> (gdb) p/x $pc
> $72 = 0x80201142
> (gdb) p/x sizeof(struct pt_regs)
> $73 = 0x120
>
> Co-developed-by: ShihPo Hung <shihpo.hung at sifive.com>
> Signed-off-by: ShihPo Hung <shihpo.hung at sifive.com>
> Co-developed-by: Vincent Chen <vincent.chen at sifive.com>
> Signed-off-by: Vincent Chen <vincent.chen at sifive.com>
> Signed-off-by: Greentime Hu <greentime.hu at sifive.com>
> ---
>   arch/riscv/kernel/head.S | 2 ++
>   1 file changed, 2 insertions(+)
>
> diff --git a/arch/riscv/kernel/head.S b/arch/riscv/kernel/head.S
> index 2877af90b025..0c307c0bd3d6 100644
> --- a/arch/riscv/kernel/head.S
> +++ b/arch/riscv/kernel/head.S
> @@ -299,6 +299,7 @@ clear_bss_done:
>   	/* Initialize page tables and relocate to virtual addresses */
>   	la sp, init_thread_union + THREAD_SIZE
>   	XIP_FIXUP_OFFSET sp
> +	addi sp, sp, -PT_SIZE
>   #ifdef CONFIG_BUILTIN_DTB
>   	la a0, __dtb_start
>   	XIP_FIXUP_OFFSET a0
> @@ -316,6 +317,7 @@ clear_bss_done:
>   	/* Restore C environment */
>   	la tp, init_task
>   	la sp, init_thread_union + THREAD_SIZE
> +	addi sp, sp, -PT_SIZE
Good point, Yes we should skip PT_SIZE in stack. I suggest wrap

la sp, init_thread_union + THREAD_SIZE
addi sp, sp, -PT_SIZE

into a macro and give the comment to explain why we need skip PT_SIZE space.

Best Regards
  Guo Ren