riscv: prevent stack corruption by reserving task_pt_regs(p) early

Early function calls, such as setup_vm(), relocate_enable_mmu(),
soc_early_init() etc, are free to operate on stack. However,
PT_SIZE_ON_STACK bytes at the head of the kernel stack are purposedly
reserved for the placement of per-task register context pointed by
task_pt_regs(p). Those functions may corrupt task_pt_regs if we overlap
the $sp with it. In fact, we had accidentally corrupted sstatus.VS in some
tests, treating the kernel to save V context before V was actually
allocated, resulting in a kernel panic.

Thus, we should skip PT_SIZE_ON_STACK for $sp before making C function
calls from the top-level assembly.
Co-developed-by: ShihPo Hung <shihpo.hung@sifive.com>
Signed-off-by: ShihPo Hung <shihpo.hung@sifive.com>
Co-developed-by: Vincent Chen <vincent.chen@sifive.com>
Signed-off-by: Vincent Chen <vincent.chen@sifive.com>
Signed-off-by: Greentime Hu <greentime.hu@sifive.com>
Signed-off-by: Andy Chiu <andy.chiu@sifive.com>
Reviewed-by: Conor Dooley <conor.dooley@microchip.com>
Reviewed-by: Heiko Stuebner <heiko.stuebner@vrull.eu>
Tested-by: Heiko Stuebner <heiko.stuebner@vrull.eu>
Link: https://lore.kernel.org/r/20230605110724.21391-18-andy.chiu@sifive.comSigned-off-by: Palmer Dabbelt <palmer@rivosinc.com>

arch/riscv/kernel/head.S

@@ -301,6 +301,7 @@ clear_bss_done:
 	la tp, init_task
 	la sp, init_thread_union + THREAD_SIZE
 	XIP_FIXUP_OFFSET sp
+	addi sp, sp, -PT_SIZE_ON_STACK
 #ifdef CONFIG_BUILTIN_DTB
 	la a0, __dtb_start
 	XIP_FIXUP_OFFSET a0
@@ -318,6 +319,7 @@ clear_bss_done:
 	/* Restore C environment */
 	la tp, init_task
 	la sp, init_thread_union + THREAD_SIZE
+	addi sp, sp, -PT_SIZE_ON_STACK
 
 #ifdef CONFIG_KASAN
 	call kasan_early_init