[PATCH] riscv: probes: save original sp in rethook trampoline

[Martin Kaiser <martin@kaiser.cx>]

Reading a word from the stack in a kretprobe crashes a risc-v kernel.

$ cd /sys/kernel/tracing/
$ echo 'r n_tty_write $stack0' > dynamic_events
$ echo 1 > events/kprobes/enable
Unable to handle kernel paging request at virtual address 0000000200000128
...
[<ffffffff80016d16>] regs_get_kernel_stack_nth+0x26/0x38
[<ffffffff80177196>] process_fetch_insn+0x3ee/0x760
[<ffffffff80177836>] kretprobe_trace_func+0x116/0x1f0
[<ffffffff8017795a>] kretprobe_dispatcher+0x4a/0x58
[<ffffffff8013572e>] kretprobe_rethook_handler+0x5e/0x90
[<ffffffff80180838>] rethook_trampoline_handler+0x70/0x108
[<ffffffff8001ba32>] arch_rethook_trampoline_callback+0x12/0x1c
[<ffffffff8001ba84>] arch_rethook_trampoline+0x48/0x94
[<ffffffff8067872a>] tty_write+0x1a/0x30

In regs_get_kernel_stack_nth, regs->sp contains an arbitrary value.

arch_rethook_trampoline saves the registers from the probed function in a
struct pt_regs. sp is not saved. Instead, sp is decremented for
arch_rethook_trampoline's local stack.

Fix this crash and save the original sp along with the other registers.
Use a0 as a temporary register, it is overwritten anyway.

Signed-off-by: Martin Kaiser <martin@kaiser.cx>
---
 arch/riscv/kernel/probes/rethook_trampoline.S | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/arch/riscv/kernel/probes/rethook_trampoline.S b/arch/riscv/kernel/probes/rethook_trampoline.S
index f2cd83d9b0f0..c3aa8d8cf5af 100644
--- a/arch/riscv/kernel/probes/rethook_trampoline.S
+++ b/arch/riscv/kernel/probes/rethook_trampoline.S
@@ -41,6 +41,9 @@
 	REG_S x29, PT_T4(sp)
 	REG_S x30, PT_T5(sp)
 	REG_S x31, PT_T6(sp)
+	/* save original sp */
+	addi a0, sp, PT_SIZE_ON_STACK
+	REG_S a0, PT_SP(sp)
 	.endm
 
 	.macro restore_all_base_regs
-- 
2.43.7