Re: [PATCH] riscv: kprobe: Fixup misaligned load text

[Jessica Clarke]

On 2 Feb 2023, at 09:48, Björn Töpel <bjorn@xxxxxxxxxx> wrote:
> 
> Guo Ren <guoren@xxxxxxxxxx> writes:
> 
>> On Wed, Feb 1, 2023 at 5:40 PM Björn Töpel <bjorn@xxxxxxxxxx> wrote:
>>> 
>>> guoren@xxxxxxxxxx writes:
>>> 
>>>> From: Guo Ren <guoren@xxxxxxxxxxxxxxxxx>
>>>> 
>>>> The current kprobe would cause a misaligned load for the probe point.
>>>> This patch fixup it with two half-word loads instead.
>>>> 
>>>> Fixes: c22b0bcb1dd0 ("riscv: Add kprobes supported")
>>>> Signed-off-by: Guo Ren <guoren@xxxxxxxxxxxxxxxxx>
>>>> Signed-off-by: Guo Ren <guoren@xxxxxxxxxx>
>>>> Link: https://lore.kernel.org/linux-riscv/878rhig9zj.fsf@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxx/
>>>> Reported-by: Bjorn Topel <bjorn.topel@xxxxxxxxx>
>>>> ---
>>>> arch/riscv/kernel/probes/kprobes.c | 4 +++-
>>>> 1 file changed, 3 insertions(+), 1 deletion(-)
>>>> 
>>>> diff --git a/arch/riscv/kernel/probes/kprobes.c b/arch/riscv/kernel/probes/kprobes.c
>>>> index 41c7481afde3..c1160629cef4 100644
>>>> --- a/arch/riscv/kernel/probes/kprobes.c
>>>> +++ b/arch/riscv/kernel/probes/kprobes.c
>>>> @@ -74,7 +74,9 @@ int __kprobes arch_prepare_kprobe(struct kprobe *p)
>>>>              return -EILSEQ;
>>>> 
>>>>      /* copy instruction */
>>>> -     p->opcode = *p->addr;
>>>> +     p->opcode = (kprobe_opcode_t)(*(u16 *)probe_addr);
>>>> +     if (GET_INSN_LENGTH(p->opcode) == 4)
>>>> +             p->opcode |= (kprobe_opcode_t)(*(u16 *)(probe_addr + 2))
>>>>      << 16;
>>> 
>>> Ugh, those casts. :-( What about the memcpy variant you had in the other
>>> thread?
>> The memcpy version would force load probe_addr + 2. This one would
>> save an lh operation. The code text guarantees half-word alignment. No
>> misaligned load happened. Second, kprobe wouldn't write the last half
>> of 32b instruction.
> 
> Ok, something more readable, like:
> 
> diff --git a/arch/riscv/kernel/probes/kprobes.c b/arch/riscv/kernel/probes/kprobes.c
> index f21592d20306..3602352ba175 100644
> --- a/arch/riscv/kernel/probes/kprobes.c
> +++ b/arch/riscv/kernel/probes/kprobes.c
> @@ -50,14 +50,16 @@ static void __kprobes arch_simulate_insn(struct kprobe *p, struct pt_regs *regs)
> 
> int __kprobes arch_prepare_kprobe(struct kprobe *p)
> {
> -	unsigned long probe_addr = (unsigned long)p->addr;
> +	u16 *insn = (u16 *)p->addr;
> 
> -	if (probe_addr & 0x1)
> +	if ((uintptr_t)insn & 0x1)
> 		return -EILSEQ;
> 
> 	/* copy instruction */
> -	p->opcode = *p->addr;
> -
> +	p->opcode = *insn++;
> +	if (GET_INSN_LENGTH(p->opcode) == 4)
> +		p->opcode |= *insn << 16;

*insn gets promoted to int not unsigned so this is UB if bit 15 is set.

Jess

> +	
> 	/* decode instruction */
> 	switch (riscv_probe_decode_insn(p->addr, &p->ainsn.api)) {
> 	case INSN_REJECTED:	/* insn not supported */
> 
> 
> Björn
> 
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