Re: [PATCH] powerpc/signal64: Copy siginfo before changing regs->nip

[Nicholas Piggin <npiggin@gmail.com>]

Excerpts from Michael Ellerman's message of June 8, 2021 11:46 pm:
> In commit 96d7a4e06fab ("powerpc/signal64: Rewrite handle_rt_signal64()
> to minimise uaccess switches") the 64-bit signal code was rearranged to
> use user_write_access_begin/end().
> 
> As part of that change the call to copy_siginfo_to_user() was moved
> later in the function, so that it could be done after the
> user_write_access_end().
> 
> In particular it was moved after we modify regs->nip to point to the
> signal trampoline. That means if copy_siginfo_to_user() fails we exit
> handle_rt_signal64() with an error but with regs->nip modified, whereas
> previously we would not modify regs->nip until the copy succeeded.
> 
> Returning an error from signal delivery but with regs->nip updated
> leaves the process in a sort of half-delivered state. We do immediately
> force a SEGV in signal_setup_done(), called from do_signal(), so the
> process should never run in the half-delivered state.
> 
> However that SEGV is not delivered until we've gone around to
> do_notify_resume() again, so it's possible some tracing could observe
> the half-delivered state.
> 
> There are other cases where we fail signal delivery with regs partly
> updated, eg. the write to newsp and SA_SIGINFO, but the latter at least
> is very unlikely to fail as it reads back from the frame we just wrote
> to.
> 
> Looking at other arches they seem to be more careful about leaving regs
> unchanged until the copy operations have succeeded, and in general that
> seems like good hygenie.
> 
> So although the current behaviour is not cleary buggy, it's also not
> clearly correct. So move the call to copy_siginfo_to_user() up prior to
> the modification of regs->nip, which is closer to the old behaviour, and
> easier to reason about.

Good catch, should it still have a Fixes: tag though? Even if it's not
clearly buggy we want it to be patched.

Thanks,
Nick

> 
> Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
> ---
>  arch/powerpc/kernel/signal_64.c | 9 ++++-----
>  1 file changed, 4 insertions(+), 5 deletions(-)
> 
> diff --git a/arch/powerpc/kernel/signal_64.c b/arch/powerpc/kernel/signal_64.c
> index dca66481d0c2..f9e1f5428b9e 100644
> --- a/arch/powerpc/kernel/signal_64.c
> +++ b/arch/powerpc/kernel/signal_64.c
> @@ -902,6 +902,10 @@ int handle_rt_signal64(struct ksignal *ksig, sigset_t *set,
>  	unsafe_copy_to_user(&frame->uc.uc_sigmask, set, sizeof(*set), badframe_block);
>  	user_write_access_end();
>  
> +	/* Save the siginfo outside of the unsafe block. */
> +	if (copy_siginfo_to_user(&frame->info, &ksig->info))
> +		goto badframe;
> +
>  	/* Make sure signal handler doesn't get spurious FP exceptions */
>  	tsk->thread.fp_state.fpscr = 0;
>  
> @@ -915,11 +919,6 @@ int handle_rt_signal64(struct ksignal *ksig, sigset_t *set,
>  		regs->nip = (unsigned long) &frame->tramp[0];
>  	}
>  
> -
> -	/* Save the siginfo outside of the unsafe block. */
> -	if (copy_siginfo_to_user(&frame->info, &ksig->info))
> -		goto badframe;
> -
>  	/* Allocate a dummy caller frame for the signal handler. */
>  	newsp = ((unsigned long)frame) - __SIGNAL_FRAMESIZE;
>  	err |= put_user(regs->gpr[1], (unsigned long __user *)newsp);
> -- 
> 2.25.1
> 
>