Re: [PATCH v2] rseq: Avoid infinite recursion when delivering SIGSEGV

[Mathieu Desnoyers <mathieu.desnoyers@efficios.com>]

----- On Jun 21, 2018, at 7:54 AM, Will Deacon will.deacon@arm.com wrote:

> When delivering a signal to a task that is using rseq, we call into
> __rseq_handle_notify_resume() so that the registers pushed in the
> sigframe are updated to reflect the state of the restartable sequence
> (for example, ensuring that the signal returns to the abort handler if
> necessary).
> 
> However, if the rseq management fails due to an unrecoverable fault when
> accessing userspace or certain combinations of RSEQ_CS_* flags, then we
> will attempt to deliver a SIGSEGV. This has the potential for infinite
> recursion if the rseq code continuously fails on signal delivery.
> 
> Avoid this problem by using force_sigsegv() instead of force_sig(), which
> is explicitly designed to reset the SEGV handler to SIG_DFL in the case
> of a recursive fault. In doing so, remove rseq_signal_deliver() from the
> internal rseq API and have an optional struct ksignal * parameter to
> rseq_handle_notify_resume() instead.
> 
> Signed-off-by: Will Deacon <will.deacon@arm.com>
> ---
> 
> RFC v1 -> v2: Kill rseq_signal_deliver()

I actually meant to kill _rseq_handle_notify_resume introduced by
your patch, not rseq_signal_deliver().

Please keep rseq_signal_deliver() as a static inline, and just remove
the _rseq_handle_notify_resume() by changing the signature of
rseq_handle_notify_resume to take an extra sig argument (which can
be NULL).

Thanks,

Mathieu

> 
> arch/arm/kernel/signal.c     |  4 ++--
> arch/powerpc/kernel/signal.c |  4 ++--
> arch/x86/entry/common.c      |  2 +-
> arch/x86/kernel/signal.c     |  2 +-
> include/linux/sched.h        | 27 ++++++++++++---------------
> kernel/rseq.c                |  7 ++++---
> 6 files changed, 22 insertions(+), 24 deletions(-)
> 
> diff --git a/arch/arm/kernel/signal.c b/arch/arm/kernel/signal.c
> index f09e9d66d605..184f69dcb9f2 100644
> --- a/arch/arm/kernel/signal.c
> +++ b/arch/arm/kernel/signal.c
> @@ -544,7 +544,7 @@ static void handle_signal(struct ksignal *ksig, struct
> pt_regs *regs)
> 	 * Increment event counter and perform fixup for the pre-signal
> 	 * frame.
> 	 */
> -	rseq_signal_deliver(regs);
> +	rseq_handle_notify_resume(ksig, regs);
> 
> 	/*
> 	 * Set up the stack frame
> @@ -666,7 +666,7 @@ do_work_pending(struct pt_regs *regs, unsigned int
> thread_flags, int syscall)
> 			} else {
> 				clear_thread_flag(TIF_NOTIFY_RESUME);
> 				tracehook_notify_resume(regs);
> -				rseq_handle_notify_resume(regs);
> +				rseq_handle_notify_resume(NULL, regs);
> 			}
> 		}
> 		local_irq_disable();
> diff --git a/arch/powerpc/kernel/signal.c b/arch/powerpc/kernel/signal.c
> index 17fe4339ba59..88bdf243a5bd 100644
> --- a/arch/powerpc/kernel/signal.c
> +++ b/arch/powerpc/kernel/signal.c
> @@ -134,7 +134,7 @@ static void do_signal(struct task_struct *tsk)
> 	/* Re-enable the breakpoints for the signal stack */
> 	thread_change_pc(tsk, tsk->thread.regs);
> 
> -	rseq_signal_deliver(tsk->thread.regs);
> +	rseq_handle_notify_resume(&ksig, tsk->thread.regs);
> 
> 	if (is32) {
>         	if (ksig.ka.sa.sa_flags & SA_SIGINFO)
> @@ -170,7 +170,7 @@ void do_notify_resume(struct pt_regs *regs, unsigned long
> thread_info_flags)
> 	if (thread_info_flags & _TIF_NOTIFY_RESUME) {
> 		clear_thread_flag(TIF_NOTIFY_RESUME);
> 		tracehook_notify_resume(regs);
> -		rseq_handle_notify_resume(regs);
> +		rseq_handle_notify_resume(NULL, regs);
> 	}
> 
> 	user_enter();
> diff --git a/arch/x86/entry/common.c b/arch/x86/entry/common.c
> index 92190879b228..3b2490b81918 100644
> --- a/arch/x86/entry/common.c
> +++ b/arch/x86/entry/common.c
> @@ -164,7 +164,7 @@ static void exit_to_usermode_loop(struct pt_regs *regs, u32
> cached_flags)
> 		if (cached_flags & _TIF_NOTIFY_RESUME) {
> 			clear_thread_flag(TIF_NOTIFY_RESUME);
> 			tracehook_notify_resume(regs);
> -			rseq_handle_notify_resume(regs);
> +			rseq_handle_notify_resume(NULL, regs);
> 		}
> 
> 		if (cached_flags & _TIF_USER_RETURN_NOTIFY)
> diff --git a/arch/x86/kernel/signal.c b/arch/x86/kernel/signal.c
> index 445ca11ff863..8affeeb74935 100644
> --- a/arch/x86/kernel/signal.c
> +++ b/arch/x86/kernel/signal.c
> @@ -692,7 +692,7 @@ setup_rt_frame(struct ksignal *ksig, struct pt_regs *regs)
> 	 * Increment event counter and perform fixup for the pre-signal
> 	 * frame.
> 	 */
> -	rseq_signal_deliver(regs);
> +	rseq_handle_notify_resume(ksig, regs);
> 
> 	/* Set up the stack frame */
> 	if (is_ia32_frame(ksig)) {
> diff --git a/include/linux/sched.h b/include/linux/sched.h
> index 87bf02d93a27..fd23a540cd8d 100644
> --- a/include/linux/sched.h
> +++ b/include/linux/sched.h
> @@ -1799,20 +1799,19 @@ static inline void rseq_set_notify_resume(struct
> task_struct *t)
> 		set_tsk_thread_flag(t, TIF_NOTIFY_RESUME);
> }
> 
> -void __rseq_handle_notify_resume(struct pt_regs *regs);
> +void __rseq_handle_notify_resume(struct ksignal *sig, struct pt_regs *regs);
> 
> -static inline void rseq_handle_notify_resume(struct pt_regs *regs)
> +static inline void rseq_handle_notify_resume(struct ksignal *ksig,
> +					     struct pt_regs *regs)
> {
> -	if (current->rseq)
> -		__rseq_handle_notify_resume(regs);
> -}
> +	if (ksig) {
> +		preempt_disable();
> +		__set_bit(RSEQ_EVENT_SIGNAL_BIT, &current->rseq_event_mask);
> +		preempt_enable();
> +	}
> 
> -static inline void rseq_signal_deliver(struct pt_regs *regs)
> -{
> -	preempt_disable();
> -	__set_bit(RSEQ_EVENT_SIGNAL_BIT, &current->rseq_event_mask);
> -	preempt_enable();
> -	rseq_handle_notify_resume(regs);
> +	if (current->rseq)
> +		__rseq_handle_notify_resume(ksig, regs);
> }
> 
> /* rseq_preempt() requires preemption to be disabled. */
> @@ -1864,10 +1863,8 @@ static inline void rseq_execve(struct task_struct *t)
> static inline void rseq_set_notify_resume(struct task_struct *t)
> {
> }
> -static inline void rseq_handle_notify_resume(struct pt_regs *regs)
> -{
> -}
> -static inline void rseq_signal_deliver(struct pt_regs *regs)
> +static inline void rseq_handle_notify_resume(struct ksignal *ksig,
> +					     struct pt_regs *regs)
> {
> }
> static inline void rseq_preempt(struct task_struct *t)
> diff --git a/kernel/rseq.c b/kernel/rseq.c
> index ae306f90c514..22b6acf1ad63 100644
> --- a/kernel/rseq.c
> +++ b/kernel/rseq.c
> @@ -251,10 +251,10 @@ static int rseq_ip_fixup(struct pt_regs *regs)
>  * respect to other threads scheduled on the same CPU, and with respect
>  * to signal handlers.
>  */
> -void __rseq_handle_notify_resume(struct pt_regs *regs)
> +void __rseq_handle_notify_resume(struct ksignal *ksig, struct pt_regs *regs)
> {
> 	struct task_struct *t = current;
> -	int ret;
> +	int ret, sig;
> 
> 	if (unlikely(t->flags & PF_EXITING))
> 		return;
> @@ -268,7 +268,8 @@ void __rseq_handle_notify_resume(struct pt_regs *regs)
> 	return;
> 
> error:
> -	force_sig(SIGSEGV, t);
> +	sig = ksig ? ksig->sig : 0;
> +	force_sigsegv(sig, t);
> }
> 
> #ifdef CONFIG_DEBUG_RSEQ
> --
> 2.1.4

-- 
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com