Re: [RESEND][PATCH 3/3] rseq: Ensure SIGBUS delivered on memory failure

[Kees Cook <kees@kernel.org>]

On Tue, Jul 23, 2024 at 04:47:52PM +0200, Andrew Zaborowski wrote:
> Uncorrected memory errors for user pages are signaled to processes
> using SIGBUS or, if the error happens in a syscall, an error retval
> from the syscall.  The SIGBUS is documented in
> Documentation/mm/hwpoison.rst#failure-recovery-modes
> 
> Once a user task sets t->rseq in the rseq() syscall, if the kernel
> cannot access the memory pointed to by t->rseq->rseq_cs, that initial
> rseq() and all future syscalls should return an error so understandably
> the code just kills the task.
> 
> To ensure that SIGBUS is used set the new t->kill_on_efault flag and
> run queued task work on rseq_get_rseq_cs() errors to give memory_failure
> the chance to run.
> 
> Note: the rseq checks run inside resume_user_mode_work() so whenever
> _TIF_NOTIFY_RESUME is set.  They do not run on every syscall exit so
> I'm not concerned that these extra flag operations are in a hot path,
> except with CONFIG_DEBUG_RSEQ.
> 
> Signed-off-by: Andrew Zaborowski <andrew.zaborowski@intel.com>
> ---
>  kernel/rseq.c | 25 +++++++++++++++++++++----

Can an rseq maintainer please review this? I can carry it via the execve
tree with the related patches...

-Kees

>  1 file changed, 21 insertions(+), 4 deletions(-)
> 
> diff --git a/kernel/rseq.c b/kernel/rseq.c
> index 9de6e35fe..c5809cd13 100644
> --- a/kernel/rseq.c
> +++ b/kernel/rseq.c
> @@ -13,6 +13,7 @@
>  #include <linux/syscalls.h>
>  #include <linux/rseq.h>
>  #include <linux/types.h>
> +#include <linux/task_work.h>
>  #include <asm/ptrace.h>
>  
>  #define CREATE_TRACE_POINTS
> @@ -320,6 +321,8 @@ void __rseq_handle_notify_resume(struct ksignal *ksig, struct pt_regs *regs)
>  	if (unlikely(t->flags & PF_EXITING))
>  		return;
>  
> +	t->kill_on_efault = true;
> +
>  	/*
>  	 * regs is NULL if and only if the caller is in a syscall path.  Skip
>  	 * fixup and leave rseq_cs as is so that rseq_sycall() will detect and
> @@ -330,13 +333,18 @@ void __rseq_handle_notify_resume(struct ksignal *ksig, struct pt_regs *regs)
>  		if (unlikely(ret < 0))
>  			goto error;
>  	}
> -	if (unlikely(rseq_update_cpu_node_id(t)))
> -		goto error;
> -	return;
> +	if (likely(!rseq_update_cpu_node_id(t)))
> +		goto out;
>  
>  error:
> +	/* Allow task work to override signr */
> +	task_work_run();
> +
>  	sig = ksig ? ksig->sig : 0;
>  	force_sigsegv(sig);
> +
> +out:
> +	t->kill_on_efault = false;
>  }
>  
>  #ifdef CONFIG_DEBUG_RSEQ
> @@ -353,8 +361,17 @@ void rseq_syscall(struct pt_regs *regs)
>  
>  	if (!t->rseq)
>  		return;
> -	if (rseq_get_rseq_cs(t, &rseq_cs) || in_rseq_cs(ip, &rseq_cs))
> +
> +	t->kill_on_efault = true;
> +
> +	if (rseq_get_rseq_cs(t, &rseq_cs) || in_rseq_cs(ip, &rseq_cs)) {
> +		/* Allow task work to override signr */
> +		task_work_run();
> +
>  		force_sig(SIGSEGV);
> +	}
> +
> +	t->kill_on_efault = false;
>  }
>  
>  #endif
> -- 
> 2.43.0
> 

-- 
Kees Cook