[PATCH] MIPS: mm: abort uaccess retries upon fatal signal

[PATCH] MIPS: mm: abort uaccess retries upon fatal signal

[Thomas Bogendoerfer]

When there's a fatal signal pending, MIPS's do_page_fault()
implementation returns. The intent is that we'll return to the
faulting userspace instruction, delivering the signal on the way.

However, if we take a fatal signal during fixing up a uaccess, this
results in a return to the faulting kernel instruction, which will be
instantly retried, resulting in the same fault being taken forever. As
the task never reaches userspace, the signal is not delivered, and the
task is left unkillable. While the task is stuck in this state, it can
inhibit the forward progress of the system.

To avoid this, we must ensure that when a fatal signal is pending, we
apply any necessary fixup for a faulting kernel instruction. Thus we
will return to an error path, and it is up to that code to make forward
progress towards delivering the fatal signal.

[ Description taken from commit 746a272e4414 ("ARM: 8692/1: mm: abort
   uaccess retries upon fatal signal") ]

Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>
---
 arch/mips/mm/fault.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/arch/mips/mm/fault.c b/arch/mips/mm/fault.c
index 7c871b14e74a..e7abda9c013f 100644
--- a/arch/mips/mm/fault.c
+++ b/arch/mips/mm/fault.c
@@ -156,8 +156,11 @@ static void __kprobes __do_page_fault(struct pt_regs *regs, unsigned long write,
 	 */
 	fault = handle_mm_fault(vma, address, flags, regs);
 
-	if (fault_signal_pending(fault, regs))
+	if (fault_signal_pending(fault, regs)) {
+		if (!user_mode(regs))
+			goto no_context;
 		return;
+	}
 
 	if (unlikely(fault & VM_FAULT_ERROR)) {
 		if (fault & VM_FAULT_OOM)
-- 
2.29.2

Re: [PATCH] MIPS: mm: abort uaccess retries upon fatal signal

[Mark Rutland]

On Thu, Jan 21, 2021 at 05:04:16PM +0100, Thomas Bogendoerfer wrote:
> When there's a fatal signal pending, MIPS's do_page_fault()
> implementation returns. The intent is that we'll return to the
> faulting userspace instruction, delivering the signal on the way.
> 
> However, if we take a fatal signal during fixing up a uaccess, this
> results in a return to the faulting kernel instruction, which will be
> instantly retried, resulting in the same fault being taken forever. As
> the task never reaches userspace, the signal is not delivered, and the
> task is left unkillable. While the task is stuck in this state, it can
> inhibit the forward progress of the system.
> 
> To avoid this, we must ensure that when a fatal signal is pending, we
> apply any necessary fixup for a faulting kernel instruction. Thus we
> will return to an error path, and it is up to that code to make forward
> progress towards delivering the fatal signal.
> 
> [ Description taken from commit 746a272e4414 ("ARM: 8692/1: mm: abort
>    uaccess retries upon fatal signal") ]
> 
> Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>

FWIW, this looks right to me, from a scan of the no_context path. I
don't have any MIPS system to test on, but FWIW:

Acked-by: Mark Rutland <mark.rutland@arm.com>

Thanks for spinning this!

Mark.

> ---
>  arch/mips/mm/fault.c | 5 ++++-
>  1 file changed, 4 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/mips/mm/fault.c b/arch/mips/mm/fault.c
> index 7c871b14e74a..e7abda9c013f 100644
> --- a/arch/mips/mm/fault.c
> +++ b/arch/mips/mm/fault.c
> @@ -156,8 +156,11 @@ static void __kprobes __do_page_fault(struct pt_regs *regs, unsigned long write,
>  	 */
>  	fault = handle_mm_fault(vma, address, flags, regs);
>  
> -	if (fault_signal_pending(fault, regs))
> +	if (fault_signal_pending(fault, regs)) {
> +		if (!user_mode(regs))
> +			goto no_context;
>  		return;
> +	}
>  
>  	if (unlikely(fault & VM_FAULT_ERROR)) {
>  		if (fault & VM_FAULT_OOM)
> -- 
> 2.29.2
> 

Re: [PATCH] MIPS: mm: abort uaccess retries upon fatal signal

[Thomas Bogendoerfer]

On Fri, Jan 22, 2021 at 10:11:57AM +0000, Mark Rutland wrote:
> On Thu, Jan 21, 2021 at 05:04:16PM +0100, Thomas Bogendoerfer wrote:
> > When there's a fatal signal pending, MIPS's do_page_fault()
> > implementation returns. The intent is that we'll return to the
> > faulting userspace instruction, delivering the signal on the way.
> > 
> > However, if we take a fatal signal during fixing up a uaccess, this
> > results in a return to the faulting kernel instruction, which will be
> > instantly retried, resulting in the same fault being taken forever. As
> > the task never reaches userspace, the signal is not delivered, and the
> > task is left unkillable. While the task is stuck in this state, it can
> > inhibit the forward progress of the system.
> > 
> > To avoid this, we must ensure that when a fatal signal is pending, we
> > apply any necessary fixup for a faulting kernel instruction. Thus we
> > will return to an error path, and it is up to that code to make forward
> > progress towards delivering the fatal signal.
> > 
> > [ Description taken from commit 746a272e4414 ("ARM: 8692/1: mm: abort
> >    uaccess retries upon fatal signal") ]
> > 
> > Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>
> 
> FWIW, this looks right to me, from a scan of the no_context path. I
> don't have any MIPS system to test on, but FWIW:

no worry, I've tested it ;-)

Thomas.

-- 
Crap can work. Given enough thrust pigs will fly, but it's not necessarily a
good idea.                                                [ RFC1925, 2.3 ]

Re: [PATCH] MIPS: mm: abort uaccess retries upon fatal signal

[Thomas Bogendoerfer]

On Thu, Jan 21, 2021 at 05:04:16PM +0100, Thomas Bogendoerfer wrote:
> When there's a fatal signal pending, MIPS's do_page_fault()
> implementation returns. The intent is that we'll return to the
> faulting userspace instruction, delivering the signal on the way.
> 
> However, if we take a fatal signal during fixing up a uaccess, this
> results in a return to the faulting kernel instruction, which will be
> instantly retried, resulting in the same fault being taken forever. As
> the task never reaches userspace, the signal is not delivered, and the
> task is left unkillable. While the task is stuck in this state, it can
> inhibit the forward progress of the system.
> 
> To avoid this, we must ensure that when a fatal signal is pending, we
> apply any necessary fixup for a faulting kernel instruction. Thus we
> will return to an error path, and it is up to that code to make forward
> progress towards delivering the fatal signal.
> 
> [ Description taken from commit 746a272e4414 ("ARM: 8692/1: mm: abort
>    uaccess retries upon fatal signal") ]
> 
> Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>
> ---
>  arch/mips/mm/fault.c | 5 ++++-
>  1 file changed, 4 insertions(+), 1 deletion(-)

applied to mips-next.

Thomas.

-- 
Crap can work. Given enough thrust pigs will fly, but it's not necessarily a
good idea.                                                [ RFC1925, 2.3 ]