linux-kernel - Re: [PATCH] ARM: don't enter kgdb when userspace executes a kgdb break instruction.

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Date:	Fri, 1 Aug 2014 09:10:38 +0100
From:	Will Deacon <will.deacon@....com>
To:	Omar Sandoval <osandov@...ndov.com>
Cc:	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	"linux@....linux.org.uk" <linux@....linux.org.uk>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Catalin Marinas <Catalin.Marinas@....com>
Subject: Re: [PATCH] ARM: don't enter kgdb when userspace executes a kgdb
 break instruction.

On Fri, Aug 01, 2014 at 04:05:20AM +0100, Omar Sandoval wrote:
> The kgdb breakpoint hooks (kgdb_brk_fn and kgdb_compiled_brk_fn) should only be
> entered when a kgdb break instruction is executed from the kernel. Otherwise,
> if kgdb is enabled, a userspace program can cause the kernel to drop into the
> debugger by executing either KGDB_BREAKINST or KGDB_COMPILED_BREAK.
> 
> Signed-off-by: Omar Sandoval <osandov@...ndov.com>
> ---
> On a kernel running with kgdb enabled, this program reproduces the problem:
> .globl _start
> _start:
> 	udf	#65006	@ KGDB_BREAKINST
> 
> The same problem has been fixed in ARM64.
> 
>  arch/arm/kernel/kgdb.c | 4 ++++
>  1 file changed, 4 insertions(+)
> 
> diff --git a/arch/arm/kernel/kgdb.c b/arch/arm/kernel/kgdb.c
> index 778c2f7..a74b53c 100644
> --- a/arch/arm/kernel/kgdb.c
> +++ b/arch/arm/kernel/kgdb.c
> @@ -160,12 +160,16 @@ static int kgdb_compiled_brk_fn(struct pt_regs *regs, unsigned int instr)
>  static struct undef_hook kgdb_brkpt_hook = {
>  	.instr_mask		= 0xffffffff,
>  	.instr_val		= KGDB_BREAKINST,
> +	.cpsr_mask		= MODE_MASK,
> +	.cpsr_val		= SVC_MODE,
>  	.fn			= kgdb_brk_fn
>  };
>  
>  static struct undef_hook kgdb_compiled_brkpt_hook = {
>  	.instr_mask		= 0xffffffff,
>  	.instr_val		= KGDB_COMPILED_BREAK,
> +	.cpsr_mask		= MODE_MASK,
> +	.cpsr_val		= SVC_MODE,
>  	.fn			= kgdb_compiled_brk_fn
>  };

Acked-by: Will Deacon <will.deacon@....com>

Please put this into rmk's patch system.

Cheers,

Will
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