Re: [PATCH v7 06/11] arm64: BTI: Decode BYTPE bits when printing PSTATE

[Kees Cook]

On Wed, Feb 26, 2020 at 03:57:09PM +0000, Mark Brown wrote:
> From: Dave Martin <Dave.Martin@xxxxxxx>
> 
> The current code to print PSTATE symbolically when generating
> backtraces etc., does not include the BYTPE field used by Branch
> Target Identification.
> 
> So, decode BYTPE and print it too.
> 
> In the interests of human-readability, print the classes of BTI
> matched.  The symbolic notation, BYTPE (PSTATE[11:10]) and
> permitted classes of subsequent instruction are:
> 
>     -- (BTYPE=0b00): any insn
>     jc (BTYPE=0b01): BTI jc, BTI j, BTI c, PACIxSP
>     -c (BYTPE=0b10): BTI jc, BTI c, PACIxSP
>     j- (BTYPE=0b11): BTI jc, BTI j
> 
> Signed-off-by: Dave Martin <Dave.Martin@xxxxxxx>

Reviewed-by: Kees Cook <keescook@xxxxxxxxxxxx>

-Kees

> Signed-off-by: Mark Brown <broonie@xxxxxxxxxx>
> ---
>  arch/arm64/kernel/process.c | 17 +++++++++++++++--
>  1 file changed, 15 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/arm64/kernel/process.c b/arch/arm64/kernel/process.c
> index b8e3faa8d406..24af13d7bde6 100644
> --- a/arch/arm64/kernel/process.c
> +++ b/arch/arm64/kernel/process.c
> @@ -211,6 +211,15 @@ void machine_restart(char *cmd)
>  	while (1);
>  }
>  
> +#define bstr(suffix, str) [PSR_BTYPE_ ## suffix >> PSR_BTYPE_SHIFT] = str
> +static const char *const btypes[] = {
> +	bstr(NONE, "--"),
> +	bstr(  JC, "jc"),
> +	bstr(   C, "-c"),
> +	bstr(  J , "j-")
> +};
> +#undef bstr
> +
>  static void print_pstate(struct pt_regs *regs)
>  {
>  	u64 pstate = regs->pstate;
> @@ -229,7 +238,10 @@ static void print_pstate(struct pt_regs *regs)
>  			pstate & PSR_AA32_I_BIT ? 'I' : 'i',
>  			pstate & PSR_AA32_F_BIT ? 'F' : 'f');
>  	} else {
> -		printk("pstate: %08llx (%c%c%c%c %c%c%c%c %cPAN %cUAO)\n",
> +		const char *btype_str = btypes[(pstate & PSR_BTYPE_MASK) >>
> +					       PSR_BTYPE_SHIFT];
> +
> +		printk("pstate: %08llx (%c%c%c%c %c%c%c%c %cPAN %cUAO BTYPE=%s)\n",
>  			pstate,
>  			pstate & PSR_N_BIT ? 'N' : 'n',
>  			pstate & PSR_Z_BIT ? 'Z' : 'z',
> @@ -240,7 +252,8 @@ static void print_pstate(struct pt_regs *regs)
>  			pstate & PSR_I_BIT ? 'I' : 'i',
>  			pstate & PSR_F_BIT ? 'F' : 'f',
>  			pstate & PSR_PAN_BIT ? '+' : '-',
> -			pstate & PSR_UAO_BIT ? '+' : '-');
> +			pstate & PSR_UAO_BIT ? '+' : '-',
> +			btype_str);
>  	}
>  }
>  
> -- 
> 2.20.1
> 

-- 
Kees Cook