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Date: Tue, 7 Aug 2018 08:12:42 -0700
From: Kees Cook <keescook@...omium.org>
To: Petr Mladek <pmladek@...e.com>
Cc: Joe Perches <joe@...ches.com>, Oleg Nesterov <oleg@...hat.com>, Palmer Dabbelt <palmer@...ive.com>, 
	Albert Ou <aou@...s.berkeley.edu>, linux-riscv@...ts.infradead.org, 
	LKML <linux-kernel@...r.kernel.org>, 
	Linus Torvalds <torvalds@...ux-foundation.org>, Steven Rostedt <rostedt@...dmis.org>, 
	Tejun Heo <tj@...nel.org>, Greg KH <gregkh@...uxfoundation.org>, 
	Kernel Hardening <kernel-hardening@...ts.openwall.com>
Subject: Re: [PATCH V2] riscv: Convert uses of REG_FMT to %p

On Tue, Aug 7, 2018 at 7:18 AM, Petr Mladek <pmladek@...e.com> wrote:
> On Sat 2018-07-28 09:39:57, Joe Perches wrote:
>> Use %p pointer output instead of REG_FMT and cast the unsigned longs to
>> (void *) to avoid exposing kernel addresses.
>>
>> Miscellanea:
>>
>> o Convert pr_cont to printk(KERN_DEFAULT as these uses are
>>   new logging lines and not previous line continuations
>> o Remove the now unused REG_FMT defines
>>
>> Signed-off-by: Joe Perches <joe@...ches.com>
>> ---
>>
>> v2: sigh: Add missing fault.c
>>
>>  arch/riscv/include/asm/ptrace.h |  6 -----
>>  arch/riscv/kernel/process.c     | 52 +++++++++++++++++++++--------------------
>>  arch/riscv/kernel/traps.c       |  4 ++--
>>  arch/riscv/mm/fault.c           |  6 ++---
>>  4 files changed, 32 insertions(+), 36 deletions(-)
>>
>> diff --git a/arch/riscv/include/asm/ptrace.h b/arch/riscv/include/asm/ptrace.h
>> index 2c5df945d43c..b123e723f8fa 100644
>> --- a/arch/riscv/include/asm/ptrace.h
>> +++ b/arch/riscv/include/asm/ptrace.h
>> @@ -60,12 +60,6 @@ struct pt_regs {
>>          unsigned long orig_a0;
>>  };
>>
>> -#ifdef CONFIG_64BIT
>> -#define REG_FMT "%016lx"
>> -#else
>> -#define REG_FMT "%08lx"
>> -#endif
>> -
>>  #define user_mode(regs) (((regs)->sstatus & SR_SPP) == 0)
>>
>>
>> diff --git a/arch/riscv/kernel/process.c b/arch/riscv/kernel/process.c
>> index d7c6ca7c95ae..7223f6715ff3 100644
>> --- a/arch/riscv/kernel/process.c
>> +++ b/arch/riscv/kernel/process.c
>> @@ -36,7 +36,7 @@
>>  extern asmlinkage void ret_from_fork(void);
>>  extern asmlinkage void ret_from_kernel_thread(void);
>>
>> -void arch_cpu_idle(void)
>> +void arch_yycpu_idle(void)
>>  {
>>       wait_for_interrupt();
>>       local_irq_enable();
>> @@ -46,31 +46,33 @@ void show_regs(struct pt_regs *regs)
>>  {
>>       show_regs_print_info(KERN_DEFAULT);
>>
>> -     pr_cont("sepc: " REG_FMT " ra : " REG_FMT " sp : " REG_FMT "\n",
>> -             regs->sepc, regs->ra, regs->sp);
>> -     pr_cont(" gp : " REG_FMT " tp : " REG_FMT " t0 : " REG_FMT "\n",
>> -             regs->gp, regs->tp, regs->t0);
>> -     pr_cont(" t1 : " REG_FMT " t2 : " REG_FMT " s0 : " REG_FMT "\n",
>> -             regs->t1, regs->t2, regs->s0);
>> -     pr_cont(" s1 : " REG_FMT " a0 : " REG_FMT " a1 : " REG_FMT "\n",
>> -             regs->s1, regs->a0, regs->a1);
>> -     pr_cont(" a2 : " REG_FMT " a3 : " REG_FMT " a4 : " REG_FMT "\n",
>> -             regs->a2, regs->a3, regs->a4);
>> -     pr_cont(" a5 : " REG_FMT " a6 : " REG_FMT " a7 : " REG_FMT "\n",
>> -             regs->a5, regs->a6, regs->a7);
>> -     pr_cont(" s2 : " REG_FMT " s3 : " REG_FMT " s4 : " REG_FMT "\n",
>> -             regs->s2, regs->s3, regs->s4);
>> -     pr_cont(" s5 : " REG_FMT " s6 : " REG_FMT " s7 : " REG_FMT "\n",
>> -             regs->s5, regs->s6, regs->s7);
>> -     pr_cont(" s8 : " REG_FMT " s9 : " REG_FMT " s10: " REG_FMT "\n",
>> -             regs->s8, regs->s9, regs->s10);
>> -     pr_cont(" s11: " REG_FMT " t3 : " REG_FMT " t4 : " REG_FMT "\n",
>> -             regs->s11, regs->t3, regs->t4);
>> -     pr_cont(" t5 : " REG_FMT " t6 : " REG_FMT "\n",
>> -             regs->t5, regs->t6);
>> +     printk(KERN_DEFAULT "sepc: %p ra : %p sp : %p\n",
>> +            (void *)regs->sepc, (void *)regs->ra, (void *)regs->sp);
>> +     printk(KERN_DEFAULT " gp : %p tp : %p t0 : %p\n",
>> +            (void *)regs->gp, (void *)regs->tp, (void *)regs->t0);
>> +     printk(KERN_DEFAULT " t1 : %p t2 : %p s0 : %p\n",
>> +            (void *)regs->t1, (void *)regs->t2, (void *)regs->s0);
>> +     printk(KERN_DEFAULT " s1 : %p a0 : %p a1 : %p\n",
>> +            (void *)regs->s1, (void *)regs->a0, (void *)regs->a1);
>> +     printk(KERN_DEFAULT " a2 : %p a3 : %p a4 : %p\n",
>> +            (void *)regs->a2, (void *)regs->a3, (void *)regs->a4);
>> +     printk(KERN_DEFAULT " a5 : %p a6 : %p a7 : %p\n",
>> +            (void *)regs->a5, (void *)regs->a6, (void *)regs->a7);
>> +     printk(KERN_DEFAULT " s2 : %p s3 : %p s4 : %p\n",
>> +            (void *)regs->s2, (void *)regs->s3, (void *)regs->s4);
>> +     printk(KERN_DEFAULT " s5 : %p s6 : %p s7 : %p\n",
>> +            (void *)regs->s5, (void *)regs->s6, (void *)regs->s7);
>> +     printk(KERN_DEFAULT " s8 : %p s9 : %p s10: %p\n",
>> +            (void *)regs->s8, (void *)regs->s9, (void *)regs->s10);
>> +     printk(KERN_DEFAULT " s11: %p t3 : %p t4 : %p\n",
>> +            (void *)regs->s11, (void *)regs->t3, (void *)regs->t4);
>> +     printk(KERN_DEFAULT " t5 : %p t6 : %p\n",
>> +            (void *)regs->t5, (void *)regs->t6);
>>
>> -     pr_cont("sstatus: " REG_FMT " sbadaddr: " REG_FMT " scause: " REG_FMT "\n",
>> -             regs->sstatus, regs->sbadaddr, regs->scause);
>> +     printk(KERN_DEFAULT "sstatus: %p sbadaddr: %p scause: %p\n",
>> +            (void *)regs->sstatus,
>> +            (void *)regs->sbadaddr,
>> +            (void *)regs->scause);
>>  }
>
> This change makes the dump almost unusable. Note that registers contain any
> kind of information, not only pointers.
>
> My understanding is that %px was introduced because printing the
> pointer directly is sometimes worth the security risk. IMHO, this
> is one place where we want to risk printing the real value.
>
> Anyway, it needs to be decided by security gurus. Adding some more
> people into CC.

If these are trap or fault dumps, I think %px (or %llx) is correct.
Before riscv existed, I went through the fault handlers and fixed this
already:

10a7e9d84915 ("Do not hash userspace addresses in fault handlers")

and dump_stack() is calling show_regs(), so I think that should be
left readable as well. (If not, we have a lot more than riscv to fix.)

-Kees

-- 
Kees Cook
Pixel Security

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