Re: [PATCH v3] arm64: print a fault message when attempting to write RO memory

[Laura Abbott]

On 04/03/2017 11:58 PM, Stephen Boyd wrote:
> If a page is marked read only we should print out that fact,
> instead of printing out that there was a page fault. Right now we
> get a cryptic error message that something went wrong with an
> unhandled fault, but we don't evaluate the esr to figure out that
> it was a read/write permission fault.
> 
> Instead of seeing:
> 
>   Unable to handle kernel paging request at virtual address ffff000008e460d8
>   pgd = ffff800003504000
>   [ffff000008e460d8] *pgd=0000000083473003, *pud=0000000083503003, *pmd=0000000000000000
>   Internal error: Oops: 9600004f [#1] PREEMPT SMP
> 
> we'll see:
> 
>   Unable to handle kernel write to read-only memory at virtual address ffff000008e760d8
>   pgd = ffff80003d3de000
>   [ffff000008e760d8] *pgd=0000000083472003, *pud=0000000083435003, *pmd=0000000000000000
>   Internal error: Oops: 9600004f [#1] PREEMPT SMP
> 
> We also fold the userspace address check into is_permission_fault()
> instead of at the current callsite so that the function can't be
> abused with software PAN and a kernel space address.
> 
> Cc: James Morse <james.morse@xxxxxxx>
> Cc: Laura Abbott <labbott@xxxxxxxxxx>
> Cc: Mark Rutland <mark.rutland@xxxxxxx>
> Signed-off-by: Stephen Boyd <stephen.boyd@xxxxxxxxxx>

Acked-by: Laura Abbott <labbott@xxxxxxxxxx>

> ---
> 
> Changes from v2:
>  * Fold addr check into is_permission_fault() (James Morse)
> 
> Changes from v1:
>  * Move into __do_kernel_fault() (Mark Rutland)
> 
>  arch/arm64/mm/fault.c | 54 +++++++++++++++++++++++++++++++++------------------
>  1 file changed, 35 insertions(+), 19 deletions(-)
> 
> diff --git a/arch/arm64/mm/fault.c b/arch/arm64/mm/fault.c
> index 156169c6981b..c6560cb4ef50 100644
> --- a/arch/arm64/mm/fault.c
> +++ b/arch/arm64/mm/fault.c
> @@ -160,12 +160,32 @@ static bool is_el1_instruction_abort(unsigned int esr)
>  	return ESR_ELx_EC(esr) == ESR_ELx_EC_IABT_CUR;
>  }
>  
> +static inline bool is_permission_fault(unsigned int esr, struct pt_regs *regs,
> +				       unsigned long addr)
> +{
> +	unsigned int ec       = ESR_ELx_EC(esr);
> +	unsigned int fsc_type = esr & ESR_ELx_FSC_TYPE;
> +
> +	if (ec != ESR_ELx_EC_DABT_CUR && ec != ESR_ELx_EC_IABT_CUR)
> +		return false;
> +
> +	if (fsc_type == ESR_ELx_FSC_PERM)
> +		return true;
> +
> +	if (addr < USER_DS && system_uses_ttbr0_pan())
> +		return fsc_type == ESR_ELx_FSC_FAULT &&
> +			(regs->pstate & PSR_PAN_BIT);
> +
> +	return false;
> +}
> +
>  /*
>   * The kernel tried to access some page that wasn't present.
>   */
>  static void __do_kernel_fault(struct mm_struct *mm, unsigned long addr,
>  			      unsigned int esr, struct pt_regs *regs)
>  {
> +	const char *msg;
>  	/*
>  	 * Are we prepared to handle this kernel fault?
>  	 * We are almost certainly not prepared to handle instruction faults.
> @@ -177,9 +197,20 @@ static void __do_kernel_fault(struct mm_struct *mm, unsigned long addr,
>  	 * No handler, we'll have to terminate things with extreme prejudice.
>  	 */
>  	bust_spinlocks(1);
> -	pr_alert("Unable to handle kernel %s at virtual address %08lx\n",
> -		 (addr < PAGE_SIZE) ? "NULL pointer dereference" :
> -		 "paging request", addr);
> +
> +	if (is_permission_fault(esr, regs, addr)) {
> +		if (esr & ESR_ELx_WNR)
> +			msg = "write to read-only memory";
> +		else
> +			msg = "read from unreadable memory";
> +	} else if (addr < PAGE_SIZE) {
> +		msg = "NULL pointer dereference";
> +	} else {
> +		msg = "paging request";
> +	}
> +
> +	pr_alert("Unable to handle kernel %s at virtual address %08lx\n", msg,
> +		 addr);
>  
>  	show_pte(mm, addr);
>  	die("Oops", regs, esr);
> @@ -269,21 +300,6 @@ static int __do_page_fault(struct mm_struct *mm, unsigned long addr,
>  	return fault;
>  }
>  
> -static inline bool is_permission_fault(unsigned int esr, struct pt_regs *regs)
> -{
> -	unsigned int ec       = ESR_ELx_EC(esr);
> -	unsigned int fsc_type = esr & ESR_ELx_FSC_TYPE;
> -
> -	if (ec != ESR_ELx_EC_DABT_CUR && ec != ESR_ELx_EC_IABT_CUR)
> -		return false;
> -
> -	if (system_uses_ttbr0_pan())
> -		return fsc_type == ESR_ELx_FSC_FAULT &&
> -			(regs->pstate & PSR_PAN_BIT);
> -	else
> -		return fsc_type == ESR_ELx_FSC_PERM;
> -}
> -
>  static bool is_el0_instruction_abort(unsigned int esr)
>  {
>  	return ESR_ELx_EC(esr) == ESR_ELx_EC_IABT_LOW;
> @@ -321,7 +337,7 @@ static int __kprobes do_page_fault(unsigned long addr, unsigned int esr,
>  		mm_flags |= FAULT_FLAG_WRITE;
>  	}
>  
> -	if (addr < USER_DS && is_permission_fault(esr, regs)) {
> +	if (is_permission_fault(esr, regs, addr)) {
>  		/* regs->orig_addr_limit may be 0 if we entered from EL0 */
>  		if (regs->orig_addr_limit == KERNEL_DS)
>  			die("Accessing user space memory with fs=KERNEL_DS", regs, esr);
>