[PATCH] powerpc/mm/hash: Hand user access of kernel address gracefully

[Breno Leitao]

hi Aneesh,

On 11/26/18 12:35 PM, Aneesh Kumar K.V wrote:
> With commit 2865d08dd9ea ("powerpc/mm: Move the DSISR_PROTFAULT sanity check")
> we moved the protection fault access check before vma lookup. That means we
> hit that WARN_ON when user space access a kernel address.  Before the commit
> this was handled by find_vma() not finding vma for the kernel address and
> considering that access as bad area access.
> 
> Avoid the confusing WARN_ON and convert that to a ratelimited printk.
> With the patch we now get
> 
> for load:
> [  187.700294] a.out[5997]: User access of kernel address (c00000000000dea0) - exploit attempt? (uid: 1000)
> [  187.700344] a.out[5997]: segfault (11) at c00000000000dea0 nip 1317c0798 lr 7fff80d6441c code 1 in a.out[1317c0000+10000]
> [  187.700429] a.out[5997]: code: 60000000 60420000 3c4c0002 38427790 4bffff20 3c4c0002 38427784 fbe1fff8
> [  187.700435] a.out[5997]: code: f821ffc1 7c3f0b78 60000000 e9228030 <89290000> 993f002f 60000000 383f0040
> 
> for exec:
> [  225.100903] a.out[6067]: User access of kernel address (c00000000000dea0) - exploit attempt? (uid: 1000)
> [  225.100938] a.out[6067]: segfault (11) at c00000000000dea0 nip c00000000000dea0 lr 129d507b0 code 1
> [  225.100943] a.out[6067]: Bad NIP, not dumping instructions.
> 
> Fixes: 2865d08dd9ea ("powerpc/mm: Move the DSISR_PROTFAULT sanity check")
> Signed-off-by: Aneesh Kumar K.V <aneesh.kumar at linux.ibm.com>

Tested-by: Breno Leitao <leitao at debian.org>

> ---
>  arch/powerpc/mm/fault.c | 29 ++++++++++++++++++++++++-----
>  1 file changed, 24 insertions(+), 5 deletions(-)
> 
> diff --git a/arch/powerpc/mm/fault.c b/arch/powerpc/mm/fault.c
> index 1697e903bbf2..46f280068c45 100644
> --- a/arch/powerpc/mm/fault.c
> +++ b/arch/powerpc/mm/fault.c
> @@ -342,8 +342,21 @@ static inline void cmo_account_page_fault(void) { }
>  #endif /* CONFIG_PPC_SMLPAR */
>  
>  #ifdef CONFIG_PPC_STD_MMU
> -static void sanity_check_fault(bool is_write, unsigned long error_code)
> +static void sanity_check_fault(bool is_write, bool is_user,
> +			       unsigned long error_code, unsigned long address)
>  {
> +	/*
> +	 * userspace trying to access kernel address, we get PROTFAULT for that.
> +	 */
> +	if (is_user && address >= TASK_SIZE) {
> +		printk_ratelimited(KERN_CRIT "%s[%d]: "
> +				   "User access of kernel address (%lx) - "
> +				   "exploit attempt? (uid: %d)\n",
> +				   current->comm, current->pid, address,
> +				   from_kuid(&init_user_ns, current_uid()));
> +		return;

Silly question: Is it OK to printk() and just return here? __do_page_fault
will continue to execute independently of this return, right?