Re: [PATCH v2 2/3] powerpc/lib: Refactor __patch_instruction() to use __put_user_asm()

[Christophe Leroy <christophe.leroy@c-s.fr>]

Hi Russel,

Le 10/12/2018 à 08:00, Russell Currey a écrit :
> __patch_instruction() is called in early boot, and uses
> __put_user_size(), which includes the locks and unlocks for KUAP,
> which could either be called too early, or in the Radix case, forced to
> use "early_" versions of functions just to safely handle this one case.

Looking at x86, I see that __put_user_size() doesn't includes the locks. 
The lock/unlock is do by callers. I'll do the same.


> 
> __put_user_asm() does not do this, and thus is safe to use both in early
> boot, and later on since in this case it should only ever be touching
> kernel memory.
> 
> __patch_instruction() was previously refactored to use __put_user_size()
> in order to be able to return -EFAULT, which would allow the kernel to
> patch instructions in userspace, which should never happen.  This has
> the functional change of causing faults on userspace addresses if KUAP
> is turned on, which should never happen in practice.
> 
> A future enhancement could be to double check the patch address is
> definitely allowed to be tampered with by the kernel.

This makes me realise that we are calling lock_user_access() with kernel 
addresses. That most likely breaks protection on kernel addresses for 
book3s/32. I'll have to work around it.

Another thing I realised also is that get_user() at least is called in 
some exceptions/trap handlers. Which means it can be called nested with 
an ongoing user access. It means that get_paca()->user_access_allowed 
might be modified during those exceptions/traps.

Christophe

> 
> Signed-off-by: Russell Currey <ruscur@russell.cc>
> ---
>   arch/powerpc/lib/code-patching.c | 4 ++--
>   1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/powerpc/lib/code-patching.c b/arch/powerpc/lib/code-patching.c
> index 89502cbccb1b..15e8c6339960 100644
> --- a/arch/powerpc/lib/code-patching.c
> +++ b/arch/powerpc/lib/code-patching.c
> @@ -26,9 +26,9 @@
>   static int __patch_instruction(unsigned int *exec_addr, unsigned int instr,
>   			       unsigned int *patch_addr)
>   {
> -	int err;
> +	int err = 0;
>   
> -	__put_user_size(instr, patch_addr, 4, err);
> +	__put_user_asm(instr, patch_addr, err, "stw");
>   	if (err)
>   		return err;
>   
>