[PATCH v5 08/10] powerpc/lib: Refactor __patch_instruction() to use __put_user_asm()

[Christophe Leroy]

Le 08/03/2019 à 02:16, Michael Ellerman a écrit :
> From: Russell Currey <ruscur at russell.cc>
> 
> __patch_instruction() is called in early boot, and uses
> __put_user_size(), which includes the allow/prevent calls to enforce
> 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.
> 
> __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.

What about modules patching, is there no risk of -EFAULT on module 
memory, as it is in vm area ?

Christophe

> 
> A future enhancement could be to double check the patch address is
> definitely allowed to be tampered with by the kernel.
> 
> Signed-off-by: Russell Currey <ruscur at russell.cc>
> Signed-off-by: Michael Ellerman <mpe at ellerman.id.au>
> ---
> v5: Unchanged.
> 
>   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 506413a2c25e..42fdadac6587 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;
>   
>