Re: [PATCH v3 1/6] powerpc64/elfv1: Validate function pointer address in the function descriptor

[Nicholas Piggin <npiggin@gmail.com>]

On Thu, 22 Jun 2017 00:08:37 +0530
"Naveen N. Rao" <naveen.n.rao@linux.vnet.ibm.com> wrote:

> Currently, we assume that the function pointer we receive in
> ppc_function_entry() points to a function descriptor. However, this is
> not always the case. In particular, assembly symbols without the right
> annotation do not have an associated function descriptor. Some of these
> symbols are added to the kprobe blacklist using _ASM_NOKPROBE_SYMBOL().
> When such addresses are subsequently processed through
> arch_deref_entry_point() in populate_kprobe_blacklist(), we see the
> below errors during bootup:
>     [    0.663963] Failed to find blacklist at 7d9b02a648029b6c
>     [    0.663970] Failed to find blacklist at a14d03d0394a0001
>     [    0.663972] Failed to find blacklist at 7d5302a6f94d0388
>     [    0.663973] Failed to find blacklist at 48027d11e8610178
>     [    0.663974] Failed to find blacklist at f8010070f8410080
>     [    0.663976] Failed to find blacklist at 386100704801f89d
>     [    0.663977] Failed to find blacklist at 7d5302a6f94d00b0
> 
> Fix this by checking if the address in the function descriptor is
> actually a valid kernel address. In the case of assembly symbols, this
> will almost always fail as this ends up being powerpc instructions. In
> that case, return pointer to the address we received, rather than the
> dereferenced value.
> 
> Signed-off-by: Naveen N. Rao <naveen.n.rao@linux.vnet.ibm.com>
> ---
>  arch/powerpc/include/asm/code-patching.h | 10 +++++++++-
>  1 file changed, 9 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/powerpc/include/asm/code-patching.h b/arch/powerpc/include/asm/code-patching.h
> index abef812de7f8..ec54050be585 100644
> --- a/arch/powerpc/include/asm/code-patching.h
> +++ b/arch/powerpc/include/asm/code-patching.h
> @@ -83,8 +83,16 @@ static inline unsigned long ppc_function_entry(void *func)
>  	 * On PPC64 ABIv1 the function pointer actually points to the
>  	 * function's descriptor. The first entry in the descriptor is the
>  	 * address of the function text.
> +	 *
> +	 * However, we may have received a pointer to an assembly symbol
> +	 * that may not be a function descriptor. Validate that the entry
> +	 * points to a valid kernel address and if not, return the pointer
> +	 * we received as is.
>  	 */
> -	return ((func_descr_t *)func)->entry;
> +	if (kernel_text_address(((func_descr_t *)func)->entry))
> +		return ((func_descr_t *)func)->entry;
> +	else
> +		return (unsigned long)func;

What if "func" is a text section label (bare asm function)?
Won't func->entry load the random instruction located there
and compare it with a kernel address?

I don't know too much about the v1 ABI, but should we check for
func belonging in the .opd section first and base the check on
that? Alternatively I if "func" is in the kernel text address,
we can recognize it's not in the .opd section... right?

Thanks,
Nick