Re: [PATCH RFC] bpf: Add support for reading user pointers

[Will Deacon <will.deacon@arm.com>]

Hi Joel,

On Sun, May 05, 2019 at 02:03:13PM -0400, Joel Fernandes wrote:
> +Mark, Will since discussion is about arm64 arch code.
> 
> The difference between observing the bug and everything just working seems to
> be the set_fs(USER_DS) as done by Masami's patch that this patch is based on.
> The following diff shows 'ret' as 255 when set_fs(KERN_DS) is used, and then
> after we retry with set_fs(USER_DS), the read succeeds.
> 
> diff --git a/mm/maccess.c b/mm/maccess.c
> index 78f9274dd49d..d3e01a33c712 100644
> --- a/mm/maccess.c
> +++ b/mm/maccess.c
> @@ -32,9 +32,20 @@ long __probe_kernel_read(void *dst, const void *src, size_t size)
>  	pagefault_disable();
>  	ret = __copy_from_user_inatomic(dst,
>  			(__force const void __user *)src, size);
> +	trace_printk("KERNEL_DS: __copy_from_user_inatomic: ret=%d\n", ret);
>  	pagefault_enable();
>  	set_fs(old_fs);
>  
> +	if (ret) {
> +	set_fs(USER_DS);
> +	pagefault_disable();
> +	ret = __copy_from_user_inatomic(dst,
> +			(__force const void __user *)src, size);
> +	trace_printk("RETRY WITH USER_DS: __copy_from_user_inatomic: ret=%d\n", ret);
> +	pagefault_enable();
> +	set_fs(old_fs);
> +	}
> +
>  	return ret ? -EFAULT : 0;
>  }
>  EXPORT_SYMBOL_GPL(probe_kernel_read);
> 
> In initially thought this was because of the addr_limit pointer masking done
> by this patch from Mark Rutland "arm64: Use pointer masking to limit uaccess
> speculation"
> 
> However removing this masking still makes it fail with KERNEL_DS.
> 
> Fwiw, I am still curious which other paths in arm64 check the addr_limit
> which might make the __copy_from_user_inatomic fail if the set_fs is not
> setup correctly.
> 
> Either way, I will resubmit the patch with the commit message fixed correctly
> as we agreed and also address Alexei's comments.

I'm coming at this with no background, so it's tricky to understand exactly
what's going on here. Some questions:

  * Are you seeing a failure with mainline and/or an official stable kernel?
  * What is the failing CPU? (so we can figure out which architectural
    extensions are implemented)
  * Do you have a .config anywhere? Particular, how are ARM64_PAN,
    ARM64_TTBR0_PAN and ARM64_UAO set?
  * Is the address being accessed a user or a kernel address?

If you're trying to dereference a pointer to userspace using
probe_kernel_read(), that clearly isn't going to work.

Will