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

From: Joel Fernandes <[email-masked]>
Date: Mon, 6 May 2019 16:58:07 -0400

On Mon, May 06, 2019 at 07:35:06PM +0100, Will Deacon wrote:
> 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:

No problem, I added you out of the blue so it is quite understandable :)

>   * Are you seeing a failure with mainline and/or an official stable kernel?

This issue is noticed on the Pixel3 kernel (4.9 based):
git clone https://android.googlesource.com/kernel/msm
(branch: android-msm-crosshatch-4.9-q-preview-1)

>   * What is the failing CPU? (so we can figure out which architectural
>     extensions are implemented)
>From cpuinfo:
AArch64 Processor rev 12 (aarch64)
(Qualcomm SDM845 SoC). It is a Pixel 3 phone.

>   * Do you have a .config anywhere? Particular, how are ARM64_PAN,
>     ARM64_TTBR0_PAN and ARM64_UAO set?

CONFIG_ARM64_SW_TTBR0_PAN is not set
CONFIG_ARM64_PAN=y
CONFIG_ARM64_UAO=y

I wanted to say I enabled SW_TTBR0_PAN config and also got the same result.

>   * Is the address being accessed a user or a kernel address?

User. It is the second argument of do_sys_open() kernel function. kprobe
gives bpf the pointer which the bpf program dereferences with
probe_kernel_read.

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

Ok. Thanks for confirming as well. The existing code has this bug and these
patches fix it.

 - Joel