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

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