Messages in this thread Patch in this message | | | Date | Sun, 5 May 2019 14:03:13 -0400 | From | Joel Fernandes <> | Subject | Re: [PATCH RFC] bpf: Add support for reading user pointers |
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On Sun, May 05, 2019 at 03:52:23PM +0000, Joel Fernandes wrote: > On Sun, May 05, 2019 at 03:46:08PM +0100, Qais Yousef wrote: > > On 05/05/19 13:29, Joel Fernandes wrote: > > > On Sun, May 05, 2019 at 12:04:24PM +0100, Qais Yousef wrote: > > > > On 05/03/19 09:49, Joel Fernandes wrote: > > > > > On Fri, May 03, 2019 at 01:12:34PM +0100, Qais Yousef wrote: > > > > > > Hi Joel > > > > > > > > > > > > On 05/02/19 16:49, Joel Fernandes (Google) wrote: > > > > > > > The eBPF based opensnoop tool fails to read the file path string passed > > > > > > > to the do_sys_open function. This is because it is a pointer to > > > > > > > userspace address and causes an -EFAULT when read with > > > > > > > probe_kernel_read. This is not an issue when running the tool on x86 but > > > > > > > is an issue on arm64. This patch adds a new bpf function call based > > > > > > > > > > > > I just did an experiment and if I use Android 4.9 kernel I indeed fail to see > > > > > > PATH info when running opensnoop. But if I run on 5.1-rc7 opensnoop behaves > > > > > > correctly on arm64. > > > > > > > > > > > > My guess either a limitation that was fixed on later kernel versions or Android > > > > > > kernel has some strict option/modifications that make this fail? > > > > > > > > > > Thanks a lot for checking, yes I was testing 4.9 kernel with this patch (pixel 3). > > > > > > > > > > I am not sure what has changed since then, but I still think it is a good > > > > > idea to make the code more robust against such future issues anyway. In > > > > > particular, we learnt with extensive discussions that user/kernel pointers > > > > > are not necessarily distinguishable purely based on their address. > > > > > > > > Yes I wasn't arguing against that. But the commit message is misleading or > > > > needs more explanation at least. I tried 4.9.y stable and arm64 worked on that > > > > too. Why do you think it's an arm64 problem? > > > > > > Well it is broken on at least on at least one arm64 device and the patch I > > > sent fixes it. We know that the bpf is using wrong kernel API so why not fix > > > it? Are you saying we should not fix it like in this patch? Or do you have > > > another fix in mind? > > > > Again I have no issue with the new API. But the claim that it's a fix for > > a broken arm64 is a big stretch. AFAICT you don't understand the root cause of > > why copy_to_user_inatomic() fails in your case. Given that Android 4.9 has > > its own patches on top of 4.9 stable, it might be something that was introduced > > in one of these patches that breaks opensnoop, and by making it use the new API > > you might be simply working around the problem. All I can see is that vanilla > > 4.9 stable works on arm64. > > Agreed that commit message could be improved. I believe issue is something to > do with differences in 4.9 PAN emulation backports. AIUI PAN was introduced > in upstream only in 4.10 so 4.9 needed backports. > > I did not root cause this completely because "doing the right thing" fixed > the issue. I will look more closely once I am home. > > Thank you.
+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.
Thank you!
- Joel
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