Messages in this thread | | | Date | Fri, 15 Feb 2019 21:08:01 -0500 | From | Steven Rostedt <> | Subject | Re: [PATCH 1/2 v2] kprobe: Do not use uaccess functions to access kernel memory that can fault |
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On Fri, 15 Feb 2019 17:32:55 -0800 Andy Lutomirski <luto@amacapital.net> wrote:
> > I added you just because I wanted help getting the change log correct, > > as that's what Linus was complaining about. I kept using "kernel > > address" when the sample bug used for the patch was really a > > non-canonical address (as Linus said, it's just garbage. Neither kernel > > or user space). But I pointed out that this can also bug if the > > address is canonical and in the kernel address space. The old code > > didn't complain about non-canonical or kernel address faulting before > > commit 9da3f2b7405, which only talks about kernel address space > > faulting (which is why I only mentioned that in my messages). > > > > Would changing all the mention of "kernel address" to "non user space" > > be accurate? > > > > I think “kernel address” is right. It’s illegal to access anything that isn’t known to be a valid kernel address while in KERNEL_DS.
But an non-canonical address is not a "kernel address", and that will cause a bug too. This patch came about because it was changed that if we do a uaccess on something other than a user space address and take a fault (either because it was a non-canonical address, or a kernel address), we BUG! Where before that one patch, it would just return a fault.
> > The old __copy seems likely to have always been a bit bogus. > > BTW, what is this probe_mem_read() thing? Some minimal inspection suggests it’s a buggy reimplementation of probe_kernel_read(). Can you delete it and just use probe_kernel_read() directly?
Well, the issue is that we have trace_probe_tmpl.h in that same directory, which does the work for kprobes and uprobes. The trace_kprobes.c defines all the functions for handling kprobes, and trace_uprobes.c does all the handling of uprobes, then they include trace_probe_tmpl.h which does the bulk of the work.
In the uprobes case, we have:
static nokprobe_inline int probe_mem_read(void *dest, void *src, size_t size) { void __user *vaddr = (void __force __user *)src;
return copy_from_user(dest, vaddr, size) ? -EFAULT : 0; }
Because that is adding probes on userspace code.
-- Steve
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