Re: [PATCH] x86/alternatives: check int3 breakpoint physical addresses

[Alexandre Chartre]

On 02/11/2019 10:57 AM, Alexandre Chartre wrote:
> On 02/11/2019 10:15 AM, Thomas Gleixner wrote:
> > On Mon, 11 Feb 2019, Alexandre Chartre wrote:
> > > On 02/10/2019 10:23 PM, Thomas Gleixner wrote:
> > > > On Fri, 25 Jan 2019, Alexandre Chartre wrote:
> > > > > Note that this issue has been observed and reproduced with a custom kernel
> > > > > with some code mapped to different virtual addresses and using jump labels
> > > > > As jump labels use text_poke_bp(), crashes were sometimes observed when
> > > > > updating jump labels.
> > > >
> > > > Rightfully so. text_poke_bp() pokes at the kernels text mapping which is
> > > > very well defined and unique. Why would you map the same text to different
> > > > virtual addresses and then use a randomly chosen one to poke at it?
> > >
> > > As an example, we used to have per-CPU SYSCALL entry trampoline [1] where the
> > > entry_SYSCALL_64_trampoline function was mapped to a different virtual address
> > > for each CPU. So, a syscall would execute entry_SYSCALL_64_trampoline() from a
> > > different virtual address depending on the CPU being used. With that code,
> > > adding a jump label in entry_SYSCALL_64_trampoline() causes the described
> > > issue.
> >
> > Right, but we knew that and there was no reason to put a jump label into
> > that. AFAICT we don't have anything like this in the kernel.
>
> In our particular case, we have introduced a jump label in JMP_NOSPEC (which
> is used by entry_SYSCALL_64_trampoline()) to have the option to dynamically
> enable/disable retpoline at runtime. So that's when we faced this issue.
>
> > That said, I'm not opposed to apply the patch as is, I just wanted to get a
> > better understanding about the background.
>
> Sure. I am aware this is a corner case, and I was precisely looking for feedback
> to check if it is worth fixing that issue. So I appreciate your reply, and I would
> understand if the patch is rejected because that's a case that we are just not
> expecting to happen.

Hi Thomas,

Do you have any final thought about this patch? Should we drop it or apply it?
As the patch is small and simple, I think it's worth applying it to prevent
any such (future?) issue.

Thanks,

alex.