Messages in this thread | | | Subject | Re: [PATCH] x86/alternatives: check int3 breakpoint physical addresses | From | Alexandre Chartre <> | Date | Thu, 21 Feb 2019 11:14:56 +0100 |
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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.
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