Messages in this thread | | | Subject | Re: [PATCH] x86/alternatives: check int3 breakpoint physical addresses | From | Alexandre Chartre <> | Date | Mon, 11 Feb 2019 10:08:11 +0100 |
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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.
This mapping was eventually removed [2]. I don't know if any other code like this is currently present in the kernel (I couldn't find any, but I might not have covered everything). But, as this past commit have shown, this is definitively something that can happen.
Thanks,
alex.
--- [1] 3386bc8aed82 ("x86/entry/64: Create a per-CPU SYSCALL entry trampoline") https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=3386bc8aed82
[2] bf904d2762ee ("x86/pti/64: Remove the SYSCALL64 entry trampoline") https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=bf904d2762ee
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