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

[Alexandre Chartre]

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