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

That said, I'm not opposed to apply the patch as is, I just wanted to get a
better understanding about the background.



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