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amd64: Avoid enabling interrupts when handling kernel mode prot faults
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Authored by markj on May 31 2021, 4:19 PM.

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Summary

When PTI is enabled, we may have been on the trampoline stack when iret
faults. So, we have to switch back to the regular stack before
re-entering trap().

trap() has the somewhat strange behaviour of re-enabling interrupts when
handling certain kernel-mode execeptions. In particular, it was doing
this for exceptions raised during execution of iret. When switching
away from the trampoline stack, however, the thread must not be migrated
to a different CPU. Fix the problem by simply leaving interrupts
disabled during the window.

Reported by: syzkaller

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