fix(guest): give page faults their own exception stack#1571
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A guest exception handler runs on the IST1 stack. If the handler writes a copy-on-write page, the first write faults. The page fault also uses IST1, so the CPU resets RSP to the top of IST1 and writes the fault frame over the live handler frame. The handler then returns to a bad address and the guest aborts. The bug stays latent until an exception handler writes a copy-on-write page. It surfaced when a memory layout change moved a counter that an existing handler increments onto a page that stays copy-on-write after a snapshot. The increment then faulted while the handler ran and crashed the guest. Send page faults to their own IST2 stack so a fault inside a handler keeps the handler frame intact. The page-fault stack uses the second of the two scratch pages already reserved at the top of the region. Add a regression test, exception_handler_nested_page_fault. It installs a handler that writes a copy-on-write page, then triggers int3. Without the fix the guest aborts with a page fault. With the fix it returns 0. Signed-off-by: Ludvig Liljenberg <4257730+ludfjig@users.noreply.github.com>
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I noticed test
exception_handler_installation_and_validationstarted failing in one of my feature branches.Turns out just adding a couple new guest functions shifted memory so a page the handler writes is now copy-on-write, and that first write from inside the handler is what trips the bug. See commit message for more details