Nit on the subject, "usermode's exception handlers" reads like KUT is handling exceptions in usermode. Maybe "usermode library's exception handlers"? On Thu, Dec 09, 2021, Aaron Lewis wrote: > When handling an exception in usermode.c the exception handler #GPs when > executing 'iret' to return from the exception handler. This happens > because the stack segment selector does not have the same privilege > level as the return code segment selector. Set the stack segment > selector to match the code segment selector's privilege level to fix the > issue. > > This problem has been disguised in kvm-unit-tests because a #GP > exception handler has been registered with run_in_user() for the tests > that are currently using this feature. With a #GP exception handler > registered, the first exception will be processed then #GP on the > return. Then, because the exception handlers run at CPL0, SS:RSP for s/return/IRET for clarity > CPL0 will be pushed onto the stack matching KERNEL_CS, which is set in > ex_regs.cs in the exception handler. The IRET from the second #GP will then succeed, and the subsequent lngjmp() will restore RSP to a sane value. But if no #GP handler is installed, e.g. if a test wants to handle only #ACs, the #GP on the initial IRET will be fatal. > This is only a problem in 64-bit mode because 64-bit mode > unconditionally pops SS:RSP (SDM vol 3, 6.14.3 "IRET in IA-32e Mode"). > In 32-bit mode SS:RSP is not popped because there is no privilege level > change when returning from the #GP. > > Signed-off-by: Aaron Lewis <aaronlewis@xxxxxxxxxx> Reviewed-by: Sean Christopherson <seanjc@xxxxxxxxxx>
