[PAL/Linux-SGX] AEX-Notify 4/5: Do not clobber RBX reg in stage-1 signal handler #2036
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If the enclave is entered with CSSA=1, then the enclave starts executing the stage-1 signal handler asssembly code. This assembly code prepares the context for the stage-2 signal handler, which will be executed in regular context with CSSA=0 and using C code. This stage-1 signal handler uses the RBX register as a base pointer to the SSA[0].GPRSGX region. Closer to the end of the stage-1 handler flow, in particular before the `.Lcssa1_exception_eexit` label, this SSA[0] base pointer is not needed anymore, so the RBX register is used for other purposes. In particular, RBX is used to hold the stashed RDX value (where-to-exit address in untrusted runtime). However, a future commit that will introduce AEX-Notify flows inside the enclave needs to access the SSA[0].GPRSGX region at this stage (in the `.Lcssa1_exception_eexit` label). So the RBX register must not be overwritten. Thus, this preparatory commit does not clobber RBX, but instead stashes RDX into another unused register, R10. Signed-off-by: Dmitrii Kuvaiskii <[email protected]>
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Oct 22, 2024
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Reviewable status: 0 of 18 files reviewed, 1 unresolved discussion, not enough approvals from maintainers (1 more required), not enough approvals from different teams (1 more required, approved so far: Intel)
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Must be applied on top of #2034. Blocking.
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Description of the changes
Part 4 in AEX-Notify series.
If the enclave is entered with CSSA=1, then the enclave starts executing the stage-1 signal handler asssembly code. This assembly code prepares the context for the stage-2 signal handler, which will be executed in regular context with CSSA=0 and using C code.
This stage-1 signal handler uses the RBX register as a base pointer to the SSA[0].GPRSGX region. Closer to the end of the stage-1 handler flow, in particular before the
.Lcssa1_exception_eexitlabel, this SSA[0] base pointer is not needed anymore, so the RBX register is used for other purposes. In particular, RBX is used to hold the stashed RDX value (where-to-exit address in untrusted runtime).However, a future commit that will introduce AEX-Notify flows inside the enclave needs to access the SSA[0].GPRSGX region at this stage (in the
.Lcssa1_exception_eexitlabel). So the RBX register must not be overwritten. Thus, this preparatory commit does not clobber RBX, but instead stashes RDX into another unused register, R10.See also related PRs and discussions:
How to test this PR?
CI is enough.
This change is