On 15/10/2024 17:01, Catalin Marinas wrote:
We also still need to resolve Kevin's concern, which probably means keeping the thread's original POR around someplace.
If we fail to allocate context for POR_EL0 (or anything else), we'll deliver a SIGSEGV. I think it's quite likely that the SIGSEGV will also fail to allocate context we end up with a fatal SIGSEGV. Not sure the user can affect the allocation/layout, though it can change stack attributes where the frame is written.
Assuming that the user tricks the kernel into failing to write the context but allows it to succeed on the resulting SIGSEGV, POR_EL0 wouldn't have been reset and the SIGSEGV context will still have the original value. I don't think we need to do anything here for 6.12.
However, in for-next/core, we have gcs_signal_entry() called after resetting POR_EL0. If this fails, we can end up with a new POR_EL0 on sigreturn (subject to the above user toggling permissions). I think this needs to be fixed, POR_EL0 only reset when we know we are going to deliver the signal.
In the series I've just posted [1], POR_EL0 is reset to "allow all" before we do anything, so it sounds like we may have a problem there. However, it does keep track of that state, so I think the fix may be simple. If any error occurs in setup_rt_frame(), we could call restore_unpriv_access_state() to restore the original value of POR_EL0, like in sigreturn(). Otherwise we call set_handler_unpriv_access_state() to set POR_EL0 to POR_EL0_INIT as we do today. I can make that change in v2 if that sounds helpful.
Kevin
[1] https://lore.kernel.org/linux-arm-kernel/20241017133909.3837547-4-kevin.brod...