arm64: debug: always unmask interrupts in el0_softstp()

We intend that EL0 exception handlers unmask all DAIF exceptions
before calling exit_to_user_mode().

When completing single-step of a suspended breakpoint, we do not call
local_daif_restore(DAIF_PROCCTX) before calling exit_to_user_mode(),
leaving all DAIF exceptions masked.

When pseudo-NMIs are not in use this is benign.

When pseudo-NMIs are in use, this is unsound. At this point interrupts
are masked by both DAIF.IF and PMR_EL1, and subsequent irq flag
manipulation may not work correctly. For example, a subsequent
local_irq_enable() within exit_to_user_mode_loop() will only unmask
interrupts via PMR_EL1 (leaving those masked via DAIF.IF), and
anything depending on interrupts being unmasked (e.g. delivery of
signals) will not work correctly.

This was detected by CONFIG_ARM64_DEBUG_PRIORITY_MASKING.

Move the call to `try_step_suspended_breakpoints()` outside of the check
so that interrupts can be unmasked even if we don't call the step handler.

Fixes: 0ac7584c08ce ("arm64: debug: split single stepping exception entry")
Cc: <[email protected]> # 6.17
Signed-off-by: Ada Couprie Diaz <[email protected]>
Acked-by: Mark Rutland <[email protected]>
[[email protected]: added Mark's rewritten commit log and some whitespace]
Signed-off-by: Catalin Marinas <[email protected]>

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