arm64: debug: split single stepping exception entry

Currently all debug exceptions share common entry code and are routed
to `do_debug_exception()`, which calls dynamically-registered
handlers for each specific debug exception. This is unfortunate as
different debug exceptions have different entry handling requirements,
and it would be better to handle these distinct requirements earlier.

The single stepping exception has the most constraints : it can be
exploited to train branch predictors and it needs special handling at EL1
for the Cortex-A76 erratum #1463225. We need to conserve all those
mitigations.
However, it does not write an address at FAR_EL1, as only hardware
watchpoints do so.

The single-step handler does its own signaling if it needs to and only
returns 0, so we can call it directly from `entry-common.c`.

Split the single stepping exception entry, adjust the function signature,
keep the security mitigation and erratum handling.
Further, as the EL0 and EL1 code paths are cleanly separated, we can split
`do_softstep()` into `do_el0_softstep()` and `do_el1_softstep()` and
call them directly from the relevant entry paths.
We can also remove `NOKPROBE_SYMBOL` for the EL0 path, as it cannot
lead to a kprobe recursion.

Move the call to `arm64_apply_bp_hardening()` to `entry-common.c` so that
we can do it as early as possible, and only for the exceptions coming
from EL0, where it is needed.
This is safe to do as it is `noinstr`, as are all the functions it
may call. `el0_ia()` and `el0_pc()` already call it this way.

When taking a soft-step exception from EL0, most of the single stepping
handling is safely preemptible : the only possible handler is
`uprobe_single_step_handler()`. It only operates on task-local data and
properly checks its validity, then raises a Thread Information Flag,
processed before returning to userspace in `do_notify_resume()`, which
is already preemptible.
However, the soft-step handler first calls `reinstall_suspended_bps()`
to check if there is any hardware breakpoint or watchpoint pending
or already stepped through.
This cannot be preempted as it manipulates the hardware breakpoint and
watchpoint registers.

Move the call to `try_step_suspended_breakpoints()` to `entry-common.c`
and adjust the relevant comments.
We can now safely unmask interrupts before handling the step itself,
fixing a PREEMPT_RT issue where the handler could call a sleeping function
with preemption disabled.

Signed-off-by: Ada Couprie Diaz <ada.coupriediaz@arm.com>
Closes: https://lore.kernel.org/linux-arm-kernel/Z6YW_Kx4S2tmj2BP@uudg.org/
Tested-by: Luis Claudio R. Goncalves <lgoncalv@redhat.com>
Reviewed-by: Will Deacon <will@kernel.org>
Acked-by: Mark Rutland <mark.rutland@arm.com>
Link: https://lore.kernel.org/r/20250707114109.35672-10-ada.coupriediaz@arm.com
Signed-off-by: Will Deacon <will@kernel.org>

0 files changed, 0 insertions, 0 deletions