In the Linux kernel, the following vulnerability has been resolved: arm64: kprobes: Allow reentering kprobes while single-stepping A kprob

In the Linux kernel, the following vulnerability has been resolved:

arm64: kprobes: Allow reentering kprobes while single-stepping

A kprobe can be hit while another kprobe is in KPROBE_HIT_SS state. This
can happen when tracing or perf code runs from the debug exception path
while the first kprobe is preparing or executing its out-of-line
single-step instruction.

Currently arm64 treats a kprobe hit in KPROBE_HIT_SS as unrecoverable,
the same as a hit in KPROBE_REENTER. This is too strict. A hit in
KPROBE_HIT_SS is still a one-level reentry and can be handled by saving
the current kprobe state and setting up single-step for the new probe,
just like reentry from KPROBE_HIT_ACTIVE or KPROBE_HIT_SSDONE.

The truly unrecoverable case is hitting another kprobe while already in
KPROBE_REENTER, because the reentry save area has already been consumed.

Move KPROBE_HIT_SS to the recoverable reentry cases and leave
KPROBE_REENTER as the unrecoverable nested reentry case.

This change also requires saving saved_irqflag in struct prev_kprobe.
When a nested kprobe calls kprobes_save_local_irqflag(), it overwrites
kcb->saved_irqflag with the currently masked DAIF value, losing the
outer kprobe's original DAIF state. Without this fix, when the outer
kprobe's single-step finishes, kprobes_restore_local_irqflag() applies
the wrong DAIF mask and leaves interrupts permanently disabled.

Extend struct prev_kprobe with a saved_irqflag field and save/restore it
alongside kp and status. This ensures the outer kprobe's original
interrupt state is preserved across reentry.

This mirrors the x86 fix in commit 6a5022a56ac3
("kprobes/x86: Allow to handle reentered kprobe on single-stepping").