On arm64, if a nested kprobes hit, it can crash the kernel with below
error message.

[  152.118921] Unexpected kernel single-step exception at EL1

This is because commit 7419333fa15e ("arm64: kprobe: Always clear
pstate.D in breakpoint exception handler") clears pstate.D always in
the nested kprobes. That is correct *unless* any nested kprobes
(single-stepping) runs inside other kprobes (including kprobes in
 user handler).

When the 1st kprobe hits, do_debug_exception() will be called. At this
point, debug exception (= pstate.D) must be masked (=1). When the 2nd
 (nested) kprobe is hit before single-step of the first kprobe, it
modifies debug exception clear (pstate.D = 0).
Then, when the 1st kprobe setting up single-step, it saves current
DAIF, mask DAIF, enable single-step, and restore DAIF.
However, since "D" flag in DAIF is cleared by the 2nd kprobe, the
single-step exception happens soon after restoring DAIF.

To solve this issue, this refers saved pstate register to check the
previous pstate.D and recover it if needed.

Reported-by: Naresh Kamboju <redacted>
Fixes: commit 7419333fa15e ("arm64: kprobe: Always clear pstate.D in breakpoint exception handler")
Signed-off-by: Masami Hiramatsu <mhiramat@kernel.org>
---
 arch/arm64/kernel/probes/kprobes.c |    9 ++++++---
 1 file changed, 6 insertions(+), 3 deletions(-)

diff --git a/arch/arm64/kernel/probes/kprobes.c b/arch/arm64/kernel/probes/kprobes.c
index bd5dfffca272..6e1dc0bb4c82 100644
--- a/arch/arm64/kernel/probes/kprobes.c
+++ b/arch/arm64/kernel/probes/kprobes.c

@@ -201,12 +201,14 @@ spsr_set_debug_flag(struct pt_regs *regs, int mask)
  * interrupt occurrence in the period of exception return and  start of
  * out-of-line single-step, that result in wrongly single stepping
  * into the interrupt handler.
+ * This also controls debug flag, so that we can refer the saved pstate.
  */
 static void __kprobes kprobes_save_local_irqflag(struct kprobe_ctlblk *kcb,
 						struct pt_regs *regs)
 {
 	kcb->saved_irqflag = regs->pstate;
 	regs->pstate |= PSR_I_BIT;
+	spsr_set_debug_flag(regs, 0);
 }
 
 static void __kprobes kprobes_restore_local_irqflag(struct kprobe_ctlblk *kcb,

@@ -216,6 +218,10 @@ static void __kprobes kprobes_restore_local_irqflag(struct kprobe_ctlblk *kcb,
 		regs->pstate |= PSR_I_BIT;
 	else
 		regs->pstate &= ~PSR_I_BIT;
+
+	/* Recover pstate.D mask if needed */
+	if (kcb->saved_irqflag & PSR_D_BIT)
+		spsr_set_debug_flag(regs, 1);
 }
 
 static void __kprobes

@@ -245,15 +251,12 @@ static void __kprobes setup_singlestep(struct kprobe *p,
 		kcb->kprobe_status = KPROBE_HIT_SS;
 	}
 
-
 	if (p->ainsn.api.insn) {
 		/* prepare for single stepping */
 		slot = (unsigned long)p->ainsn.api.insn;
 
 		set_ss_context(kcb, slot);	/* mark pending ss */
 
-		spsr_set_debug_flag(regs, 0);
-
 		/* IRQs and single stepping do not mix well. */
 		kprobes_save_local_irqflag(kcb, regs);
 		kernel_enable_single_step(regs);

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