[RFC PATCH] genirq: Enforce monotonic increase contract in irq_get_next_irq()

[Aaron Tomlin <atomlin@atomlin.com>]

When an IRQ descriptor is corrupted in memory (e.g., via an out-of-bounds
write by a rogue driver), the descriptor's internal IRQ number may be
zeroed out.

During iteration via for_each_active_irq(), irq_get_next_irq() relies on
irq_desc_get_irq(desc) to retrieve the next IRQ number. If a descriptor is
corrupted, this can result in returning an IRQ number (e.g., 0) that is
strictly less than the requested offset. This breaks the fundamental
forward-progress guarantee of the iterator.

This contract violation causes catastrophic unsigned integer underflows in
callers. For instance, show_all_irqs() in fs/proc/stat.c calculates
padding using (i - next). A corrupted descriptor returning 0 forces a
massive unsigned underflow, trapping the CPU in an extensive loop inside
show_irq_gap() and triggering a soft lockup watchdog.

While the underlying issue is a memory corruption bug, core iterators
should be resilient against returning values that violate their own
mathematical boundaries and induce lockups in other subsystems.

Introduce a lightweight boundary check in irq_get_next_irq() to verify
the returned IRQ is greater than or equal to the offset. If corruption
is detected, raise a WARN_ONCE() to pinpoint the invalid state and
return nr_irqs to safely abort the iteration.

Signed-off-by: Aaron Tomlin <atomlin@atomlin.com>
---
 kernel/irq/irqdesc.c | 17 ++++++++++++++++-
 1 file changed, 16 insertions(+), 1 deletion(-)

diff --git a/kernel/irq/irqdesc.c b/kernel/irq/irqdesc.c
index 7173b8b634f2..47a9dedb36b3 100644
--- a/kernel/irq/irqdesc.c
+++ b/kernel/irq/irqdesc.c
@@ -927,7 +927,22 @@ EXPORT_SYMBOL_GPL(__irq_alloc_descs);
  */
 unsigned int irq_get_next_irq(unsigned int offset)
 {
-	return irq_find_at_or_after(offset);
+	unsigned int irq;
+	const unsigned int nr_irqs = irq_get_nr_irqs();
+
+	irq = irq_find_at_or_after(offset);
+
+	/*
+	 * Defend against corrupted IRQ descriptors violating the monotonic
+	 * iterator contract. Returning a value lower than the offset will
+	 * cause catastrophic unsigned underflows in callers.
+	 */
+	if (WARN_ONCE(irq < offset && irq < nr_irqs,
+		      "genirq: Corrupted IRQ descriptor detected: irq %u < offset %u\n",
+		      irq, offset))
+		return nr_irqs;
+
+	return irq;
 }
 
 struct irq_desc *__irq_get_desc_lock(unsigned int irq, unsigned long *flags, bool bus,
-- 
2.51.0