genirq: Prevent proc race against freeing of irq descriptors

Since the rework of the sparse interrupt code to actually free the
unused interrupt descriptors there exists a race between the /proc
interfaces to the irq subsystem and the code which frees the interrupt
descriptor.

CPU0				CPU1
				show_interrupts()
				  desc = irq_to_desc(X);
free_desc(desc)
  remove_from_radix_tree();
  kfree(desc);
				  raw_spinlock_irq(&desc->lock);

/proc/interrupts is the only interface which can actively corrupt
kernel memory via the lock access. /proc/stat can only read from freed
memory. Extremly hard to trigger, but possible.

The interfaces in /proc/irq/N/ are not affected by this because the
removal of the proc file is serialized in procfs against concurrent
readers/writers. The removal happens before the descriptor is freed.

For architectures which have CONFIG_SPARSE_IRQ=n this is a non issue
as the descriptor is never freed. It's merely cleared out with the irq
descriptor lock held. So any concurrent proc access will either see
the old correct value or the cleared out ones.

Protect the lookup and access to the irq descriptor in
show_interrupts() with the sparse_irq_lock.

Provide kstat_irqs_usr() which is protecting the lookup and access
with sparse_irq_lock and switch /proc/stat to use it.

Document the existing kstat_irqs interfaces so it's clear that the
caller needs to take care about protection. The users of these
interfaces are either not affected due to SPARSE_IRQ=n or already
protected against removal.

Fixes: 1f5a5b87f78f "genirq: Implement a sane sparse_irq allocator"
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: stable@vger.kernel.org

1 files changed, 4 insertions, 0 deletions

diff --git a/kernel/irq/internals.h b/kernel/irq/internals.h
index 4332d766619d..df553b0af936 100644
--- a/kernel/irq/internals.h
+++ b/kernel/irq/internals.h
@@ -78,8 +78,12 @@ extern void unmask_threaded_irq(struct irq_desc *desc);
 
 #ifdef CONFIG_SPARSE_IRQ
 static inline void irq_mark_irq(unsigned int irq) { }
+extern void irq_lock_sparse(void);
+extern void irq_unlock_sparse(void);
 #else
 extern void irq_mark_irq(unsigned int irq);
+static inline void irq_lock_sparse(void) { }
+static inline void irq_unlock_sparse(void) { }
 #endif
 
 extern void init_kstat_irqs(struct irq_desc *desc, int node, int nr);