io_uring: free fixed_file_data after RCU grace period
The percpu refcount protects this structure, and we can have an atomic
switch in progress when exiting. This makes it unsafe to just free the
struct normally, and can trigger the following KASAN warning:
BUG: KASAN: use-after-free in percpu_ref_switch_to_atomic_rcu+0xfa/0x1b0
Read of size 1 at addr ffff888181a19a30 by task swapper/0/0
CPU: 0 PID: 0 Comm: swapper/0 Not tainted 5.6.0-rc4+ #5747
Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.10.2-1ubuntu1 04/01/2014
Call Trace:
<IRQ>
dump_stack+0x76/0xa0
print_address_description.constprop.0+0x3b/0x60
? percpu_ref_switch_to_atomic_rcu+0xfa/0x1b0
? percpu_ref_switch_to_atomic_rcu+0xfa/0x1b0
__kasan_report.cold+0x1a/0x3d
? percpu_ref_switch_to_atomic_rcu+0xfa/0x1b0
percpu_ref_switch_to_atomic_rcu+0xfa/0x1b0
rcu_core+0x370/0x830
? percpu_ref_exit+0x50/0x50
? rcu_note_context_switch+0x7b0/0x7b0
? run_rebalance_domains+0x11d/0x140
__do_softirq+0x10a/0x3e9
irq_exit+0xd5/0xe0
smp_apic_timer_interrupt+0x86/0x200
apic_timer_interrupt+0xf/0x20
</IRQ>
RIP: 0010:default_idle+0x26/0x1f0
Fix this by punting the final exit and free of the struct to RCU, then
we know that it's safe to do so. Jann suggested the approach of using a
double rcu callback to achieve this. It's important that we do a nested
call_rcu() callback, as otherwise the free could be ordered before the
atomic switch, even if the latter was already queued.
Reported-by: syzbot+e017e49c39ab484ac87a@syzkaller.appspotmail.com
Suggested-by: Jann Horn <jannh@google.com>
Reviewed-by: Paul E. McKenney <paulmck@kernel.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
1 file changed
