Re: [PATCH] block/mirror: fix NULL pointer dereference in mirror_wait_on_conflicts()

[Stefano Garzarella <sgarzare@redhat.com>]

On Fri, Sep 10, 2021 at 01:37:40PM +0300, Vladimir Sementsov-Ogievskiy wrote:
>10.09.2021 11:56, Stefano Garzarella wrote:
>>In mirror_iteration() we call mirror_wait_on_conflicts() with
>>`self` parameter set to NULL.
>>
>>Starting from commit d44dae1a7c we dereference `self` pointer in
>>mirror_wait_on_conflicts() without checks if it is not NULL.
>>
>>Backtrace:
>>   Program terminated with signal SIGSEGV, Segmentation fault.
>>   #0  mirror_wait_on_conflicts (self=0x0, s=<optimized out>, offset=<optimized out>, bytes=<optimized out>)
>>       at ../block/mirror.c:172
>>   172	                self->waiting_for_op = op;
>>   [Current thread is 1 (Thread 0x7f0908931ec0 (LWP 380249))]
>>   (gdb) bt
>>   #0  mirror_wait_on_conflicts (self=0x0, s=<optimized out>, offset=<optimized out>, bytes=<optimized out>)
>>       at ../block/mirror.c:172
>>   #1  0x00005610c5d9d631 in mirror_run (job=0x5610c76a2c00, errp=<optimized out>) at ../block/mirror.c:491
>>   #2  0x00005610c5d58726 in job_co_entry (opaque=0x5610c76a2c00) at ../job.c:917
>>   #3  0x00005610c5f046c6 in coroutine_trampoline (i0=<optimized out>, i1=<optimized out>)
>>       at ../util/coroutine-ucontext.c:173
>>   #4  0x00007f0909975820 in ?? () at ../sysdeps/unix/sysv/linux/x86_64/__start_context.S:91
>>       from /usr/lib64/libc.so.6
>>
>>Buglink: https://bugzilla.redhat.com/show_bug.cgi?id=2001404
>>Fixes: d44dae1a7c ("block/mirror: fix active mirror dead-lock in mirror_wait_on_conflicts")
>>Signed-off-by: Stefano Garzarella <sgarzare@redhat.com>
>>---
>>I'm not familiar with this code so maybe we can fix the bug differently.
>>
>>Running iotests and the test in bugzilla, everything seems okay.
>>
>>Thanks,
>>Stefano
>>---
>>  block/mirror.c | 11 +++++++++--
>>  1 file changed, 9 insertions(+), 2 deletions(-)
>>
>>diff --git a/block/mirror.c b/block/mirror.c
>>index 98fc66eabf..6c834d6a7b 100644
>>--- a/block/mirror.c
>>+++ b/block/mirror.c
>>@@ -169,9 +169,16 @@ static void coroutine_fn mirror_wait_on_conflicts(MirrorOp *self,
>>                      continue;
>>                  }
>>-                self->waiting_for_op = op;
>>+                if (self) {
>>+                    self->waiting_for_op = op;
>>+                }
>>+
>>                  qemu_co_queue_wait(&op->waiting_requests, NULL);
>>-                self->waiting_for_op = NULL;
>>+
>>+                if (self) {
>>+                    self->waiting_for_op = NULL;
>>+                }
>>+
>>                  break;
>>              }
>>          }
>>
>
>Hi Stefano!
>
>The patch is almost OK. The only thing is, I think, you should also put 
>"if (op->waiting_for_op) {continue;}" code above into "if (self)" too.  
>Look at the comment above: if we don't have "self", than we are not in 
>the list and nobody will wait for us. This means that we should wait 
>for all.
>
>So, with my suggested fix, you'll simply roll-back d44dae1a7c for the 
>case of self==NULL, which is an additional point of safety.
>

Right, I'll send a v2 with this change.

>
>Still, I wonder, where we check for conflicting requests when create 
>usual MirrorOp. We don't call mirror_wait_on_conflicts() in 
>mirror_perform...

IIUC in mirror_iteration() we call mirror_wait_on_conflicts() at the 
beginning, then in the cycle we call mirror_perform() where we create a 
new MirrorOp and we add it in the `ops_in_flight` list.

At this point, should we call mirror_wait_on_conflicts()?

I need to understand the code better to follow you... :-)

Thanks,
Stefano