Re: [RFC PATCH] md raid10: fix NULL deference in handle_write_completed()

[NeilBrown <neilb@suse.com>]

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On Mon, Feb 05 2018, Yufen Yu wrote:

> When an io error in recovery, end_sync_write will set R10BIO_WriteError.
> After reschedule_retry(), this r10_bio will be inserted to retry_list,
> and progressed by handle_write_completed(), which will traverse all 
> r10_bio->devs[]. If devs[m].repl_bio != NULL, it thinks 
> conf->mirrors[dev].replacement is also not NULL.
> In fact, it is not true, resulting in replacement NULL deference.
>
> For example,
> in raid10, only rdev[1] has replacement, rdev[0], rdev[2] and rdev[3]
> don't have replacement. And, r10buf_pool_alloc() will allocate 
> repl_bio for every r10bio. raid10_sync_request() gets a r10bio from
> the pool. But, it doesn't pay attention to repl_bio for 
> read bio(r10bio->devs[0], rdev[2]), or sets repl_bio=NULL in 
> write bio(r10bio->devs[1], rdev[3]), if replacement == NULL.
>
> Signed-off-by: Yufen Yu <yuyufen@huawei.com>

Thanks for this.  You're analysis is correct.
This bug was introduced when replacement support for raid10 was added
in Linux 3.3, so

Fixes: 9ad1aefc8ae8 ("md/raid10:  Handle replacement devices during resync.")

I'm not sure you patch is correct though.  Just because rdev is not NULL
and bio is not NULL, that doesn't mean we tried to write - particularly
in the 'recover' case.

Elsewhere the determination of "is this device part of the
resync/recovery" is made by resting bio->bi_end_io.
If this is end_sync_write, then we tried to write here.
If it is NULL, then we didn't try to write.

So I think a correct patch would be more like.

 if (r10_bio->devs[m].bio == NULL ||
     r10_bio->devs[m].bio->bi_end_io == NULL)
         continue;

Thanks,
NeilBrown


> ---
>  drivers/md/raid10.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/drivers/md/raid10.c b/drivers/md/raid10.c
> index 99c9207899a7..9cbc1a193c8e 100644
> --- a/drivers/md/raid10.c
> +++ b/drivers/md/raid10.c
> @@ -2655,7 +2655,7 @@ static void handle_write_completed(struct r10conf *conf, struct r10bio *r10_bio)
>  		for (m = 0; m < conf->copies; m++) {
>  			int dev = r10_bio->devs[m].devnum;
>  			rdev = conf->mirrors[dev].rdev;
> -			if (r10_bio->devs[m].bio == NULL)
> +			if (rdev == NULL || r10_bio->devs[m].bio == NULL)
>  				continue;
>  			if (!r10_bio->devs[m].bio->bi_status) {
>  				rdev_clear_badblocks(
> @@ -2670,7 +2670,7 @@ static void handle_write_completed(struct r10conf *conf, struct r10bio *r10_bio)
>  					md_error(conf->mddev, rdev);
>  			}
>  			rdev = conf->mirrors[dev].replacement;
> -			if (r10_bio->devs[m].repl_bio == NULL)
> +			if (rdev == NULL || r10_bio->devs[m].repl_bio == NULL)
>  				continue;
>  
>  			if (!r10_bio->devs[m].repl_bio->bi_status) {
> -- 
> 2.13.6

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