linux-kernel - Re: [Cluster-devel] [PATCH] gfs2: gfs2_read_sb: put gfs2

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Message-ID: <ca694fc8-5f1b-ebf0-b891-c9a20225704f@redhat.com>
Date:   Mon, 5 Oct 2020 14:23:03 +0100
From:   Andrew Price <anprice@...hat.com>
To:     Fox Chen <foxhlchen@...il.com>
Cc:     rpeterso@...hat.com, agruenba@...hat.com, cluster-devel@...hat.com,
        gregkh@...uxfoundation.org, linux-kernel@...r.kernel.org
Subject: Re: [Cluster-devel] [PATCH] gfs2: gfs2_read_sb: put gfs2_assert
 inside the loop

On 03/10/2020 07:31, Fox Chen wrote:
> for (x = 2;; x++) {
>          ...
>          gfs2_assert(sdp, x <= GFS2_MAX_META_HEIGHT);  <--- after
>          ...
>          if (d != sdp->sd_heightsize[x - 1] || m)
>                  break;
>          sdp->sd_heightsize[x] = space;
> }
> 
> sdp->sd_max_height = x
> gfs2_assert(sdp, sdp->sd_max_height <= GFS2_MAX_META_HEIGHT) <--- before
> 
> Before this patch, gfs2_assert is put outside of the loop of
> sdp->sd_heightsize[x] calculation. When something goes wrong,

So this looks related to one of the recent syzbot reports, where the 
"something goes wrong" is the block size in the on-disk superblock was 
zeroed and that leads eventually to this out-of-bounds write. The 
correct fix in that case would be to add a validity check for the block 
size in gfs2_check_sb().

Andy

> x exceeds the size of GFS2_MAX_META_HEIGHT, it may already crash inside
> the loop when
> 
> sdp->sd_heightsize[x] = space
> 
> tries to reach the out-of-bound
> location, gfs2_assert won't help here.
> 
> This patch fixes this by moving gfs2_assert into the loop.
> We will check x value each time to see if it exceeds GFS2_MAX_META_HEIGHT.
> 
> Signed-off-by: Fox Chen <foxhlchen@...il.com>
> ---
>   fs/gfs2/ops_fstype.c | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/fs/gfs2/ops_fstype.c b/fs/gfs2/ops_fstype.c
> index 6d18d2c91add..6cc32e3010f2 100644
> --- a/fs/gfs2/ops_fstype.c
> +++ b/fs/gfs2/ops_fstype.c
> @@ -333,6 +333,7 @@ static int gfs2_read_sb(struct gfs2_sbd *sdp, int silent)
>   		u64 space, d;
>   		u32 m;
>   
> +		gfs2_assert(sdp, x <= GFS2_MAX_META_HEIGHT);
>   		space = sdp->sd_heightsize[x - 1] * sdp->sd_inptrs;
>   		d = space;
>   		m = do_div(d, sdp->sd_inptrs);
> @@ -343,7 +344,6 @@ static int gfs2_read_sb(struct gfs2_sbd *sdp, int silent)
>   	}
>   	sdp->sd_max_height = x;
>   	sdp->sd_heightsize[x] = ~0;
> -	gfs2_assert(sdp, sdp->sd_max_height <= GFS2_MAX_META_HEIGHT);
>   
>   	sdp->sd_max_dents_per_leaf = (sdp->sd_sb.sb_bsize -
>   				      sizeof(struct gfs2_leaf)) /
>