Re: [PATCH] ocfs2: mount fails with buffer overflow in strlen

[Joseph Qi]

On 9/27/21 11:44 PM, Valentin Vidic wrote:
> Starting with kernel v5.11 mouting an ocfs2 filesystem with either o2cb
> or pcmk cluster stack fails with the trace below. Problem seems to be
> that strings for cluster stack and cluster name are not guaranteed to be
> null terminated in the disk representation, while strlcpy assumes that
> the source string is always null terminated. This causes a read outside
> of the source string triggering the buffer overflow detection.
> 

strlcpy in ocfs2_initialize_super() is introduced 8 years ago, so I
don't understand why you've mentioned that the issues starts from
v5.11.

osb->osb_cluster_stack and osb->osb_cluster_name is always larger by
1 than which in ocfs2_cluster_info, and the input size of strlcpy does
the same, so I don't see how it overflows.

Thanks,
Joseph

> detected buffer overflow in strlen
> ------------[ cut here ]------------
> kernel BUG at lib/string.c:1149!
> invalid opcode: 0000 [#1] SMP PTI
> CPU: 1 PID: 910 Comm: mount.ocfs2 Not tainted 5.14.0-1-amd64 #1
>   Debian 5.14.6-2
> RIP: 0010:fortify_panic+0xf/0x11
> ...
> Call Trace:
>  ocfs2_initialize_super.isra.0.cold+0xc/0x18 [ocfs2]
>  ocfs2_fill_super+0x359/0x19b0 [ocfs2]
>  mount_bdev+0x185/0x1b0
>  ? ocfs2_remount+0x440/0x440 [ocfs2]
>  legacy_get_tree+0x27/0x40
>  vfs_get_tree+0x25/0xb0
>  path_mount+0x454/0xa20
>  __x64_sys_mount+0x103/0x140
>  do_syscall_64+0x3b/0xc0
>  entry_SYSCALL_64_after_hwframe+0x44/0xae
> 
> Signed-off-by: Valentin Vidic <vvidic@xxxxxxxxxxxxxxxxxxxxxx>
> ---
>  fs/ocfs2/super.c | 10 ++++++----
>  1 file changed, 6 insertions(+), 4 deletions(-)
> 
> diff --git a/fs/ocfs2/super.c b/fs/ocfs2/super.c
> index c86bd4e60e20..1dea535224df 100644
> --- a/fs/ocfs2/super.c
> +++ b/fs/ocfs2/super.c
> @@ -2169,9 +2169,10 @@ static int ocfs2_initialize_super(struct super_block *sb,
>  	if (ocfs2_clusterinfo_valid(osb)) {
>  		osb->osb_stackflags =
>  			OCFS2_RAW_SB(di)->s_cluster_info.ci_stackflags;
> -		strlcpy(osb->osb_cluster_stack,
> +		memcpy(osb->osb_cluster_stack,
>  		       OCFS2_RAW_SB(di)->s_cluster_info.ci_stack,
> -		       OCFS2_STACK_LABEL_LEN + 1);
> +		       OCFS2_STACK_LABEL_LEN);
> +		osb->osb_cluster_stack[OCFS2_STACK_LABEL_LEN] = '\0';
>  		if (strlen(osb->osb_cluster_stack) != OCFS2_STACK_LABEL_LEN) {
>  			mlog(ML_ERROR,
>  			     "couldn't mount because of an invalid "
> @@ -2180,9 +2181,10 @@ static int ocfs2_initialize_super(struct super_block *sb,
>  			status = -EINVAL;
>  			goto bail;
>  		}
> -		strlcpy(osb->osb_cluster_name,
> +		memcpy(osb->osb_cluster_name,
>  			OCFS2_RAW_SB(di)->s_cluster_info.ci_cluster,
> -			OCFS2_CLUSTER_NAME_LEN + 1);
> +			OCFS2_CLUSTER_NAME_LEN);
> +		osb->osb_cluster_name[OCFS2_CLUSTER_NAME_LEN] = '\0';
>  	} else {
>  		/* The empty string is identical with classic tools that
>  		 * don't know about s_cluster_info. */
>