Re: [RFC PATCH v2] ceph: ceph: fix out-of-bound array access when doing a file read

[Xiubo Li <xiubli@redhat.com>]

On 9/5/24 21:57, Luis Henriques (SUSE) wrote:
> __ceph_sync_read() does not correctly handle reads when the inode size is
> zero.  It is easy to hit a NULL pointer dereference by continuously reading
> a file while, on another client, we keep truncating and writing new data
> into it.
>
> The NULL pointer dereference happens when the inode size is zero but the
> read op returns some data (ceph_osdc_wait_request()).  This will lead to
> 'left' being set to a huge value due to the overflow in:
>
> 	left = i_size - off;
>
> and, in the loop that follows, the pages[] array being accessed beyond
> num_pages.
>
> This patch fixes the issue simply by checking the inode size and returning
> if it is zero, even if there was data from the read op.
>
> Link: https://tracker.ceph.com/issues/67524
> Fixes: 1065da21e5df ("ceph: stop copying to iter at EOF on sync reads")
> Signed-off-by: Luis Henriques (SUSE) <luis.henriques@linux.dev>
> ---
>   fs/ceph/file.c | 5 ++++-
>   1 file changed, 4 insertions(+), 1 deletion(-)
>
> diff --git a/fs/ceph/file.c b/fs/ceph/file.c
> index 4b8d59ebda00..41d4eac128bb 100644
> --- a/fs/ceph/file.c
> +++ b/fs/ceph/file.c
> @@ -1066,7 +1066,7 @@ ssize_t __ceph_sync_read(struct inode *inode, loff_t *ki_pos,
>   	if (ceph_inode_is_shutdown(inode))
>   		return -EIO;
>   
> -	if (!len)
> +	if (!len || !i_size)
>   		return 0;
>   	/*
>   	 * flush any page cache pages in this range.  this
> @@ -1154,6 +1154,9 @@ ssize_t __ceph_sync_read(struct inode *inode, loff_t *ki_pos,
>   		doutc(cl, "%llu~%llu got %zd i_size %llu%s\n", off, len,
>   		      ret, i_size, (more ? " MORE" : ""));
>   
> +		if (i_size == 0)
> +			ret = 0;
> +
>   		/* Fix it to go to end of extent map */
>   		if (sparse && ret >= 0)
>   			ret = ceph_sparse_ext_map_end(op);
>
Hi Luis,

BTW, so in the following code:

1202                 idx = 0;
1203                 if (ret <= 0)
1204                         left = 0;
1205                 else if (off + ret > i_size)
1206                         left = i_size - off;
1207                 else
1208                         left = ret;

The 'ret' should be larger than '0', right ?

If so we do not check anf fix it in the 'else if' branch instead?

Because currently the read path code won't exit directly and keep 
retrying to read if it found that the real content length is longer than 
the local 'i_size'.

Again I am afraid your current fix will break the MIX filelock semantic ?

Thanks

- Xiubo