Re: [PATCH] fix: general protection fault in iter_file_splice_write

[Al Viro <viro@zeniv.linux.org.uk>]

On Mon, Nov 04, 2024 at 12:42:39AM -0800, Daniel Yang wrote:
> The function iter_file_splice_write() calls pipe_buf_release() which has
> a nullptr dereference in ops->release. Add check for buf->ops not null
> before calling pipe_buf_release().
> 
> Signed-off-by: Daniel Yang <danielyangkang@gmail.com>
> Reported-by: syzbot+d2125fcb6aa8c4276fd2@syzkaller.appspotmail.com
> Closes: https://syzkaller.appspot.com/bug?extid=d2125fcb6aa8c4276fd2
> Fixes: 2df86547b23d ("netfs: Cut over to using new writeback code")
> ---
>  fs/splice.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/fs/splice.c b/fs/splice.c
> index 06232d7e5..b8c503e47 100644
> --- a/fs/splice.c
> +++ b/fs/splice.c
> @@ -756,7 +756,8 @@ iter_file_splice_write(struct pipe_inode_info *pipe, struct file *out,
>  			if (ret >= buf->len) {
>  				ret -= buf->len;
>  				buf->len = 0;
> -				pipe_buf_release(pipe, buf);
> +				if (buf->ops)
> +					pipe_buf_release(pipe, buf);
>  				tail++;
>  				pipe->tail = tail;
>  				if (pipe->files)

Wait a minute.  If nothing else, all those buffers should've passed through
pipe_buf_confirm() just prior to the call of ->write_iter(); just what had
managed to zero their ->ops and what else had that whatever it had been
done to them?

Note that pipe must've been held locked all along, so I suspect that we
ended up with ->write_iter() claiming to have consumed more than it had
been given.  That could've ended up with the second loop running around
the pipe->bufs[], having already emptied each of them and trying to
find where the hell had that extra data come from.

I'd suggest checking which ->write_iter() instance had been called and
hunting for bogus return values in there.  Again, ->write_iter(iocb, from)
should never return more than the value of iov_iter_count(from) prior
to the call; any instance told "write those 42 bytes" should never
reply with "here, I've written 69 of them", lest it confuses the living
fuck out of the callers.