linux-kernel - Re: [PATCH 0/2] printk: replace ringbuffer

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Date:   Thu, 6 Feb 2020 15:31:10 +0900
From:   Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
To:     John Ogness <john.ogness@...utronix.de>
Cc:     Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
        lijiang <lijiang@...hat.com>, Petr Mladek <pmladek@...e.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Andrea Parri <parri.andrea@...il.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        kexec@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 0/2] printk: replace ringbuffer

On (20/02/05 16:48), John Ogness wrote:
> On 2020-02-05, Sergey Senozhatsky <sergey.senozhatsky@...il.com> wrote:
> > 3BUG: KASAN: wild-memory-access in copy_data+0x129/0x220>
> > 3Write of size 4 at addr 5a5a5a5a5a5a5a5a by task cat/474>
> 
> The problem was due to an uninitialized pointer.
> 
> Very recently the ringbuffer API was expanded so that it could
> optionally count lines in a record. This made it possible for me to
> implement record_print_text_inline(), which can do all the kmsg_dump
> multi-line madness without requiring a temporary buffer. Rather than
> passing an extra argument around for the optional line count, I added
> the text_line_count pointer to the printk_record struct. And since line
> counting is rarely needed, it is only performed if text_line_count is
> non-NULL.
> 
> I oversaw that devkmsg_open() setup a printk_record and so I did not see
> to add the extra NULL initialization of text_line_count. There should be
> be an initializer function/macro to avoid this danger.
> 
> John Ogness
> 
> The quick fixup:
> 
> diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
> index d0d24ee1d1f4..5ad67ff60cd9 100644
> --- a/kernel/printk/printk.c
> +++ b/kernel/printk/printk.c
> @@ -883,6 +883,7 @@ static int devkmsg_open(struct inode *inode, struct file *file)
>  	user->record.text_buf_size = sizeof(user->text_buf);
>  	user->record.dict_buf = &user->dict_buf[0];
>  	user->record.dict_buf_size = sizeof(user->dict_buf);
> +	user->record.text_line_count = NULL;
>  
>  	logbuf_lock_irq();
>  	user->seq = prb_first_seq(prb);

Yes. That should do. It seems that /dev/kmsg reads/writes happen very early in
my system and all the backtraces I saw were from completely unrelated paths -
either a NULL deref at sys_clone()->do_fork()->copy_creds()->prepare_creads(),
or general protection fault in sys_keyctl()->join_session_keyring()->prepare_creds(),
or some weird crashes in ext4. And so on.

I see some more unexplainable lockups on one on my test boards, but I
can't provide more details at this time. Might not be related to the
patch set. Need to investigate further.

	-ss