linux-kernel - Re: [BUG] ALSA: seq: a possible sleep-in-atomic-context bug in snd_virmidi_dev_receive

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Message-ID: <s5hh81z9iob.wl-tiwai@suse.de>
Date:   Tue, 17 Dec 2019 14:37:56 +0100
From:   Takashi Iwai <tiwai@...e.de>
To:     Jia-Ju Bai <baijiaju1990@...il.com>
Cc:     perex@...ex.cz, tiwai@...e.com, rfontana@...hat.com,
        gregkh@...uxfoundation.org, allison@...utok.net,
        tglx@...utronix.de, alsa-devel@...a-project.org,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [BUG] ALSA: seq: a possible sleep-in-atomic-context bug in snd_virmidi_dev_receive_event()

On Tue, 17 Dec 2019 14:24:21 +0100,
Jia-Ju Bai wrote:
> 
> The driver may sleep while holding a read lock.
> The function call path (from bottom to top) in Linux 4.19 is:
> 
> sound/core/seq/seq_memory.c, 96:
>     copy_from_user in snd_seq_dump_var_event
> sound/core/seq/seq_virmidi.c, 97:
>     snd_seq_dump_var_event in snd_virmidi_dev_receive_event
> sound/core/seq/seq_virmidi.c, 88:
>     _raw_read_lock in snd_virmidi_dev_receive_event

This can't happen.  snd_virmidi_dev_receive_event() takes
conditionally either read_lock or rw_sem depending on the atomic
argument.  And the data including user-space pointer is handled always
with atomic=1, hence down_read() is used instead of read_lock() here.


thanks,

Takashi

> copy_from_user() can sleep at runtime.
> 
> I am not sure how to properly fix this possible bug, so I only report it.
> 
> This bug is found by a static analysis tool STCheck written by myself.
> 
> 
> Best wishes,
> Jia-Ju Bai
>