linux-kernel - Re: refcount_t: underflow; use-after-free with CIFS umount after scsi-misc commit ef2cc88e2a205b8a11a19e78db63a70d3728cdf5

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Date:   Tue, 10 Dec 2019 01:15:46 +1030
From:   Arthur Marsh <arthur.marsh@...ernode.on.net>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
CC:     SCSI development list <linux-scsi@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        CIFS <linux-cifs@...r.kernel.org>,
        "James E.J. Bottomley" <jejb@...ux.ibm.com>
Subject: Re: refcount_t: underflow; use-after-free with CIFS umount after scsi-misc commit ef2cc88e2a205b8a11a19e78db63a70d3728cdf5

Hi, I ran the last good kernel with several boot-up, cifs mount, un-mount, shut down cycles without encountering the problem.

After applying the patch from <ronniesahlberg@...il.com>:

diff --git a/fs/cifs/smb2pdu.c b/fs/cifs/smb2pdu.c
index 0ab6b1200288..d2658f51ff60 100644
--- a/fs/cifs/smb2pdu.c
+++ b/fs/cifs/smb2pdu.c
@@ -1847,7 +1847,8 @@ SMB2_tdis(const unsigned int xid, struct cifs_tcon *tcon)
if ((tcon->need_reconnect) || (tcon->ses->need_reconnect))
return 0;

- close_shroot(&tcon->crfid);
+ if (tcon->crfid.is_valid)
+ close_shroot(&tcon->crfid);


 to kernel 5.5.0-rc1 I no longer experience the problem.

Regards,

Arthur. 

On 9 December 2019 12:53:02 pm ACDT, Linus Torvalds <torvalds@...ux-foundation.org> wrote:
>On Sun, Dec 8, 2019 at 5:49 PM Arthur Marsh
><arthur.marsh@...ernode.on.net> wrote:
>>
>> This still happens with 5.5.0-rc1:
>
>Does it happen 100% of the time?
>
>Your bisection result looks pretty nonsensical - not that it's
>impossible (anything is possible), but it really doesn't look very
>likely. Which makes me think maybe it's slightly timing-sensitive or
>something?
>
>Would you mind trying to re-do the bisection, and for each kernel try
>the mount thing at least a few times before you decide a kernel is
>good?
>
>Bisection is very powerful, but if _any_ of the kernels you marked
>good weren't really good (they just happened to not trigger the
>problem), bisection ends up giving completely the wrong answer. And
>with that bisection commit, there's not even a hint of what could have
>gone wrong.
>
>             Linus

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