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From: Dmitry Vyukov <>
To: Dominique Martinet <>
Cc: syzbot <>,
	David Miller <>,
	Eric Van Hensbergen <>,
	LKML <>,
	Latchesar Ionkov <>,
	netdev <>,
	syzkaller-bugs <>,
Subject: Re: KASAN: invalid-free in p9stat_free
Date: Mon, 27 Aug 2018 07:25:18 -0700
Message-ID: <> (raw)
In-Reply-To: <20180827052412.GA26294@nautica>

On Sun, Aug 26, 2018 at 10:24 PM, Dominique Martinet
<> wrote:
> syzbot wrote on Sun, Aug 26, 2018:
>> HEAD commit:    e27bc174c9c6 Add linux-next specific files for 20180824
>> git tree:       linux-next
>> console output:
>> kernel config:
>> dashboard link:
>> compiler:       gcc (GCC) 8.0.1 20180413 (experimental)
>> syz repro:
>> C reproducer:
>> IMPORTANT: if you fix the bug, please add the following tag to the commit:
>> Reported-by:
>> random: sshd: uninitialized urandom read (32 bytes read)
>> random: sshd: uninitialized urandom read (32 bytes read)
>> random: sshd: uninitialized urandom read (32 bytes read)
>> random: sshd: uninitialized urandom read (32 bytes read)
>> ==================================================================
>> BUG: KASAN: double-free or invalid-free in p9stat_free+0x35/0x100
>> net/9p/protocol.c:48
> That looks straight-forward enough, p9pdu_vreadf does p9stat_free on
> error then v9fs_dir_readdir does the same ; there is nothing else that
> could return an error without going through the first free so we could
> just remove the later one...
> There are a couple other users of the 'S' pdu read (that reads the stat
> struct and frees it on error), so it's probably best to keep the current
> behaviour as far as this is concerned, what we could do though is make
> the free function idempotent (write NULLs in the freed fields), but I do
> not see this being done often, do you know what the policy is about
> this kind of pattern nowadays?

Hi Dominique,

kfree and then null pointer is pretty common, try to run:

find -name "*.c" -exec grep -A 1 "kfree(" {} \; | grep -B 1 " = NULL;"

Leaving dangling pointers behind is not the best idea.
And from what I remember a bunch of similar double frees were fixed by
nulling the pointer after the first kfree.

> The struct is cleanly zeroed before being read so there is no risk of
> double-frees between iterations so zeroing pointers is not strictly
> required, but it does make things safer in general.

  reply index

Thread overview: 6+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2018-08-26 13:50 syzbot
2018-08-27  5:24 ` Dominique Martinet
2018-08-27 14:25   ` Dmitry Vyukov [this message]
2018-08-27 22:40     ` Dominique Martinet
2018-08-27 22:48 ` [PATCH 1/2] v9fs_dir_readdir: fix double-free on p9stat_read error Dominique Martinet
2018-08-27 22:48   ` [PATCH 2/2] 9p: clear dangling pointers in p9stat_free Dominique Martinet

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