Re: [syzbot] KASAN: null-ptr-deref Read in filp_close (2)

[Christian Brauner <christian.brauner@ubuntu.com>]

On Wed, Jul 14, 2021 at 09:59:25AM +0200, Christian Brauner wrote:
> On Tue, Jul 13, 2021 at 11:49:14AM -0700, Linus Torvalds wrote:
> > On Mon, Jul 12, 2021 at 9:12 PM syzbot
> > <syzbot+283ce5a46486d6acdbaf@syzkaller.appspotmail.com> wrote:
> > >
> > > syzbot has found a reproducer for the following issue on:
> > 
> > Hmm.
> > 
> > This issue is reported to have been already fixed:
> > 
> >     Fix commit: 9b5b8722 file: fix close_range() for unshare+cloexec
> > 
> > and that fix is already in the reported HEAD commit:
> > 
> > > HEAD commit:    7fef2edf sd: don't mess with SD_MINORS for CONFIG_DEBUG_BL..
> > 
> > and the oops report clearly is from that:
> > 
> > > CPU: 1 PID: 8445 Comm: syz-executor493 Not tainted 5.14.0-rc1-syzkaller #0
> > 
> > so the alleged fix is already there.
> > 
> > So clearly commit 9b5b872215fe ("file: fix close_range() for
> > unshare+cloexec") does *NOT* fix the issue.
> > 
> > This was originally bisected to that 582f1fb6b721 ("fs, close_range:
> > add flag CLOSE_RANGE_CLOEXEC") in
> > 
> >      https://syzkaller.appspot.com/bug?id=1bef50bdd9622a1969608d1090b2b4a588d0c6ac
> > 
> > which is where the "fix" is from.
> > 
> > It would probably be good if sysbot made this kind of "hey, it was
> > reported fixed, but it's not" very clear.
> > 
> > The KASAN report looks like a use-after-free, and that "use" is
> > actually the sanity check that the file count is non-zero, so it's
> > really a "struct file *" that has already been free'd.
> > 
> > That bogus free is a regular close() system call
> > 
> > >  filp_close+0x22/0x170 fs/open.c:1306
> > >  close_fd+0x5c/0x80 fs/file.c:628
> > >  __do_sys_close fs/open.c:1331 [inline]
> > >  __se_sys_close fs/open.c:1329 [inline]
> > 
> > And it was opened by a "creat()" system call:
> > 
> > > Allocated by task 8445:
> > >  __alloc_file+0x21/0x280 fs/file_table.c:101
> > >  alloc_empty_file+0x6d/0x170 fs/file_table.c:150
> > >  path_openat+0xde/0x27f0 fs/namei.c:3493
> > >  do_filp_open+0x1aa/0x400 fs/namei.c:3534
> > >  do_sys_openat2+0x16d/0x420 fs/open.c:1204
> > >  do_sys_open fs/open.c:1220 [inline]
> > >  __do_sys_creat fs/open.c:1294 [inline]
> > >  __se_sys_creat fs/open.c:1288 [inline]
> > >  __x64_sys_creat+0xc9/0x120 fs/open.c:1288
> > >  do_syscall_x64 arch/x86/entry/common.c:50 [inline]
> > >  do_syscall_64+0x35/0xb0 arch/x86/entry/common.c:80
> > >  entry_SYSCALL_64_after_hwframe+0x44/0xae
> > 
> > But it has apparently already been closed from a workqueue:
> > 
> > > Freed by task 8445:
> > >  __fput+0x288/0x920 fs/file_table.c:280
> > >  task_work_run+0xdd/0x1a0 kernel/task_work.c:164
> > 
> > So it's some kind of confusion and re-use of a struct file pointer.
> > 
> > Which is certainly consistent with the "fix" in 9b5b872215fe ("file:
> > fix close_range() for unshare+cloexec"), but it very much looks like
> > that fix was incomplete and not the full story.
> > 
> > Some fdtable got re-allocated? The fix that wasn't a fix ends up
> > re-checking the maximum file number under the file_lock, but there's
> > clearly something else going on too.
> > 
> > Christian?
> 
> Looking into this now.
> 
> I have to say I'm very confused by the syzkaller report here.
> 
> If I go to
> 
> https://syzkaller.appspot.com/bug?extid=283ce5a46486d6acdbaf
> 
> which is the original link in the report it shows me
> 
> android-54	KASAN: use-after-free Read in filp_close	C			2	183d	183d	0/1	upstream: reported C repro on 2021/01/11 12:38
> 
> which seems to indicate that this happened on an Android specific 5.4
> kernel?
> 
> But ok, so I click on the link "upstream: reported C repro on 2021/01/11 12:38"
> which takes me to a google group
> 
> https://groups.google.com/g/syzkaller-android-bugs/c/FQj0qcRSy_M/m/wrY70QFzBAAJ
> 
> which again strongly indicates that this is an Android specific kernel?
> 
> HEAD commit: c9951e5d Merge 5.4.88 into android12-5.4
> git tree: android12-5.4
> 
> but then I can click on the dashboard link for that crash report and it
> takes me to:
> 
> https://syzkaller.appspot.com/bug?extid=53897bcb31b82c7a08fe
> 
> which seems to be the upstream report?
> 
> So I'm a bit confused whether I'm even looking at the correct bug report
> but I'll just give the repro a try and see what's going on.

Ok, reproduced and I think I found the issue. It's not related to
close_fd((), I think it's caused by a UAF when FSCONFIG_SET_FD is with
the key "source" and a valid fd passed through "aux".

Briefly, fs_parameter is a union:

struct fs_parameter {
	const char		*key;		/* Parameter name */
	enum fs_value_type	type:8;		/* The type of value here */
	union {
		char		*string;
		void		*blob;
		struct filename	*name;
		struct file	*file;
	};
	size_t	size;
	int	dirfd;
};

and cgroup1_parse_param is copying out param->string when the param's
key is "source" without verifying that param->type is actually
fs_value_is_string.

I'll explain in detail in the commit once I've confirmed and tested that
my suspicion is correct.

Christian