linux-kernel - [PATCH v2] tomoyo: Don't check open/getattr permission on sockets.

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Message-ID: <8f874b03-b129-205f-5f05-125479701275@i-love.sakura.ne.jp>
Date:   Sat, 22 Jun 2019 13:45:30 +0900
From:   Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>
To:     Al Viro <viro@...iv.linux.org.uk>
Cc:     linux-fsdevel@...r.kernel.org,
        syzbot <syzbot+0341f6a4d729d4e0acf1@...kaller.appspotmail.com>,
        jmorris@...ei.org, linux-kernel@...r.kernel.org,
        linux-security-module@...r.kernel.org, serge@...lyn.com,
        syzkaller-bugs@...glegroups.com, takedakn@...data.co.jp,
        "David S. Miller" <davem@...emloft.net>
Subject: [PATCH v2] tomoyo: Don't check open/getattr permission on sockets.

On 2019/06/19 5:49, Al Viro wrote:
> On Sun, Jun 16, 2019 at 03:49:00PM +0900, Tetsuo Handa wrote:
>> Hello, Al.
>>
>> Q1: Do you agree that we should fix TOMOYO side rather than SOCKET_I()->sk
>>     management.
> 
> You do realize that sockets are not unique in that respect, right?
> All kinds of interesting stuff can be accessed via /proc/*/fd/*, and
> it _can_ be closed under you.  So I'd suggest checking how your code
> copes with similar for pipes, FIFOs, epoll, etc., accessed that way...

I know all kinds of interesting stuff can be accessed via /proc/*/fd/*,
and it _can_ be closed under me.

Regarding sockets, I was accessing "struct socket" memory and
"struct sock" memory which are outside of "struct inode" memory.

But regarding other objects, I am accessing "struct dentry" memory,
"struct super_block" memory and "struct inode" memory. I'm expecting
that these memory can't be kfree()d as long as "struct path" holds
a reference. Is my expectation correct?

> 
> We are _not_ going to be checking that in fs/open.c - the stuff found
> via /proc/*/fd/* can have the associated file closed by the time
> we get to calling ->open() and we won't know that until said call.

OK. Then, fixing TOMOYO side is the correct way.

> 
>> Q2: Do you see any problem with using f->f_path.dentry->d_inode ?
>>     Do we need to use d_backing_inode() or d_inode() ?
> 
> Huh?  What's wrong with file_inode(f), in the first place?  And
> just when can that be NULL, while we are at it?

Oh, I was not aware of file_inode(). Thanks.

> 
>>>  static int tomoyo_inode_getattr(const struct path *path)
>>>  {
>>> +	/* It is not safe to call tomoyo_get_socket_name(). */
>>> +	if (path->dentry->d_inode && S_ISSOCK(path->dentry->d_inode->i_mode))
>>> +		return 0;
> 
> Can that be called for a negative?
> 

I check for NULL when I'm not sure it is guaranteed to hold a valid pointer.
You meant "we are sure that path->dentry->d_inode is valid", don't you?

By the way, "negative" associates with IS_ERR() range. I guess that
"NULL" is the better name...

Anyway, here is V2 patch.

>From c63c4074300921d6d1c33c3b8dc9c84ebfededf5 Mon Sep 17 00:00:00 2001
From: Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
Date: Sat, 22 Jun 2019 13:14:26 +0900
Subject: [PATCH v2] tomoyo: Don't check open/getattr permission on sockets.

syzbot is reporting that use of SOCKET_I()->sk from open() can result in
use after free problem [1], for socket's inode is still reachable via
/proc/pid/fd/n despite destruction of SOCKET_I()->sk already completed.

But there is no point with calling security_file_open() on sockets
because open("/proc/pid/fd/n", !O_PATH) on sockets fails with -ENXIO.

There is some point with calling security_inode_getattr() on sockets
because stat("/proc/pid/fd/n") and fstat(open("/proc/pid/fd/n", O_PATH))
are valid. If we want to access "struct sock"->sk_{family,type,protocol}
fields, we will need to use security_socket_post_create() hook and
security_inode_free() hook in order to remember these fields because
security_sk_free() hook is called before the inode is destructed. But
since information which can be protected by checking
security_inode_getattr() on sockets is trivial, let's not be bothered by
"struct inode"->i_security management.

There is point with calling security_file_ioctl() on sockets. Since
ioctl(open("/proc/pid/fd/n", O_PATH)) is invalid, security_file_ioctl()
on sockets should remain safe.

[1] https://syzkaller.appspot.com/bug?id=73d590010454403d55164cca23bd0565b1eb3b74

Signed-off-by: Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
Reported-by: syzbot <syzbot+0341f6a4d729d4e0acf1@...kaller.appspotmail.com>
---
 security/tomoyo/tomoyo.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/security/tomoyo/tomoyo.c b/security/tomoyo/tomoyo.c
index 716c92e..8ea3f5d 100644
--- a/security/tomoyo/tomoyo.c
+++ b/security/tomoyo/tomoyo.c
@@ -126,6 +126,9 @@ static int tomoyo_bprm_check_security(struct linux_binprm *bprm)
  */
 static int tomoyo_inode_getattr(const struct path *path)
 {
+	/* It is not safe to call tomoyo_get_socket_name(). */
+	if (S_ISSOCK(d_inode(path->dentry)->i_mode))
+		return 0;
 	return tomoyo_path_perm(TOMOYO_TYPE_GETATTR, path, NULL);
 }
 
@@ -316,6 +319,9 @@ static int tomoyo_file_open(struct file *f)
 	/* Don't check read permission here if called from do_execve(). */
 	if (current->in_execve)
 		return 0;
+	/* Sockets can't be opened by open(). */
+	if (S_ISSOCK(file_inode(f)->i_mode))
+		return 0;
 	return tomoyo_check_open_permission(tomoyo_domain(), &f->f_path,
 					    f->f_flags);
 }
-- 
1.8.3.1