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SubjectRe: [PATCH] tomoyo: Don't check open/getattr permission on sockets.
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Hello, Al.

Q1: Do you agree that we should fix TOMOYO side rather than SOCKET_I()->sk
management.

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() ?

Regards.

On 2019/06/09 15:41, Tetsuo Handa wrote:
> 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@I-love.SAKURA.ne.jp>
> Reported-by: syzbot <syzbot+0341f6a4d729d4e0acf1@syzkaller.appspotmail.com>
> ---
> security/tomoyo/tomoyo.c | 7 +++++++
> 1 file changed, 7 insertions(+)
>
> diff --git a/security/tomoyo/tomoyo.c b/security/tomoyo/tomoyo.c
> index 716c92e..9661b86 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 (path->dentry->d_inode && S_ISSOCK(path->dentry->d_inode->i_mode))
> + return 0;
> return tomoyo_path_perm(TOMOYO_TYPE_GETATTR, path, NULL);
> }
>
> @@ -316,6 +319,10 @@ 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 (f->f_path.dentry->d_inode &&
> + S_ISSOCK(f->f_path.dentry->d_inode->i_mode))
> + return 0;
> return tomoyo_check_open_permission(tomoyo_domain(), &f->f_path,
> f->f_flags);
> }
>

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