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    SubjectRe: [PATCH 2/2] PM / Sleep: Check the file capability when writing wake lock interface
    On Mon, Dec 31, 2018 at 11:41 AM Greg Kroah-Hartman
    <gregkh@linuxfoundation.org> wrote:
    >
    > On Mon, Dec 31, 2018 at 05:38:51PM +0800, joeyli wrote:
    > > Hi Greg,
    > >
    > > On Sun, Dec 30, 2018 at 03:48:35PM +0100, Greg Kroah-Hartman wrote:
    > > > On Sun, Dec 30, 2018 at 09:28:56PM +0800, Lee, Chun-Yi wrote:
    > > > > The wake lock/unlock sysfs interfaces check that the writer must has
    > > > > CAP_BLOCK_SUSPEND capability. But the checking logic can be bypassed
    > > > > by opening sysfs file within an unprivileged process and then writing
    > > > > the file within a privileged process. The tricking way has been exposed
    > > > > by Andy Lutomirski in CVE-2013-1959.
    > > >
    > > > Don't you mean "open by privileged and then written by unprivileged?"
    > > > Or if not, exactly how is this a problem? You check the capabilities
    > > > when you do the write and if that is not allowed then, well
    > > >
    > >
    > > Sorry for I didn't provide clear explanation.
    > >
    > > The privileged means CAP_BLOCK_SUSPEND but not file permission. The file permission
    > > has already relaxed for non-root user. Then the expected behavior is that non-root
    > > process must has CAP_BLOCK_SUSPEND capability for writing wake_lock sysfs.
    > >
    > > But, the CAP_BLOCK_SUSPEND restrict can be bypassed:
    > >
    > > int main(int argc, char* argv[])
    > > {
    > > int fd, ret = 0;
    > >
    > > fd = open("/sys/power/wake_lock", O_RDWR);
    > > if (fd < 0)
    > > err(1, "open wake_lock");
    > >
    > > if (dup2(fd, 1) != 1) // overwrite the stdout with wake_lock
    > > err(1, "dup2");
    > > sleep(1);
    > > execl("./string", "string"); //string has capability
    > >
    > > return ret;
    > > }
    > >
    > > This program is an unpriviledged process (has no CAP_BLOCK_SUSPEND), it opened
    > > wake_lock sysfs and overwrited stdout. Then it executes the "string" program
    > > that has CAP_BLOCK_SUSPEND.
    >
    > That's the problem right there, do not give CAP_BLOCK_SUSPEND rights to
    > "string". If any user can run that program, there's nothing the kernel
    > can do about this, right? Just don't allow that program on the system :)
    >
    > > The string program writes to stdout, which means that it writes to
    > > wake_lock. So an unpriviledged opener can trick an priviledged writer
    > > for writing sysfs.
    >
    > That sounds like a userspace program that was somehow given incorrect
    > rights by the admin, and a user is taking advantage of it. That's not
    > the kernel's fault.

    Isn't it? Pretty much any setuid program will write to stdout or
    stderr; even the glibc linker code does so if you set LD_DEBUG.
    (Normally the output isn't entirely attacker-controlled, but it is in
    the case of stuff like "procmail", which I think Debian still ships as
    setuid root.) setuid programs should always be able to safely call
    read() and write() on caller-provided file descriptors. Also, you're
    supposed to be able to receive file descriptors over unix domain
    sockets and then write to them without trusting the sender. Basically,
    the ->read and ->write VFS handlers should never look at the caller's
    credentials, only the opener's (with the exception of LSMs, which tend
    to do weird things to the system's security model).

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