Re: [PATCH bpf] bpf: respect CAP_IPC_LOCK in RLIMIT_MEMLOCK check

From: Daniel Borkmann <daniel@iogearbox.net>
To: Christian Barcenas <christian@cbarcenas.com>,
	Alexei Starovoitov <ast@kernel.org>,
	netdev@vger.kernel.org
Cc: Martin KaFai Lau <kafai@fb.com>, Song Liu <songliubraving@fb.com>,
	Yonghong Song <yhs@fb.com>,
	bpf@vger.kernel.org
Subject: Re: [PATCH bpf] bpf: respect CAP_IPC_LOCK in RLIMIT_MEMLOCK check
Date: Mon, 16 Sep 2019 11:26:53 +0200	[thread overview]
Message-ID: <678ba696-4b20-5f06-7c4f-ec68a9229620@iogearbox.net> (raw)
In-Reply-To: <20190911181816.89874-1-christian@cbarcenas.com>

On 9/11/19 8:18 PM, Christian Barcenas wrote:
> A process can lock memory addresses into physical RAM explicitly
> (via mlock, mlockall, shmctl, etc.) or implicitly (via VFIO,
> perf ring-buffers, bpf maps, etc.), subject to RLIMIT_MEMLOCK limits.
> 
> CAP_IPC_LOCK allows a process to exceed these limits, and throughout
> the kernel this capability is checked before allowing/denying an attempt
> to lock memory regions into RAM.
> 
> Because bpf locks its programs and maps into RAM, it should respect
> CAP_IPC_LOCK. Previously, bpf would return EPERM when RLIMIT_MEMLOCK was
> exceeded by a privileged process, which is contrary to documented
> RLIMIT_MEMLOCK+CAP_IPC_LOCK behavior.

Do you have a link/pointer where this is /clearly/ documented?

Uapi header is not overly clear ...

include/uapi/linux/capability.h says:

   /* Allow locking of shared memory segments */
   /* Allow mlock and mlockall (which doesn't really have anything to do
      with IPC) */

   #define CAP_IPC_LOCK         14

   [...]

   /* Override resource limits. Set resource limits. */
   /* Override quota limits. */
   /* Override reserved space on ext2 filesystem */
   /* Modify data journaling mode on ext3 filesystem (uses journaling
      resources) */
   /* NOTE: ext2 honors fsuid when checking for resource overrides, so
      you can override using fsuid too */
   /* Override size restrictions on IPC message queues */
   /* Allow more than 64hz interrupts from the real-time clock */
   /* Override max number of consoles on console allocation */
   /* Override max number of keymaps */

   #define CAP_SYS_RESOURCE     24

... but my best guess is you are referring to `man 2 mlock`:

    Limits and permissions

        In Linux 2.6.8 and earlier, a process must be privileged (CAP_IPC_LOCK)
        in order to lock memory and the RLIMIT_MEMLOCK soft resource limit defines
        a limit on how much memory the process may lock.

        Since  Linux  2.6.9, no limits are placed on the amount of memory that a
        privileged process can lock and the RLIMIT_MEMLOCK soft resource limit
        instead defines a limit on how much memory an unprivileged process may lock.

Thanks,
Daniel