Re: Access to CMSG_DATA

[Arnd Bergmann <arnd@arndb.de>]

On Tue, Dec 17, 2019 at 3:36 PM Rich Felker <dalias@libc.org> wrote:
>
> It came to my attention while reviewing possible breakage with move to
> 64-bit time_t that some applications are dereferencing data in socket
> control messages (particularly SCM_TIMESTAMP*) in-place as the message
> type, rather than memcpy'ing it to appropriate storage. This
> necessarily does not work and is not supportable if the message
> contains data with greater alignment requirement than the header. In
> particular, on 32-bit archs, cmsghdr has size 12 and alignment 4, but
> struct timeval and timespec may have alignment requirement 8.
>
> I found at least ptpd, socat, and ssmping doing this via Debian Code
> Search:
>
> https://sources.debian.org/src/ptpd/2.3.1-debian1-4/src/dep/net.c/?hl=1578#L1578
> https://sources.debian.org/src/socat/1.7.3.3-2/xio-socket.c/?hl=1839#L1839
> https://sources.debian.org/src/ssmping/0.9.1-3/ssmpngcl.c/?hl=307#L307
>
> and I suspect there are a good deal more out there. On most archs they
> won't break, or will visibly break with SIGBUS, but in theory it's
> possible that they silently read wrong data and this might happen on
> some older and more tiny-embedded-oriented archs.

Good find. I suppose this is going to be particularly annoying for
architectures that are affected because all systems that are in
widespread use are not affected:

- x86, riscv, ppc and s390 always allow unaligned loads
- ARMv6+ mostly allows unaligned loads. Some instructions such as ldrd
  require alignment of four bytes, which is ok, and ARMv5 requires natural
  alignment up to 32 bits, so this is also ok
- On MIPS I think that o32 is fine since there are no 64-bit loads, but
  n64  would likely be affected, if there are still users remaining (musl
  supports it, so I assume there are some users).
- m68k only requires 16-bit alignment
- For the other 32-bit architectures that musl supports (microblaze, sh,
  openrisc), none advertise unaligned-access capability  to the kernel,
  but I also don't think any of them have a native 64-bit load instruction.
  armv5, microblaze, sh and nds32 fix up unaligned accesses in an
  exception handler; openrisc and csky require aligned accesses in user
  space.

> I think it's clear to someone who understands alignment and who's
> thought about it that applications just can't do this, but it doesn't
> seem to be documented, and an example in cmsg(3) even shows access to
> int payload via *(int *)CMSG_DATA(cmsg) (of course int is safe because
> its alignment is <= header alignment, but this is not mentioned).
>
> Could we add text, and perhaps change the example, to indicate that in
> general memcpy needs to be used to copy the payload to/from a suitable
> object?

Yes, I think that would be a good idea.

         Arnd