Re: [PATCH v2] x86: Fix x32 System V message queue syscalls

[Jessica Clarke]

On 1 Nov 2020, at 18:15, Jessica Clarke <jrtc27@xxxxxxxxxx> wrote:
> 
> On 1 Nov 2020, at 18:07, Andy Lutomirski <luto@xxxxxxxxxx> wrote:
>> 
>> On Sat, Oct 31, 2020 at 6:50 PM Rich Felker <dalias@xxxxxxxx> wrote:
>>> 
>>> On Sun, Nov 01, 2020 at 01:27:35AM +0000, Jessica Clarke wrote:
>>>> On 1 Nov 2020, at 01:22, Rich Felker <dalias@xxxxxxxx> wrote:
>>>>> On Sat, Oct 31, 2020 at 04:30:44PM -0700, Andy Lutomirski wrote:
>>>>>> cc: some libc folks
>>>>>> 
>>>>>> On Mon, Oct 12, 2020 at 6:45 AM Jessica Clarke <jrtc27@xxxxxxxxxx> wrote:
>>>>>>> 
>>>>>>> POSIX specifies that the first field of the supplied msgp, namely mtype,
>>>>>>> is a long, not a __kernel_long_t, and it's a user-defined struct due to
>>>>>>> the variable-length mtext field so we can't even bend the spec and make
>>>>>>> it a __kernel_long_t even if we wanted to. Thus we must use the compat
>>>>>>> syscalls on x32 to avoid buffer overreads and overflows in msgsnd and
>>>>>>> msgrcv respectively.
>>>>>> 
>>>>>> This is a mess.
>>>>>> 
>>>>>> include/uapi/linux/msg.h has:
>>>>>> 
>>>>>> /* message buffer for msgsnd and msgrcv calls */
>>>>>> struct msgbuf {
>>>>>>      __kernel_long_t mtype;          /* type of message */
>>>>>>      char mtext[1];                  /* message text */
>>>>>> };
>>>>>> 
>>>>>> Your test has:
>>>>>> 
>>>>>> struct msg_long {
>>>>>>  long mtype;
>>>>>>  char mtext[8];
>>>>>> };
>>>>>> 
>>>>>> struct msg_long_ext {
>>>>>>  struct msg_long msg_long;
>>>>>>  char mext[4];
>>>>>> };
>>>>>> 
>>>>>> and I'm unclear as to exactly what you're trying to do there with the
>>>>>> "mext" part.
>>>>>> 
>>>>>> POSIX says:
>>>>>> 
>>>>>>     The application shall ensure that the argument msgp points to  a  user-
>>>>>>     defined  buffer that contains first a field of type long specifying the
>>>>>>     type of the message, and then a data portion that holds the data  bytes
>>>>>>     of the message. The structure below is an example of what this user-de‐
>>>>>>     fined buffer might look like:
>>>>>> 
>>>>>>         struct mymsg {
>>>>>>             long   mtype;       /* Message type. */
>>>>>>             char   mtext[1];    /* Message text. */
>>>>>>         }
>>>>>> 
>>>>>> NTP has this delightful piece of code:
>>>>>> 
>>>>>> 44 typedef union {
>>>>>> 45   struct msgbuf msgp;
>>>>>> 46   struct {
>>>>>> 47     long mtype;
>>>>>> 48     int code;
>>>>>> 49     struct timeval tv;
>>>>>> 50   } msgb;
>>>>>> 51 } MsgBuf;
>>>>>> 
>>>>>> bluefish has:
>>>>>> 
>>>>>> struct small_msgbuf {
>>>>>> long mtype;
>>>>>> char mtext[MSQ_QUEUE_SMALL_SIZE];
>>>>>> } small_msgp;
>>>>>> 
>>>>>> 
>>>>>> My laptop has nothing at all in /dev/mqueue.
>>>>>> 
>>>>>> So I don't really know what the right thing to do is.  Certainly if
>>>>>> we're going to apply this patch, we should also fix the header.  I
>>>>>> almost think we should *delete* struct msgbuf from the headers, since
>>>>>> it's all kinds of busted, but that will break the NTP build.  Ideally
>>>>>> we would go back in time and remove it from the headers.
>>>>>> 
>>>>>> Libc people, any insight?  We can probably fix the bug without
>>>>>> annoying anyone given how lightly x32 is used and how lightly POSIX
>>>>>> message queues are used.
>>>>> 
>>>>> If it's that outright wrong and always has been, I feel like the old
>>>>> syscall numbers should just be deprecated and new ones assigned.
>>>>> Otherwise, there's no way for userspace to be safe against data
>>>>> corruption when run on older kernels. If there's a new syscall number,
>>>>> libc can just use the new one unconditionally (giving ENOSYS on
>>>>> kernels where it would be broken) or have a x32-specific
>>>>> implementation that makes the old syscall and performs translation if
>>>>> the new one fails with ENOSYS.
>>>> 
>>>> That doesn't really help broken code continue to work reliably, as
>>>> upgrading libc will just pull in the new syscall for a binary that's
>>>> expecting the broken behaviour, unless you do symbol versioning, but
>>>> then it'll just break when you next recompile the code, and there's no
>>>> way for that to be diagnosed given the *application* has to define the
>>>> type. But given it's application-defined I really struggle to see how
>>>> any code out there is actually expecting the current x32 behaviour as
>>>> you'd have to go really out of your way to find out that x32 is broken
>>>> and needs __kernel_long_t. I don't think there's any way around just
>>>> technically breaking ABI whilst likely really fixing ABI in 99.999% of
>>>> cases (maybe 100%).
>>> 
>>> I'm not opposed to "breaking ABI" here because the current syscall
>>> doesn't work unless someone wrote bogus x32-specific code to work
>>> around it being wrong. I don't particularly want to preserve any of
>>> the current behavior.
>>> 
>>> What I am somewhat opposed to is making a situation where an updated
>>> libc can't be safe against getting run on a kernel with a broken
>>> version of the syscall and silently corrupting data. I'm flexible
>>> about how avoiding tha tis achieved.
>> 
>> If we're sufficiently confident that we won't regress anything by
>> fixing the bug, I propose we do the following.  First, we commit a fix
>> that's Jessica's patch plus a fix to struct msghdr, and we mark that
>> for -stable.  Then we commit another patch that removes 'struct
>> msghdr' from uapi entirely, but we don't mark that for -stable.  If
>> people complain about the latter, we revert it.
> 
> Thinking about this more, MIPS n32 is also affected by that header. In
> fact the n32 syscalls currently do the right thing and use the compat
> implementations, so the header is currently out-of-sync with the kernel
> there*. This should be noted when committing the change to msg.h.

Never mind, it seems MIPS n32 is weird and leaves __kernel_long_t as a
normal long despite being an ILP32-on-64-bit ABI, I guess because it's
inherited from IRIX rather than being invented by the GNU world.

Jess