Re: Undefined behavior in libxenvchan

["Teddy Astie" <teddy.astie@vates.tech>]

Le 15/12/2025 à 00:08, Demi Marie Obenour a écrit :
> On 12/14/25 17:50, Andrew Cooper wrote:
>> On 14/12/2025 7:09 pm, Demi Marie Obenour wrote:
>>> I noticed that libxenvchan has undefined behavior: it passes pointers
>>> to guest memory to memcpy() even though they can be concurrently
>>> changed.
>>>
>>> Would it make sense to reuse some of Xen's copy_from_guest() code
>>> instead?  There might be a licensing problem (GPL vs LGPL), though.
>>> I think the only approach that isn't UB and has decent performance
>>> is to do the whole copy in assembly.
>>
>> memcpy() is well defined.
>
> Rich Felker wrote otherwise on the musl mailing list.  Specifically,
> it is undefined behavior if the data is changed while memcpy() is
> accessing it, either for reading or for writing.
>
>> The problem is the potential for creating TOCTOU races if suitable
>> barriers aren't used, due to the compiler being able to optimise through
>> memcpy().
>
> The concern here is about races in memcpy() itself.
>
>> Xen's copy to/from guest are not appropriate in userspace.  They're
>> guarding against pagefaults and address ranges not belonging to the
>> target context.
>>
>> If more compiler/smp barriers are needed, then that's the appropriate fix.
>
> Rich Felker suggested to use an open-coded memcpy() that used volatile
> accesses.

Do you mean that if a libc uses something like this as a memcpy.

void *memcpy(
     void *restrict dest_str,
     const void *restrict src_str,
     size_t n)
{
     const char *src = src_str;
     char *dest = dest_str;
     size_t i = 0;

     while (i < n)
     {
         dest[i] = src[i];
         i++;
     }

     return dest_str;
}

that the compiler is free to optimize inside this function in ways that
conflict with the "actual volatile-ness" of dest/src ?

Anything said regarding regarding TOCTOU can also happens from within
the memcpy (even though most memcpy() implementations and what compiler
would emit here is very unlikely unaffected by this).

Unfortunately, there is no available volatile memcpy in C, LLVM has a
volatile memcpy, but not usable from C, Rust exposes it through
"unstable" volatile_copy_non_overlapping, which got discussed in [1],
there is also something regarding "atomic memcpy" [2], but I don't know
the exact status of all this.

[1]
https://doc.rust-lang.org/stable/core/intrinsics/fn.volatile_copy_nonoverlapping_memory.html

[2] https://github.com/rust-lang/rfcs/pull/3301

Teddy


--
Teddy Astie | Vates XCP-ng Developer

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