Re: [PATCH for-4.18] x86/time: Fix UBSAN failure in __update_vcpu_system_time()

[Jan Beulich <jbeulich@xxxxxxxx>]

On 01.11.2023 21:37, Andrew Cooper wrote:
> As reported:
> 
>   (XEN) 
> ================================================================================
>   (XEN) UBSAN: Undefined behaviour in arch/x86/time.c:1542:32
>   (XEN) member access within null pointer of type 'union vcpu_info_t'
>   (XEN) ----[ Xen-4.19-unstable  x86_64  debug=y ubsan=y  Not tainted ]----
>   ...
>   (XEN) Xen call trace:
>   (XEN)    [<ffff82d040345036>] R common/ubsan/ubsan.c#ubsan_epilogue+0xa/0xd2
>   (XEN)    [<ffff82d0403456e8>] F __ubsan_handle_type_mismatch+0x133/0x49b
>   (XEN)    [<ffff82d040345b4a>] F __ubsan_handle_type_mismatch_v1+0xfa/0xfc
>   (XEN)    [<ffff82d040623356>] F 
> arch/x86/time.c#__update_vcpu_system_time+0x212/0x30f
>   (XEN)    [<ffff82d040623461>] F update_vcpu_system_time+0xe/0x10
>   (XEN)    [<ffff82d04062389d>] F 
> arch/x86/time.c#local_time_calibration+0x1f7/0x523
>   (XEN)    [<ffff82d0402a64b5>] F common/softirq.c#__do_softirq+0x1f4/0x31a
>   (XEN)    [<ffff82d0402a67ad>] F do_softirq+0x13/0x15
>   (XEN)    [<ffff82d0405a95dc>] F arch/x86/domain.c#idle_loop+0x2e0/0x367
>   (XEN)
>   (XEN) 
> ================================================================================
> 
> It is not valid to derive a pointer from vcpu_info() prior to checking that
> the underlying map pointer is good.
> 
> Reorder actions so the NULL pointer check is first.
> 
> Fixes: 20279afd7323 ("x86: split populating of struct vcpu_time_info into a 
> separate function")
> Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>

In the interest of silencing the checker
Acked-by: Jan Beulich <jbeulich@xxxxxxxx>
However, ...

> 4.18 blocker, or we'll need to issue an XSA/CVE.

... I dare to disagree (or to at least be uncertain) here, and I further
dare to question correctness of the checker (which suggests that it might
be helpful to point out in the description which version of the compiler
it was). Quoting part of a footnote of the respective part of the C99
spec, describing the unary * operator:

"Thus, &*E is equivalent to E (even if E is a null pointer), ..."

This imo leaves it at best ambiguous whether there's any actual UB here.

Furthermore, if you deem this XSA-worthy despite the generated code not
resulting in any real misbehavior, code patterns like
(found in free_heap_pages())

            struct page_info *predecessor = pg - mask;

            /* Merge with predecessor block? */
            if ( !mfn_valid(page_to_mfn(predecessor)) ||

or (found in get_page_from_l1e())

    struct page_info *page = mfn_to_page(_mfn(mfn));
    ...
    valid = mfn_valid(_mfn(mfn));

    if ( !valid ||

would be in the same class (access outside of array bounds), just that the
checker cannot spot those without producing false positives (simply because
it doesn't know frame_table[]'s bounds). We're fully aware of the existence
of such code, and we've decided to - if at all - only eliminate such cases
(slowly) as respective code is touched anyway.

Jan

PS: argo.c:find_ring_mfn() has

    *mfn = page_to_mfn(page);
    if ( !mfn_valid(*mfn) )
        ret = -EINVAL;

which, while not at risk of yielding any UB, is an entirely pointless
check (as the underlying assumption needs to be that the struct
page_info * must already be a valid pointer, or else _its_ obtaining was
possibly UB). Having such checks in the code is at best misleading.