[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index] Re: [PATCH for-4.18] x86/time: Fix UBSAN failure in __update_vcpu_system_time()
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.
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