| Message ID | 20231101203737.3441425-1-andrew.cooper3@citrix.com (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
| Series | [for-4.18] x86/time: Fix UBSAN failure in __update_vcpu_system_time() | expand |
Hi Andrew, > On Nov 2, 2023, at 04:37, Andrew Cooper <andrew.cooper3@citrix.com> 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@citrix.com> > --- > CC: Jan Beulich <JBeulich@suse.com> > CC: Roger Pau Monné <roger.pau@citrix.com> > CC: Wei Liu <wl@xen.org> > CC: Henry Wang <Henry.Wang@arm.com> > > 4.18 blocker, or we'll need to issue an XSA/CVE. Release-acked-by: Henry Wang <Henry.Wang@arm.com> Kind regards, Henry
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@citrix.com> In the interest of silencing the checker Acked-by: Jan Beulich <jbeulich@suse.com> 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.
On 02/11/2023 8:33 am, Jan Beulich wrote: > 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@citrix.com> > In the interest of silencing the checker > Acked-by: Jan Beulich <jbeulich@suse.com> Thanks. > 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. I'd argue that this isn't relevant. What matters how the compiler interprets the spec, and in this case GCC is telling us explicitly that it does consider this to be UB. Whether there is a practical consequence is a different question, but ... > Furthermore, if you deem this XSA-worthy despite the generated code not > resulting in any real misbehavior ... we've issued XSAs for this class of issue before. XSA-328 is the one I specifically remember, but I'm sure we've done others too. In this case, an unprivileged guest can control the NULL-ness here, so if there's a practical consequence from the compiler then the guest can definitely tickle that consequence. Alternatively, the security team could decide to change it's stance on this class of issues. > , 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. I don't agree with describing these as the same class. NULL deference is different to OoB, even if they overlap from an underlying mechanics point of view. Nevertheless, I've raised that "valid" pattern with the security team before, and I would certainly prefer to express it differently. But neither of them trigger UBSAN. GCC can't see any wiggle room to potentially optimise, and I expect it's because __mfn_valid() is in an external call. If we had working LTO, I'd be interested to see that alters the UBSAN determination or not. > > 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. > Yeah, that does looks wonky. Luckily ARGO is still experimental. Although given the topics on the call today, I wonder whether this is really appropriate, seeing as ARGO underpins the security model on OpenXT... I did have some plans (for copious free time, of course) to investigate coccinelle and use it like Linux does, in order to spot and correct known anti-patterns. ~Andrew
On 02.11.2023 20:05, Andrew Cooper wrote: > On 02/11/2023 8:33 am, Jan Beulich wrote: >> Furthermore, if you deem this XSA-worthy despite the generated code not >> resulting in any real misbehavior > > ... we've issued XSAs for this class of issue before. XSA-328 is the > one I specifically remember, but I'm sure we've done others too. > > In this case, an unprivileged guest can control the NULL-ness here, so > if there's a practical consequence from the compiler then the guest can > definitely tickle that consequence. > > Alternatively, the security team could decide to change it's stance on > this class of issues. > >> , 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. > > I don't agree with describing these as the same class. NULL deference > is different to OoB, even if they overlap from an underlying mechanics > point of view. > > Nevertheless, I've raised that "valid" pattern with the security team > before, and I would certainly prefer to express it differently. > > But neither of them trigger UBSAN. GCC can't see any wiggle room to > potentially optimise, and I expect it's because __mfn_valid() is in an > external call. > > If we had working LTO, I'd be interested to see that alters the UBSAN > determination or not. I'm not convinced it takes as much as working LTO. With PDX_COMPRESSION=n I question __mfn_valid() needing to be an out-of-line function. Converting it (back) to an inline one would better not come with the risk of breaking certain use sites. Jan
diff --git a/xen/arch/x86/time.c b/xen/arch/x86/time.c index d0b0986509b2..6d33edd0addc 100644 --- a/xen/arch/x86/time.c +++ b/xen/arch/x86/time.c @@ -1539,12 +1539,14 @@ static void collect_time_info(const struct vcpu *v, static void __update_vcpu_system_time(struct vcpu *v, int force) { - struct vcpu_time_info *u = &vcpu_info(v, time), _u; + struct vcpu_time_info *u, _u; const struct domain *d = v->domain; if ( !v->vcpu_info_area.map ) return; + u = &vcpu_info(v, time); + collect_time_info(v, &_u); /* Don't bother unless timestamp record has changed or we are forced. */
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@citrix.com> --- CC: Jan Beulich <JBeulich@suse.com> CC: Roger Pau Monné <roger.pau@citrix.com> CC: Wei Liu <wl@xen.org> CC: Henry Wang <Henry.Wang@arm.com> 4.18 blocker, or we'll need to issue an XSA/CVE. --- xen/arch/x86/time.c | 4 +++- 1 file changed, 3 insertions(+), 1 deletion(-) base-commit: 7befef87cc9b1bb8ca15d866ce1ecd9165ccb58c