From mboxrd@z Thu Jan 1 00:00:00 1970 From: Xiao Guangrong Subject: Re: index-out-of-range ubsan warnings Date: Wed, 24 Feb 2016 14:23:19 +0800 Message-ID: <56CD4C57.5030402@linux.intel.com> References: <20160223092643.GB4396@kmu-ThinkPad-X230> <56CC2F7B.1060607@siemens.com> <56CC37F3.3060308@linux.intel.com> <56CC5CC3.5060208@redhat.com> Mime-Version: 1.0 Content-Type: text/plain; charset=utf-8; format=flowed Content-Transfer-Encoding: 7bit Cc: Marcelo Tosatti , Sasha Levin To: Paolo Bonzini , Jan Kiszka , Mike Krinkin , kvm@vger.kernel.org Return-path: Received: from mga04.intel.com ([192.55.52.120]:44341 "EHLO mga04.intel.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752038AbcBXGbh (ORCPT ); Wed, 24 Feb 2016 01:31:37 -0500 In-Reply-To: <56CC5CC3.5060208@redhat.com> Sender: kvm-owner@vger.kernel.org List-ID: On 02/23/2016 09:21 PM, Paolo Bonzini wrote: >> >> +#define INVALID_INDEX (-1) >> + >> static int mmu_unsync_walk(struct kvm_mmu_page *sp, >> struct kvm_mmu_pages *pvec) >> { >> if (!sp->unsync_children) >> return 0; >> >> - mmu_pages_add(pvec, sp, 0); >> + /* >> + * do not count the index in the parent of the sp we're >> + * walking start from. >> + */ >> + mmu_pages_add(pvec, sp, INVALID_INDEX); >> return __mmu_unsync_walk(sp, pvec); >> } >> >> @@ -1980,8 +1986,11 @@ static int mmu_pages_next(struct kvm_mmu_pages *pvec, >> return n; >> } >> >> - parents->parent[sp->role.level-2] = sp; >> - parents->idx[sp->role.level-1] = pvec->page[n].idx; >> + parents->parent[sp->role.level - 2] = sp; >> + >> + /* skip setting idex of the sp we start from. */ >> + if (pvec->page[n].idx != INVALID_INDEX) >> + parents->idx[sp->role.level - 1] = pvec->page[n].idx; > > So far so good. > >> } >> >> return n; >> @@ -1999,6 +2008,7 @@ static void mmu_pages_clear_parents(struct mmu_page_path *parents) >> if (!sp) >> return; >> >> + WARN_ON(idx != INVALID_INDEX); > > Shouldn't this warn if idx is *equal* to INVALID_INDEX? Yes, indeed. It seems i am spending so much time on QEMU and mixed it with assert() :( > >> clear_unsync_child_bit(sp, idx); >> level++; >> } while (level < PT64_ROOT_LEVEL-1 && !sp->unsync_children); >> @@ -2008,7 +2018,7 @@ static void kvm_mmu_pages_init(struct kvm_mmu_page *parent, >> struct mmu_page_path *parents, >> struct kvm_mmu_pages *pvec) >> { >> - parents->parent[parent->role.level-1] = NULL; >> + parents->parent[parent->role.level - 2] = NULL; > > This is meant to stop mmu_pages_clear_parents _after_ it has > processed sp, so the "-1" is correct. The right fix would be: > > if (parent->role.level < PT64_ROOT_LEVEL-1) > parents->parent[parent->role.level - 1] = NULL; > it is okay as mmu_pages_next() will refill the highest level. > and no one has noticed it so far because if the bug hits you just write > over the beginning of ->idx (which so far is uninitialized) and nothing > bad happens. > > I'm thinking of the following (untested) patch instead: > > -------- 8< -------------- > From: Paolo Bonzini > Subject: [PATCH] KVM: MMU: Fix ubsan warnings > > kvm_mmu_pages_init is doing some really yucky stuff. It is setting > up a sentinel for mmu_page_clear_parents; however, because of a) the > way levels are numbered starting from 1 and b) the way mmu_page_path > sizes its arrays with PT64_ROOT_LEVEL-1 elements, the access can be > out of bounds. This is harmless because the code overwrites up to the > first two elements of parents->idx and these are initialized, and > because the sentinel is not needed in this case---mmu_page_clear_parents > exits anyway when it gets to the end of the array. However ubsan > complains, and everyone else should too. > > This fix does three things. First it makes the mmu_page_path arrays > PT64_ROOT_LEVEL elements in size, so that we can write to them without > checking the level in advance. Second it disintegrates kvm_mmu_pages_init > between mmu_unsync_walk (which resets struct kvm_mmu_pages) and > mmu_pages_next (which unconditionally places a NULL sentinel at the > end of the current path). This is okay because the mmu_page_path is > only used in mmu_pages_clear_parents; mmu_pages_clear_parents itself > is called within a for_each_sp iterator, and hence always after a > call to mmu_pages_next. Third it changes mmu_pages_clear_parents to > just use the sentinel to stop iteration, without checking the bounds > on level. > > Reported-by: Sasha Levin > Reported-by: Mike Krinkin > Signed-off-by: Paolo Bonzini > > diff --git a/arch/x86/kvm/mmu.c b/arch/x86/kvm/mmu.c > index 33cc9f3f5b02..f4f5e7ca14dd 100644 > --- a/arch/x86/kvm/mmu.c > +++ b/arch/x86/kvm/mmu.c > @@ -1843,6 +1843,7 @@ static int __mmu_unsync_walk(struct kvm_mmu_page *sp, > static int mmu_unsync_walk(struct kvm_mmu_page *sp, > struct kvm_mmu_pages *pvec) > { > + pvec->nr = 0; > if (!sp->unsync_children) > return 0; > > @@ -1956,8 +1957,8 @@ static void kvm_sync_pages(struct kvm_vcpu *vcpu, gfn_t gfn) > } > > struct mmu_page_path { > - struct kvm_mmu_page *parent[PT64_ROOT_LEVEL-1]; > - unsigned int idx[PT64_ROOT_LEVEL-1]; > + struct kvm_mmu_page *parent[PT64_ROOT_LEVEL]; > + unsigned int idx[PT64_ROOT_LEVEL]; So that it wastes 1 entry as we only use it to save parent sp. > }; > > #define for_each_sp(pvec, sp, parents, i) \ > @@ -1971,19 +1972,21 @@ static int mmu_pages_next(struct kvm_mmu_pages *pvec, > int i) > { > int n; > + int level = PT_PAGE_TABLE_LEVEL; > > for (n = i+1; n < pvec->nr; n++) { > struct kvm_mmu_page *sp = pvec->page[n].sp; > > - if (sp->role.level == PT_PAGE_TABLE_LEVEL) { > - parents->idx[0] = pvec->page[n].idx; > - return n; > - } > + level = sp->role.level; > + parents->idx[level-1] = pvec->page[n].idx; > > - parents->parent[sp->role.level-2] = sp; > - parents->idx[sp->role.level-1] = pvec->page[n].idx; > + if (level == PT_PAGE_TABLE_LEVEL) > + break; > + > + parents->parent[level-2] = sp; > } > > + parents->parent[level-1] = NULL; Why? Considering we have already got a unsync page, so break the loop and reach here. It results parents->parent[ 0 ] = NULL and see below...... > return n; > } > > @@ -1994,22 +1996,13 @@ static void mmu_pages_clear_parents(struct mmu_page_path *parents) > > do { > unsigned int idx = parents->idx[level]; > - > sp = parents->parent[level]; > if (!sp) > return; So the first level is NULL, it stops clearing unsync_children. > > clear_unsync_child_bit(sp, idx); > level++; > - } while (level < PT64_ROOT_LEVEL-1 && !sp->unsync_children); > -} > - > -static void kvm_mmu_pages_init(struct kvm_mmu_page *parent, > - struct mmu_page_path *parents, > - struct kvm_mmu_pages *pvec) > -{ > - parents->parent[parent->role.level-1] = NULL; > - pvec->nr = 0; > + } while (!sp->unsync_children); > } > > static void mmu_sync_children(struct kvm_vcpu *vcpu, > @@ -2021,7 +2014,6 @@ static void mmu_sync_children(struct kvm_vcpu *vcpu, > struct kvm_mmu_pages pages; > LIST_HEAD(invalid_list); > > - kvm_mmu_pages_init(parent, &parents, &pages); > while (mmu_unsync_walk(parent, &pages)) { > bool protected = false; > > @@ -2037,7 +2029,6 @@ static void mmu_sync_children(struct kvm_vcpu *vcpu, > } > kvm_mmu_commit_zap_page(vcpu->kvm, &invalid_list); > cond_resched_lock(&vcpu->kvm->mmu_lock); > - kvm_mmu_pages_init(parent, &parents, &pages); > } > } > > @@ -2269,7 +2260,6 @@ static int mmu_zap_unsync_children(struct kvm *kvm, > if (parent->role.level == PT_PAGE_TABLE_LEVEL) > return 0; > > - kvm_mmu_pages_init(parent, &parents, &pages); > while (mmu_unsync_walk(parent, &pages)) { > struct kvm_mmu_page *sp; > > @@ -2278,7 +2268,6 @@ static int mmu_zap_unsync_children(struct kvm *kvm, > mmu_pages_clear_parents(&parents); > zapped++; > } > - kvm_mmu_pages_init(parent, &parents, &pages); > } > > return zapped; > Each time i looked into these functions, it costs my several hours. The trick is that the item, sp and idx, saved to kvm_mmu_pages is the sp and the index in the _parent_ level. How about clarify/simplify it as this patch: diff --git a/arch/x86/kvm/mmu.c b/arch/x86/kvm/mmu.c index 57cf30b..14b2800 100644 --- a/arch/x86/kvm/mmu.c +++ b/arch/x86/kvm/mmu.c @@ -1848,7 +1848,7 @@ static int __mmu_unsync_walk(struct kvm_mmu_page *sp, child = page_header(ent & PT64_BASE_ADDR_MASK); if (child->unsync_children) { - if (mmu_pages_add(pvec, child, i)) + if (mmu_pages_add(pvec, sp, i)) return -ENOSPC; ret = __mmu_unsync_walk(child, pvec); @@ -1861,7 +1861,14 @@ static int __mmu_unsync_walk(struct kvm_mmu_page *sp, return ret; } else if (child->unsync) { nr_unsync_leaf++; - if (mmu_pages_add(pvec, child, i)) + if (mmu_pages_add(pvec, sp, i)) + return -ENOSPC; + + /* + * the unsync is on the last level so its 'idx' is + * useless, we set it to 0 to catch potential bugs. + */ + if (mmu_pages_add(pvec, child, 0)) return -ENOSPC; } else clear_unsync_child_bit(sp, i); @@ -1876,7 +1883,6 @@ static int mmu_unsync_walk(struct kvm_mmu_page *sp, if (!sp->unsync_children) return 0; - mmu_pages_add(pvec, sp, 0); return __mmu_unsync_walk(sp, pvec); } @@ -2002,16 +2008,18 @@ static int mmu_pages_next(struct kvm_mmu_pages *pvec, { int n; - for (n = i+1; n < pvec->nr; n++) { + for (n = i + 1; n < pvec->nr; n++) { struct kvm_mmu_page *sp = pvec->page[n].sp; + unsigned int idx = pvec->page[n].idx; if (sp->role.level == PT_PAGE_TABLE_LEVEL) { - parents->idx[0] = pvec->page[n].idx; + /* we always set the idex as 0 for the unsync page. */ + WARN_ON(idx != 0); return n; } - parents->parent[sp->role.level-2] = sp; - parents->idx[sp->role.level-1] = pvec->page[n].idx; + parents->parent[sp->role.level - 2] = sp; + parents->idx[sp->role.level - 2] = idx; } return n; @@ -2031,14 +2039,18 @@ static void mmu_pages_clear_parents(struct mmu_page_path *parents) clear_unsync_child_bit(sp, idx); level++; - } while (level < PT64_ROOT_LEVEL-1 && !sp->unsync_children); + } while (level < PT64_ROOT_LEVEL - 1 && !sp->unsync_children); } static void kvm_mmu_pages_init(struct kvm_mmu_page *parent, struct mmu_page_path *parents, struct kvm_mmu_pages *pvec) { - parents->parent[parent->role.level-1] = NULL; + /* + * set the highest level to NULL to stop mmu_pages_clear_parents() + * handing it on 32 bit guest. + */ + parents->parent[parent->role.level - 2] = NULL; pvec->nr = 0; }