| Message ID | 20220627161123.1386853-1-pgonda@google.com (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
| Series | KVM: SEV: Clear the pages pointer in sev_unpin_memory | expand |
On Mon, Jun 27, 2022, Peter Gonda wrote: > Clear to the @pages array pointer in sev_unpin_memory to avoid leaving a > dangling pointer to invalid memory. > > Signed-off-by: Peter Gonda <pgonda@google.com> > Cc: Greg Thelen <gthelen@google.com> > Cc: Paolo Bonzini <pbonzini@redhat.com> > Cc: Sean Christopherson <seanjc@google.com> > Cc: Tom Lendacky <thomas.lendacky@amd.com> > Cc: kvm@vger.kernel.org > Cc: linux-kernel@vger.kernel.org > --- > arch/x86/kvm/svm/sev.c | 1 + > 1 file changed, 1 insertion(+) > > diff --git a/arch/x86/kvm/svm/sev.c b/arch/x86/kvm/svm/sev.c > index 309bcdb2f929..485ad86c01c6 100644 > --- a/arch/x86/kvm/svm/sev.c > +++ b/arch/x86/kvm/svm/sev.c > @@ -452,6 +452,7 @@ static void sev_unpin_memory(struct kvm *kvm, struct page **pages, > unpin_user_pages(pages, npages); > kvfree(pages); > sev->pages_locked -= npages; > + *pages = NULL; Would this have helped detect a real bug? I generally like cleaning up, but this leaves things in a somewhat inconsistent state, e.g. when unpinning a kvm_enc_region, pages will be NULL but npages will be non-zero. It's somewhat moot because the region is immediately freed in that case, but that begs the question of what real benefit this provides. sev_dbg_crypt() is the only flow where there's much danger of a use-after-free. > } > > static void sev_clflush_pages(struct page *pages[], unsigned long npages) > -- > 2.37.0.rc0.161.g10f37bed90-goog >
On Mon, Jun 27, 2022 at 12:10 PM Sean Christopherson <seanjc@google.com> wrote: > > On Mon, Jun 27, 2022, Peter Gonda wrote: > > Clear to the @pages array pointer in sev_unpin_memory to avoid leaving a > > dangling pointer to invalid memory. > > > > Signed-off-by: Peter Gonda <pgonda@google.com> > > Cc: Greg Thelen <gthelen@google.com> > > Cc: Paolo Bonzini <pbonzini@redhat.com> > > Cc: Sean Christopherson <seanjc@google.com> > > Cc: Tom Lendacky <thomas.lendacky@amd.com> > > Cc: kvm@vger.kernel.org > > Cc: linux-kernel@vger.kernel.org > > --- > > arch/x86/kvm/svm/sev.c | 1 + > > 1 file changed, 1 insertion(+) > > > > diff --git a/arch/x86/kvm/svm/sev.c b/arch/x86/kvm/svm/sev.c > > index 309bcdb2f929..485ad86c01c6 100644 > > --- a/arch/x86/kvm/svm/sev.c > > +++ b/arch/x86/kvm/svm/sev.c > > @@ -452,6 +452,7 @@ static void sev_unpin_memory(struct kvm *kvm, struct page **pages, > > unpin_user_pages(pages, npages); > > kvfree(pages); > > sev->pages_locked -= npages; > > + *pages = NULL; > > Would this have helped detect a real bug? I generally like cleaning up, but this > leaves things in a somewhat inconsistent state, e.g. when unpinning a kvm_enc_region, > pages will be NULL but npages will be non-zero. It's somewhat moot because the > region is immediately freed in that case, but that begs the question of what real > benefit this provides. sev_dbg_crypt() is the only flow where there's much danger > of a use-after-free. > No strong opinion here, I just thought since this is a helper that takes a 'struct page **pages" we may as well clear this. While there are no bugs caught now if someone were to introduce something wrong this would make it more clear. We could update sev_unpin_memory() to take a int *npages so it can be cleared as well. Since kvm_enc_region describes a region with u64 instead of pointers that seemed "safer" give you'd have to cast them to dereference. Totally fine with the NACK of course. =]
On Mon, Jun 27, 2022 at 1:37 PM Sean Christopherson <seanjc@google.com> wrote: > > On Mon, Jun 27, 2022, Peter Gonda wrote: > > Clear to the @pages array pointer in sev_unpin_memory to avoid leaving a > > dangling pointer to invalid memory. > > > > Signed-off-by: Peter Gonda <pgonda@google.com> > > Cc: Greg Thelen <gthelen@google.com> > > Cc: Paolo Bonzini <pbonzini@redhat.com> > > Cc: Sean Christopherson <seanjc@google.com> > > Cc: Tom Lendacky <thomas.lendacky@amd.com> > > Cc: kvm@vger.kernel.org > > Cc: linux-kernel@vger.kernel.org > > --- > > arch/x86/kvm/svm/sev.c | 1 + > > 1 file changed, 1 insertion(+) > > > > diff --git a/arch/x86/kvm/svm/sev.c b/arch/x86/kvm/svm/sev.c > > index 309bcdb2f929..485ad86c01c6 100644 > > --- a/arch/x86/kvm/svm/sev.c > > +++ b/arch/x86/kvm/svm/sev.c > > @@ -452,6 +452,7 @@ static void sev_unpin_memory(struct kvm *kvm, struct page **pages, > > unpin_user_pages(pages, npages); > > kvfree(pages); > > sev->pages_locked -= npages; > > + *pages = NULL; > > Would this have helped detect a real bug? I generally like cleaning up, but this > leaves things in a somewhat inconsistent state, e.g. when unpinning a kvm_enc_region, > pages will be NULL but npages will be non-zero. It's somewhat moot because the > region is immediately freed in that case, but that begs the question of what real > benefit this provides. sev_dbg_crypt() is the only flow where there's much danger > of a use-after-free. +1 We could send thousands of patches like this. But I think none of them would be directly helpful. Sometimes, things could be even worse, i.e., imagine when those NULL pointers got deferenced, they become BUG_ON(true). If there are bugs related with those pointers, better to fix those bugs instead. Thanks. -Mingwei
diff --git a/arch/x86/kvm/svm/sev.c b/arch/x86/kvm/svm/sev.c index 309bcdb2f929..485ad86c01c6 100644 --- a/arch/x86/kvm/svm/sev.c +++ b/arch/x86/kvm/svm/sev.c @@ -452,6 +452,7 @@ static void sev_unpin_memory(struct kvm *kvm, struct page **pages, unpin_user_pages(pages, npages); kvfree(pages); sev->pages_locked -= npages; + *pages = NULL; } static void sev_clflush_pages(struct page *pages[], unsigned long npages)
Clear to the @pages array pointer in sev_unpin_memory to avoid leaving a dangling pointer to invalid memory. Signed-off-by: Peter Gonda <pgonda@google.com> Cc: Greg Thelen <gthelen@google.com> Cc: Paolo Bonzini <pbonzini@redhat.com> Cc: Sean Christopherson <seanjc@google.com> Cc: Tom Lendacky <thomas.lendacky@amd.com> Cc: kvm@vger.kernel.org Cc: linux-kernel@vger.kernel.org --- arch/x86/kvm/svm/sev.c | 1 + 1 file changed, 1 insertion(+)