From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753101AbeBARMM (ORCPT ); Thu, 1 Feb 2018 12:12:12 -0500 Received: from mx1.redhat.com ([209.132.183.28]:48473 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753083AbeBARMK (ORCPT ); Thu, 1 Feb 2018 12:12:10 -0500 Subject: Re: [PATCH] KVM/x86: remove WARN_ON() for when vm_munmap() fails To: =?UTF-8?B?UmFkaW0gS3LEjW3DocWZ?= , Eric Biggers Cc: kvm@vger.kernel.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org, syzkaller-bugs@googlegroups.com, Eric Biggers References: <001a1141c71c13f559055d1b28eb@google.com> <20180201013021.151884-1-ebiggers3@gmail.com> <20180201153310.GD31080@flask> From: Paolo Bonzini Message-ID: <584ef475-21cc-9ef5-8ac9-d6b00e93134e@redhat.com> Date: Thu, 1 Feb 2018 12:12:00 -0500 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.5.2 MIME-Version: 1.0 In-Reply-To: <20180201153310.GD31080@flask> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 8bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 01/02/2018 10:33, Radim Krčmář wrote: > 2018-01-31 17:30-0800, Eric Biggers: >> From: Eric Biggers >> >> On x86, special KVM memslots such as the TSS region have anonymous >> memory mappings created on behalf of userspace, and these mappings are >> removed when the VM is destroyed. >> >> It is however possible for removing these mappings via vm_munmap() to >> fail. This can most easily happen if the thread receives SIGKILL while >> it's waiting to acquire ->mmap_sem. This triggers the 'WARN_ON(r < 0)' >> in __x86_set_memory_region(). syzkaller was able to hit this, using >> 'exit()' to send the SIGKILL. Note that while the vm_munmap() failure >> results in the mapping not being removed immediately, it is not leaked >> forever but rather will be freed when the process exits. >> >> It's not really possible to handle this failure properly, so almost > > We could check "r < 0 && r != -EINTR" to get rid of the easily > triggerable warning. Considering that vm_munmap uses down_write_killable, that would be preferrable I think. Paolo >> every other caller of vm_munmap() doesn't check the return value. It's >> a limitation of having the kernel manage these mappings rather than >> userspace. >> >> So just remove the WARN_ON() so that users can't spam the kernel log >> with this warning. >> >> Fixes: f0d648bdf0a5 ("KVM: x86: map/unmap private slots in __x86_set_memory_region") >> Reported-by: syzbot >> Signed-off-by: Eric Biggers >> --- > > Removing it altogether doesn't sound that bad, though ... > Queued, thanks. > >> arch/x86/kvm/x86.c | 6 ++---- >> 1 file changed, 2 insertions(+), 4 deletions(-) >> >> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c >> index c53298dfbf50..53b57f18baec 100644 >> --- a/arch/x86/kvm/x86.c >> +++ b/arch/x86/kvm/x86.c >> @@ -8272,10 +8272,8 @@ int __x86_set_memory_region(struct kvm *kvm, int id, gpa_t gpa, u32 size) >> return r; >> } >> >> - if (!size) { >> - r = vm_munmap(old.userspace_addr, old.npages * PAGE_SIZE); >> - WARN_ON(r < 0); >> - } >> + if (!size) >> + vm_munmap(old.userspace_addr, old.npages * PAGE_SIZE); >> >> return 0; >> } >> -- >> 2.16.0.rc1.238.g530d649a79-goog >>