On Wed, Nov 15, 2023 at 4:53 PM Peter Xu <peterx@xxxxxxxxxx> wrote: > > Hi, Andrei, Muhammad, > > I had a look (as it triggered the guard I added before..), and I think I > know what happened. So far I think it's a question to the new ioctl() > interface, which I'd like to double check with you all. See below. > > On Wed, Nov 15, 2023 at 01:07:18PM -0800, Andrei Vagin wrote: > > Cc: Peter and Muhammad > > > > On Wed, Nov 15, 2023 at 6:41 AM syzbot > > <syzbot+e94c5aaf7890901ebf9b@xxxxxxxxxxxxxxxxxxxxxxxxx> wrote: > > > > > > Hello, > > > > > > syzbot found the following issue on: > > > > > > HEAD commit: c42d9eeef8e5 Merge tag 'hardening-v6.7-rc2' of git://git.k.. > > > git tree: upstream > > > console+strace: https://syzkaller.appspot.com/x/log.txt?x=13626650e80000 > > > kernel config: https://syzkaller.appspot.com/x/.config?x=84217b7fc4acdc59 > > > dashboard link: https://syzkaller.appspot.com/bug?extid=e94c5aaf7890901ebf9b > > > compiler: gcc (Debian 12.2.0-14) 12.2.0, GNU ld (GNU Binutils for Debian) 2.40 > > > syz repro: https://syzkaller.appspot.com/x/repro.syz?x=15d73be0e80000 > > > C reproducer: https://syzkaller.appspot.com/x/repro.c?x=13670da8e80000 > > > > > > Downloadable assets: > > > disk image: https://storage.googleapis.com/syzbot-assets/a595d90eb9af/disk-c42d9eee.raw.xz > > > vmlinux: https://storage.googleapis.com/syzbot-assets/c1e726fedb94/vmlinux-c42d9eee.xz > > > kernel image: https://storage.googleapis.com/syzbot-assets/cb43ae262d09/bzImage-c42d9eee.xz > > > > > > IMPORTANT: if you fix the issue, please add the following tag to the commit: > > > Reported-by: syzbot+e94c5aaf7890901ebf9b@xxxxxxxxxxxxxxxxxxxxxxxxx > > > > > > ------------[ cut here ]------------ > > > WARNING: CPU: 1 PID: 5071 at arch/x86/include/asm/pgtable.h:403 pte_uffd_wp arch/x86/include/asm/pgtable.h:403 [inline] > > This is the guard I added to detect writable bit set even if uffd-wp bit is > not yet cleared. It means something obviously wrong happened. > > Here afaict the wrong thing is ioctl(PAGEMAP_SCAN) allows applying uffd-wp > bit to VMA that is not even registered with userfault. Then what happened > is when the page is written, do_wp_page() will try to reuse the anonymous > page with the uffd-wp bit set, set W bit on top of it. Thank you for looking at this. > > Below change works for me: > > ===8<=== > diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c > index ef2eb12906da..8a2500fa4580 100644 > --- a/fs/proc/task_mmu.c > +++ b/fs/proc/task_mmu.c > @@ -1987,6 +1987,12 @@ static int pagemap_scan_test_walk(unsigned long start, unsigned long end, > vma_category |= PAGE_IS_WPALLOWED; > else if (p->arg.flags & PM_SCAN_CHECK_WPASYNC) > return -EPERM; > + else > + /* > + * Neither has the VMA enabled WP tracking, nor does the > + * user want to explicit fail the walk. Skip the vma. > + */ > + return 1; In this case, I think we need to check the PM_SCAN_WP_MATCHING flag and skip these vma-s only if it is set. If PM_SCAN_WP_MATCHING isn't set, this ioctl returns page flags and can be used without the intention of tracking memory changes. > > if (vma->vm_flags & VM_PFNMAP) > return 1; > ===8<=== > > This is based on my reading of the pagemap scan flags: > > - Write-protect the pages. The ``PM_SCAN_WP_MATCHING`` is used to write-protect > the pages of interest. The ``PM_SCAN_CHECK_WPASYNC`` aborts the operation if > non-Async Write Protected pages are found. The ``PM_SCAN_WP_MATCHING`` can be > used with or without ``PM_SCAN_CHECK_WPASYNC``. > > If PM_SCAN_CHECK_WPASYNC is used to enforce the check, we need to skip the > vma that is not registered properly. Does it look reasonable to you? I think the idea here could be to report page flags but doesn't write-protect such pages. Thanks, Andrei
