Re: [syzbot] [mm?] WARNING in unmap_page_range (2)

[David Hildenbrand]

On 15.11.23 23:00, Andrew Morton wrote:
> On Wed, 15 Nov 2023 05:32:19 -0800 syzbot <syzbot+7ca4b2719dc742b8d0a4@xxxxxxxxxxxxxxxxxxxxxxxxx> wrote:
>
> > Hello,
> >
> > syzbot found the following issue on:
> >
> > HEAD commit:    ac347a0655db Merge tag 'arm64-fixes' of git://git.kernel.o..
> > git tree:       upstream
> > console+strace: https://syzkaller.appspot.com/x/log.txt?x=15ff3057680000
> > kernel config:  https://syzkaller.appspot.com/x/.config?x=287570229f5c0a7c
> > dashboard link: https://syzkaller.appspot.com/bug?extid=7ca4b2719dc742b8d0a4
> > 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=162a25ff680000
> > C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=13d62338e80000
> >
> > Downloadable assets:
> > disk image: https://storage.googleapis.com/syzbot-assets/00e30e1a5133/disk-ac347a06.raw.xz
> > vmlinux: https://storage.googleapis.com/syzbot-assets/07c43bc37935/vmlinux-ac347a06.xz
> > kernel image: https://storage.googleapis.com/syzbot-assets/c6690c715398/bzImage-ac347a06.xz
> >
> > The issue was bisected to:
> >
> > commit 12f6b01a0bcbeeab8cc9305673314adb3adf80f7
> > Author: Muhammad Usama Anjum <usama.anjum@xxxxxxxxxxxxx>
> > Date:   Mon Aug 21 14:15:15 2023 +0000
> >
> >      fs/proc/task_mmu: add fast paths to get/clear PAGE_IS_WRITTEN flag
>
> Thanks.  The bisection is surprising, but the mentioned patch does
> mess with pagemap.
>
> How about we add this?
>
> From: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> Subject: mm/memory.c:zap_pte_range() print bad swap entry
> Date: Wed Nov 15 01:54:18 PM PST 2023
>
> We have a report of this WARN() triggering.  Let's print the offending
> swp_entry_t to help diagnosis.
>
> Link: https://lkml.kernel.org/r/000000000000b0e576060a30ee3b@xxxxxxxxxx
> Cc: Muhammad Usama Anjum <usama.anjum@xxxxxxxxxxxxx>
> Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> ---
>
>   mm/memory.c |    1 +
>   1 file changed, 1 insertion(+)
>
> --- a/mm/memory.c~a
> +++ a/mm/memory.c
> @@ -1521,6 +1521,7 @@ static unsigned long zap_pte_range(struc
>   				continue;
>   		} else {
>   			/* We should have covered all the swap entry types */
> +			pr_alert("unrecognized swap entry 0x%lx\n", entry.val);
>   			WARN_ON_ONCE(1);
>   		}
>   		pte_clear_not_present_full(mm, addr, pte, tlb->fullmm);
> _

I'm curious if

1) make_uffd_wp_pte() won't end up overwriting existing pte markers, for
   example, if PTE_MARKER_POISONED is set. [unrelated to this bug]

2) We get the error on arm64, which does *not* support uffd-wp. Do we
   maybe end up calling make_uffd_wp_pte() and place a pte marker, even
   though we don't have CONFIG_PTE_MARKER_UFFD_WP?


static inline bool pte_marker_entry_uffd_wp(swp_entry_t entry)
{
#ifdef CONFIG_PTE_MARKER_UFFD_WP
	return is_pte_marker_entry(entry) &&
	    (pte_marker_get(entry) & PTE_MARKER_UFFD_WP);
#else
	return false;
#endif
}

Will always return false without CONFIG_PTE_MARKER_UFFD_WP.

But make_uffd_wp_pte() might just happily place an entry. Hm.


The following might fix the problem:

diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c
index 51e0ec658457..ae1cf19918d3 100644
--- a/fs/proc/task_mmu.c
+++ b/fs/proc/task_mmu.c
@@ -1830,8 +1830,10 @@ static void make_uffd_wp_pte(struct 
vm_area_struct *vma,
                ptent = pte_swp_mkuffd_wp(ptent);
                set_pte_at(vma->vm_mm, addr, pte, ptent);
        } else {
+#ifdef CONFIG_PTE_MARKER_UFFD_WP
                set_pte_at(vma->vm_mm, addr, pte,
                           make_pte_marker(PTE_MARKER_UFFD_WP));
+#endif
        }
 }


But I am *pretty* sure that that whole machinery should be fenced off. 
It does make 0 sense to mess with uffd-wp if there is no uffd-wp support.

--
Cheers,

David / dhildenb