Re: WARNING in wp_page_copy

[Magnus Karlsson <magnus.karlsson@gmail.com>]

On Tue, Dec 17, 2019 at 4:40 PM Catalin Marinas <catalin.marinas@arm.com> wrote:
>
> Hi Magnus,
>
> Thanks for investigating this. I have more questions below rather than a
> solution.
>
> On Tue, Dec 17, 2019 at 02:27:22PM +0100, Magnus Karlsson wrote:
> > On Mon, Dec 16, 2019 at 4:10 PM Magnus Karlsson
> > <magnus.karlsson@gmail.com> wrote:
> > > On Mon, Dec 16, 2019 at 4:00 PM Daniel Borkmann <daniel@iogearbox.net> wrote:
> > > >
> > > > On Sat, Dec 14, 2019 at 08:20:07AM -0800, syzbot wrote:
> > > > > syzbot has found a reproducer for the following crash on:
> > > > >
> > > > > HEAD commit:    1d1997db Revert "nfp: abm: fix memory leak in nfp_abm_u32_..
> > > > > git tree:       net-next
> > > > > console output: https://syzkaller.appspot.com/x/log.txt?x=1029f851e00000
> > > > > kernel config:  https://syzkaller.appspot.com/x/.config?x=cef1fd5032faee91
> > > > > dashboard link: https://syzkaller.appspot.com/bug?extid=9301f2f33873407d5b33
> > > > > compiler:       gcc (GCC) 9.0.0 20181231 (experimental)
> > > > > syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=119d9fb1e00000
> > > > >
> > > > > IMPORTANT: if you fix the bug, please add the following tag to the commit:
> > > > > Reported-by: syzbot+9301f2f33873407d5b33@syzkaller.appspotmail.com
> > > >
> > > > Bjorn / Magnus, given xsk below, PTAL, thanks!
> > >
> > > Thanks. I will take a look at it right away.
> > >
> > > /Magnus
> >
> > After looking through the syzcaller report, I have the following
> > hypothesis that would dearly need some comments from MM-savy people
> > out there. Syzcaller creates, using mmap, a memory area that is
>
> I guess that's not an anonymous mmap() since we don't seem to have a
> struct page for src in cow_user_page() (the WARN_ON_ONCE path). Do you
> have more information on the mmap() call?

I have this from the syzcaller logs:

mmap(&(0x7f0000001000/0x2000)=nil, 0x2000, 0xfffffe, 0x12, r8, 0x0)
getsockopt$XDP_MMAP_OFFSETS(r8, 0x11b, 0x7, &(0x7f0000001300),
&(0x7f0000000100)=0x60)

The full log can be found at:
https://syzkaller.appspot.com/x/repro.syz?x=119d9fb1e00000

Hope this helps.

> > write-only and supplies this to a getsockopt call (in this case
> > XDP_STATISTICS, but probably does not matter really) as the area where
> > it wants the values to be stored. When the getsockopt implementation
> > gets to copy_to_user() to write out the values to user space, it
> > encounters a page fault when accessing this write-only page. When
> > servicing this, it gets to the following piece of code that triggers
> > the warning that syzcaller reports:
> >
> > static inline bool cow_user_page(struct page *dst, struct page *src,
> >                                  struct vm_fault *vmf)
> > {
> > ....
> > snip
> > ....
> >        /*
> >          * This really shouldn't fail, because the page is there
> >          * in the page tables. But it might just be unreadable,
> >          * in which case we just give up and fill the result with
> >          * zeroes.
> >          */
> >         if (__copy_from_user_inatomic(kaddr, uaddr, PAGE_SIZE)) {
> >                 /*
> >                  * Give a warn in case there can be some obscure
> >                  * use-case
> >                  */
> >                 WARN_ON_ONCE(1);
> >                 clear_page(kaddr);
> >         }
>
> So on x86, a PROT_WRITE-only private page is mapped as non-readable? I
> had the impression that write-only still allows reading by looking at
> the __P010 definition.
>
> Anyway, if it's not an anonymous mmap(), whoever handled the mapping may
> have changed the permissions (e.g. some device).
>
> > So without a warning. My hypothesis is that if we create a page in the
> > same way as syzcaller then any getsockopt that does a copy_to_user()
> > (pretty much all of them I guess) will get this warning.
>
> The copy_to_user() only triggers the do_wp_page() fault handling. If
> this is a CoW page (private read-only presumably, or at least not
> writeable), the kernel tries to copy the original page given to
> getsockopt into a new page and restart the copy_to_user(). Since the
> kernel doesn't have a struct page for this (e.g. PFN mapping), it uses
> __copy_from_user_inatomic() which fails because of the read permission.
>
> > I have not tried this, so I might be wrong. If this is true, then the
> > question is what to do about it. One possible fix would be just to
> > remove the warning to get the same behavior as before. But it was
> > probably put there for a reason.
>
> It was there for some obscure cases, as the comment says ;). If the
> above is a valid scenario that the user can trigger, we should probably
> remove the WARN_ON.
>
> --
> Catalin
>