Re: WARNING in wp_page_copy

[Catalin Marinas]

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@xxxxxxxxx> wrote:
> > On Mon, Dec 16, 2019 at 4:00 PM Daniel Borkmann <daniel@xxxxxxxxxxxxx> 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@xxxxxxxxxxxxxxxxxxxxxxxxx
> > >
> > > 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?

> 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