On Tue, Dec 17, 2019 at 4:40 PM Catalin Marinas <catalin.marinas@xxxxxxx> 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@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? 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 >
