Re: BUG: unable to handle kernel paging request in fuse_copy_do

[David Hildenbrand]

On 22.03.24 14:50, Miklos Szeredi wrote:
> [MM list + secretmem author CC-d]
>
> On Thu, 21 Mar 2024 at 08:52, xingwei lee <xrivendell7@xxxxxxxxx> wrote:
> > Hello I found a bug titled "BUG: unable to handle kernel paging
> > request in fuse_copy_do” with modified syzkaller, and maybe it is
> > related to fs/fuse.
> > I also confirmed in the latest upstream.
> >
> > If you fix this issue, please add the following tag to the commit:
> > Reported-by: xingwei lee <xrivendell7@xxxxxxxxx>
> > Reported-by: yue sun <samsun1006219@xxxxxxxxx>
>
> Thanks for the report.   This looks like a secretmem vs get_user_pages issue.
>
> I reduced the syz reproducer to a minimal one that isn't dependent on fuse:
>
> === repro.c ===
> #define _GNU_SOURCE
>
> #include <fcntl.h>
> #include <unistd.h>
> #include <sys/mman.h>
> #include <sys/syscall.h>
> #include <sys/socket.h>
>
> int main(void)
> {
>          int fd1, fd2, fd3;
>          int pip[2];
>          struct iovec iov;
>          void *addr;
>
>          fd1 = syscall(__NR_memfd_secret, 0);
>          addr = mmap(NULL, 4096, PROT_READ, MAP_SHARED, fd1, 0);
>          ftruncate(fd1, 7);
>          fd2 = socket(AF_INET, SOCK_DGRAM, 0);
>          getsockopt(fd2, 0, 0, NULL, addr);
>
>          pipe(pip);
>          iov.iov_base = addr;
>          iov.iov_len = 0x50;
>          vmsplice(pip[1], &iov, 1, 0);

pip[1] should be the write end. So it will be used as the source.

I assume we go the ITER_SOURCE path in vmsplice, and call 
vmsplice_to_pipe(). Then we call iter_to_pipe().

I would expect iov_iter_get_pages2() -> get_user_pages_fast() to fail on 
secretmem pages?

But at least the vmsplice() just seems to work. Which is weird, because 
GUP-fast should not apply (page not faulted in?) and check_vma_flags() 
bails out early on vma_is_secretmem(vma).

So something is not quite right.

>          fd3 = open("/tmp/repro-secretmem.test", O_RDWR | O_CREAT, 0x600);
>          splice(pip[0], NULL, fd3, NULL, 0x50, 0);
>
>          return 0;
> }




--
Cheers,

David / dhildenb