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

[David Hildenbrand]

On 22.03.24 22:13, Miklos Szeredi wrote:
> On Fri, 22 Mar 2024 at 22:08, David Hildenbrand <david@xxxxxxxxxx> wrote:
> > On 22.03.24 20:46, Miklos Szeredi wrote:
> > > On Fri, 22 Mar 2024 at 16:41, David Hildenbrand <david@xxxxxxxxxx> wrote:
> > >
> > > > But at least the vmsplice() just seems to work. Which is weird, because
> > > > GUP-fast should not apply (page not faulted in?)
> > >
> > > But it is faulted in, and that indeed seems to be the root cause.
> >
> > secretmem mmap() won't populate the page tables. So it's not faulted in yet.
> >
> > When we GUP via vmsplice, GUP-fast should not find it in the page tables
> > and fallback to slow GUP.
> >
> > There, we seem to pass check_vma_flags(), trigger faultin_page() to
> > fault it in, and then find it via follow_page_mask().
> >
> > ... and I wonder how we manage to skip check_vma_flags(), or otherwise
> > managed to GUP it.
> >
> > vmsplice() should, in theory, never succeed here.
> >
> > Weird :/
> >
> > > Improved repro:
> > >
> > > #define _GNU_SOURCE
> > >
> > > #include <fcntl.h>
> > > #include <unistd.h>
> > > #include <stdio.h>
> > > #include <errno.h>
> > > #include <sys/mman.h>
> > > #include <sys/syscall.h>
> > >
> > > int main(void)
> > > {
> > >           int fd1, fd2;
> > >           int pip[2];
> > >           struct iovec iov;
> > >           char *addr;
> > >           int ret;
> > >
> > >           fd1 = syscall(__NR_memfd_secret, 0);
> > >           addr = mmap(NULL, 4096, PROT_READ | PROT_WRITE, MAP_SHARED, fd1, 0);
> > >           ftruncate(fd1, 7);
> > >           addr[0] = 1; /* fault in page */
>
> Here the page is faulted in and GUP-fast will find it.  It's not in
> the kernel page table, but it is in the user page table, which is what
> matter for GUP.

Trust me, I know the GUP code very well :P

gup_pte_range -- GUP fast -- contains:

if (unlikely(folio_is_secretmem(folio))) {
	gup_put_folio(folio, 1, flags);
	goto pte_unmap;
}

So we "should" be rejecting any secretmem folios and fallback to GUP slow.


.. we don't check the same in gup_huge_pmd(), but we shouldn't ever see 
THP in secretmem code.

--
Cheers,

David / dhildenb