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Date:   Wed, 13 Feb 2019 22:11:58 +0100
From:   Jann Horn <jannh@...gle.com>
To:     Andrew Morton <akpm@...ux-foundation.org>
Cc:     Linux-MM <linux-mm@...ck.org>,
        kernel list <linux-kernel@...r.kernel.org>,
        Michal Hocko <mhocko@...e.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Pavel Tatashin <pavel.tatashin@...rosoft.com>,
        Oscar Salvador <osalvador@...e.de>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Aaron Lu <aaron.lu@...el.com>,
        Network Development <netdev@...r.kernel.org>,
        Alexander Duyck <alexander.h.duyck@...hat.com>
Subject: Re: [PATCH] mm: page_alloc: fix ref bias in page_frag_alloc() for
 1-byte allocs

On Wed, Feb 13, 2019 at 9:59 PM Andrew Morton <akpm@...ux-foundation.org> wrote:
>
> On Wed, 13 Feb 2019 21:41:57 +0100 Jann Horn <jannh@...gle.com> wrote:
>
> > The basic idea behind ->pagecnt_bias is: If we pre-allocate the maximum
> > number of references that we might need to create in the fastpath later,
> > the bump-allocation fastpath only has to modify the non-atomic bias value
> > that tracks the number of extra references we hold instead of the atomic
> > refcount. The maximum number of allocations we can serve (under the
> > assumption that no allocation is made with size 0) is nc->size, so that's
> > the bias used.
> >
> > However, even when all memory in the allocation has been given away, a
> > reference to the page is still held; and in the `offset < 0` slowpath, the
> > page may be reused if everyone else has dropped their references.
> > This means that the necessary number of references is actually
> > `nc->size+1`.
> >
> > Luckily, from a quick grep, it looks like the only path that can call
> > page_frag_alloc(fragsz=1) is TAP with the IFF_NAPI_FRAGS flag, which
> > requires CAP_NET_ADMIN in the init namespace and is only intended to be
> > used for kernel testing and fuzzing.
>
> For the net-naive, what is TAP?  It doesn't appear to mean
> drivers/net/tap.c.

It's implemented in drivers/net/tun.c; the combined functionality
implemented in there is called TUN/TAP. TUN refers to providing raw IP
packets to the kernel, TAP refers to providing raw ethernet packets.
It's documented in Documentation/networking/tuntap.txt. The code
that's interesting here is tun_get_user(), which calls into
tun_napi_alloc_frags() if tun_napi_frags_enabled(tfile) is true, which
in turn calls into netdev_alloc_frag(), which ends up in
page_frag_alloc(). This is how you can use it (except that if you were
using it legitimately, you'd be writing an ethernet header, a layer 3
header, and application data instead of writing "aaaaaaaaaaaaaaa" like
me):

================
#define _GNU_SOURCE
#include <stdlib.h>
#include <stdarg.h>
#include <net/if.h>
#include <linux/if.h>
#include <linux/if_tun.h>
#include <err.h>
#include <sys/types.h>
#include <fcntl.h>
#include <string.h>
#include <stdio.h>
#include <unistd.h>
#include <sys/ioctl.h>

void systemf(const char *command, ...) {
  char *full_command;
  va_list ap;
  va_start(ap, command);
  if (vasprintf(&full_command, command, ap) == -1)
    err(1, "vasprintf");
  va_end(ap);
  printf("systemf: <<<%s>>>\n", full_command);
  system(full_command);
}

char *devname;

int tun_alloc(char *name) {
  int fd = open("/dev/net/tun", O_RDWR);
  if (fd == -1)
    err(1, "open tun dev");
  static struct ifreq req = { .ifr_flags =
IFF_TAP|IFF_NO_PI|IFF_NAPI_FRAGS|IFF_NAPI };
  strcpy(req.ifr_name, name);
  if (ioctl(fd, TUNSETIFF, &req))
    err(1, "TUNSETIFF");
  devname = req.ifr_name;
  printf("device name: %s\n", devname);
  return fd;
}

int main(void) {
  int tun_fd = tun_alloc("inject_dev%d");
  systemf("ip link set %s up", devname);

  while (1) {
    struct iovec iov[15];
    for (int i=0; i<sizeof(iov)/sizeof(iov[0]); i++) {
      iov[i].iov_base = "a";
      iov[i].iov_len = 1;
    }
    writev(tun_fd, iov, sizeof(iov)/sizeof(iov[0]));
  }
}
================

> > To test for this issue, put a `WARN_ON(page_ref_count(page) == 0)` in the
> > `offset < 0` path, below the virt_to_page() call, and then repeatedly call
> > writev() on a TAP device with IFF_TAP|IFF_NO_PI|IFF_NAPI_FRAGS|IFF_NAPI,
> > with a vector consisting of 15 elements containing 1 byte each.
> >
> > ...
> >
> > --- a/mm/page_alloc.c
> > +++ b/mm/page_alloc.c
> > @@ -4675,11 +4675,11 @@ void *page_frag_alloc(struct page_frag_cache *nc,
> >               /* Even if we own the page, we do not use atomic_set().
> >                * This would break get_page_unless_zero() users.
> >                */
> > -             page_ref_add(page, size - 1);
> > +             page_ref_add(page, size);
> >
> >               /* reset page count bias and offset to start of new frag */
> >               nc->pfmemalloc = page_is_pfmemalloc(page);
> > -             nc->pagecnt_bias = size;
> > +             nc->pagecnt_bias = size + 1;
> >               nc->offset = size;
> >       }
> >
> > @@ -4695,10 +4695,10 @@ void *page_frag_alloc(struct page_frag_cache *nc,
> >               size = nc->size;
> >  #endif
> >               /* OK, page count is 0, we can safely set it */
> > -             set_page_count(page, size);
> > +             set_page_count(page, size + 1);
> >
> >               /* reset page count bias and offset to start of new frag */
> > -             nc->pagecnt_bias = size;
> > +             nc->pagecnt_bias = size + 1;
> >               offset = size - fragsz;
> >       }
>
> This is probably more a davem patch than a -mm one.

Ah, sorry. I assumed that I just should go by which directory the
patched code is in.

You did just add it to the -mm tree though, right? So I shouldn't
resend it to davem?

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