From: Nadav Amit <nadav.amit@gmail.com>
To: Yu Zhao <yuzhao@google.com>,
Peter Zijlstra <peterz@infradead.org>,
Minchan Kim <minchan@kernel.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>,
Peter Xu <peterx@redhat.com>,
Andrea Arcangeli <aarcange@redhat.com>,
linux-mm <linux-mm@kvack.org>,
lkml <linux-kernel@vger.kernel.org>,
Pavel Emelyanov <xemul@openvz.org>,
Mike Kravetz <mike.kravetz@oracle.com>,
Mike Rapoport <rppt@linux.vnet.ibm.com>,
stable <stable@vger.kernel.org>,
Andy Lutomirski <luto@kernel.org>, Will Deacon <will@kernel.org>
Subject: Re: [PATCH] mm/userfaultfd: fix memory corruption due to writeprotect
Date: Tue, 22 Dec 2020 04:40:32 -0800 [thread overview]
Message-ID: <D4916F41-DE4A-42C6-8702-944382631A02@gmail.com> (raw)
In-Reply-To: <X+ESkna2z3WjjniN@google.com>
> On Dec 21, 2020, at 1:24 PM, Yu Zhao <yuzhao@google.com> wrote:
>
> On Mon, Dec 21, 2020 at 12:26:22PM -0800, Linus Torvalds wrote:
>> On Mon, Dec 21, 2020 at 12:23 PM Nadav Amit <nadav.amit@gmail.com> wrote:
>>> Using mmap_write_lock() was my initial fix and there was a strong pushback
>>> on this approach due to its potential impact on performance.
>>
>> From whom?
>>
>> Somebody who doesn't understand that correctness is more important
>> than performance? And that userfaultfd is not the most important part
>> of the system?
>>
>> The fact is, userfaultfd is CLEARLY BUGGY.
>>
>> Linus
>
> Fair enough.
>
> Nadav, for your patch (you might want to update the commit message).
>
> Reviewed-by: Yu Zhao <yuzhao@google.com>
>
> While we are all here, there is also clear_soft_dirty() that could
> use a similar fix…
Just an update as for why I have still not sent v2: I fixed
clear_soft_dirty(), created a reproducer, and the reproducer kept failing.
So after some debugging, it appears that clear_refs_write() does not flush
the TLB. It indeed calls tlb_finish_mmu() but since 0758cd830494
("asm-generic/tlb: avoid potential double flush”), tlb_finish_mmu() does not
flush the TLB since there is clear_refs_write() does not call to
__tlb_adjust_range() (unless there are nested TLBs are pending).
So I have a patch for this issue too: arguably the tlb_gather interface is
not the right one for clear_refs_write() that does not clear PTEs but
changes them.
Yet, sadly, my reproducer keeps falling (less frequently, but still). So I
will keep debugging to see what goes wrong. I will send v2 once I figure out
what the heck is wrong in the code or my reproducer.
For the reference, here is my reproducer:
-- >8 --
#define _GNU_SOURCE
#include <sys/types.h>
#include <sys/stat.h>
#include <sys/mman.h>
#include <unistd.h>
#include <stdio.h>
#include <stdlib.h>
#include <assert.h>
#include <fcntl.h>
#include <string.h>
#include <threads.h>
#include <stdatomic.h>
#define PAGE_SIZE (4096)
#define TLB_SIZE (2000)
#define N_PAGES (300000)
#define ITERATIONS (100)
#define N_THREADS (2)
static int stop;
static char *m;
static int writer(void *argp)
{
unsigned long t_idx = (unsigned long)argp;
int i, cnt = 0;
while (!atomic_load(&stop)) {
cnt++;
atomic_fetch_add((atomic_int *)m, 1);
/*
* First thread only accesses the page to have it cached in the
* TLB.
*/
if (t_idx == 0)
continue;
/*
* Other threads access enough entries to cause eviction from
* the TLB and trigger #PF upon the next access (before the TLB
* flush of clear_ref actually takes place).
*/
for (i = 1; i < TLB_SIZE; i++) {
if (atomic_load((atomic_int *)(m + PAGE_SIZE * i))) {
fprintf(stderr, "unexpected error\n");
exit(1);
}
}
}
return cnt;
}
/*
* Runs mlock/munlock in the background to raise the page-count of the page and
* force copying instead of reusing the page.
*/
static int do_mlock(void *argp)
{
while (!atomic_load(&stop)) {
if (mlock(m, PAGE_SIZE) || munlock(m, PAGE_SIZE)) {
perror("mlock/munlock");
exit(1);
}
}
return 0;
}
int main(void)
{
int r, cnt, fd, total = 0;
long i;
thrd_t thr[N_THREADS];
thrd_t mlock_thr[N_THREADS];
fd = open("/proc/self/clear_refs", O_WRONLY, 0666);
if (fd < 0) {
perror("open");
exit(1);
}
/*
* Have large memory for clear_ref, so there would be some time between
* the unmap and the actual deferred flush.
*/
m = mmap(NULL, PAGE_SIZE * N_PAGES, PROT_READ|PROT_WRITE,
MAP_PRIVATE|MAP_ANONYMOUS|MAP_POPULATE, -1, 0);
if (m == MAP_FAILED) {
perror("mmap");
exit(1);
}
for (i = 0; i < N_THREADS; i++) {
r = thrd_create(&thr[i], writer, (void *)i);
assert(r == thrd_success);
}
for (i = 0; i < N_THREADS; i++) {
r = thrd_create(&mlock_thr[i], do_mlock, (void *)i);
assert(r == thrd_success);
}
for (i = 0; i < ITERATIONS; i++) {
for (i = 0; i < ITERATIONS; i++) {
r = pwrite(fd, "4", 1, 0);
if (r < 0) {
perror("pwrite");
exit(1);
}
}
}
atomic_store(&stop, 1);
for (i = 0; i < N_THREADS; i++) {
r = thrd_join(mlock_thr[i], NULL);
assert(r == thrd_success);
}
for (i = 0; i < N_THREADS; i++) {
r = thrd_join(thr[i], &cnt);
assert(r == thrd_success);
total += cnt;
}
r = atomic_load((atomic_int *)(m));
if (r != total) {
fprintf(stderr, "failed - expected=%d actual=%d\n", total, r);
exit(-1);
}
fprintf(stderr, "ok\n");
return 0;
}
next prev parent reply other threads:[~2020-12-22 12:40 UTC|newest]
Thread overview: 120+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-12-19 4:30 [PATCH] mm/userfaultfd: fix memory corruption due to writeprotect Nadav Amit
2020-12-19 19:15 ` Andrea Arcangeli
[not found] ` <EDC00345-B46E-4396-8379-98E943723809@gmail.com>
2020-12-19 22:06 ` Nadav Amit
2020-12-20 2:20 ` Andrea Arcangeli
2020-12-21 4:36 ` Nadav Amit
2020-12-21 5:12 ` Yu Zhao
2020-12-21 5:25 ` Nadav Amit
2020-12-21 5:39 ` Nadav Amit
2020-12-21 7:29 ` Yu Zhao
2020-12-22 20:34 ` Andy Lutomirski
2020-12-22 20:58 ` Nadav Amit
2020-12-22 21:34 ` Andrea Arcangeli
2020-12-20 2:01 ` Andy Lutomirski
2020-12-20 2:49 ` Andrea Arcangeli
2020-12-20 5:08 ` Andy Lutomirski
2020-12-21 18:03 ` Andrea Arcangeli
2020-12-21 18:22 ` Andy Lutomirski
2020-12-20 6:05 ` Yu Zhao
2020-12-20 8:06 ` Nadav Amit
2020-12-20 9:54 ` Yu Zhao
2020-12-21 3:33 ` Nadav Amit
2020-12-21 4:44 ` Yu Zhao
2020-12-21 17:27 ` Peter Xu
2020-12-21 18:31 ` Nadav Amit
2020-12-21 19:16 ` Yu Zhao
2020-12-21 19:55 ` Linus Torvalds
2020-12-21 20:21 ` Yu Zhao
2020-12-21 20:25 ` Linus Torvalds
2020-12-21 20:23 ` Nadav Amit
2020-12-21 20:26 ` Linus Torvalds
2020-12-21 21:24 ` Yu Zhao
2020-12-21 21:49 ` Nadav Amit
2020-12-21 22:30 ` Peter Xu
2020-12-21 22:55 ` Nadav Amit
2020-12-21 23:30 ` Linus Torvalds
2020-12-21 23:46 ` Nadav Amit
2020-12-22 19:44 ` Andrea Arcangeli
2020-12-22 20:19 ` Nadav Amit
2020-12-22 21:17 ` Andrea Arcangeli
2020-12-21 23:12 ` Yu Zhao
2020-12-21 23:33 ` Linus Torvalds
2020-12-22 0:00 ` Yu Zhao
2020-12-22 0:11 ` Linus Torvalds
2020-12-22 0:24 ` Yu Zhao
2020-12-21 23:22 ` Linus Torvalds
2020-12-22 3:19 ` Andy Lutomirski
2020-12-22 4:16 ` Linus Torvalds
2020-12-22 20:19 ` Andy Lutomirski
2021-01-05 15:37 ` Peter Zijlstra
2021-01-05 18:03 ` Andrea Arcangeli
2021-01-12 16:20 ` Peter Zijlstra
2021-01-12 11:43 ` Vinayak Menon
2021-01-12 15:47 ` Laurent Dufour
2021-01-12 16:57 ` Peter Zijlstra
2021-01-12 19:02 ` Laurent Dufour
2021-01-12 19:15 ` Nadav Amit
2021-01-12 19:56 ` Yu Zhao
2021-01-12 20:38 ` Nadav Amit
2021-01-12 20:49 ` Yu Zhao
2021-01-12 21:43 ` Will Deacon
2021-01-12 22:29 ` Nadav Amit
2021-01-12 22:46 ` Will Deacon
2021-01-13 0:31 ` Andy Lutomirski
2021-01-17 4:41 ` Yu Zhao
2021-01-17 7:32 ` Nadav Amit
2021-01-17 9:16 ` Yu Zhao
2021-01-17 10:13 ` Nadav Amit
2021-01-17 19:25 ` Yu Zhao
2021-01-18 2:49 ` Nadav Amit
2020-12-22 9:38 ` Nadav Amit
2020-12-22 19:31 ` Andrea Arcangeli
2020-12-22 20:15 ` Matthew Wilcox
2020-12-22 20:26 ` Andrea Arcangeli
2020-12-22 21:14 ` Yu Zhao
2020-12-22 22:02 ` Andrea Arcangeli
2020-12-22 23:39 ` Yu Zhao
2020-12-22 23:50 ` Linus Torvalds
2020-12-23 0:01 ` Linus Torvalds
2020-12-23 0:23 ` Yu Zhao
2020-12-23 2:17 ` Andrea Arcangeli
2020-12-23 9:44 ` Linus Torvalds
2020-12-23 10:06 ` Yu Zhao
2020-12-23 16:24 ` Peter Xu
2020-12-23 18:51 ` Andrea Arcangeli
2020-12-23 18:55 ` Andrea Arcangeli
2020-12-23 19:12 ` Yu Zhao
2020-12-23 19:32 ` Peter Xu
2020-12-23 0:20 ` Linus Torvalds
2020-12-23 2:56 ` Andrea Arcangeli
2020-12-23 3:36 ` Yu Zhao
2020-12-23 15:52 ` Peter Xu
2020-12-23 21:07 ` Andrea Arcangeli
2020-12-23 21:39 ` Andrea Arcangeli
2020-12-23 22:29 ` Yu Zhao
2020-12-23 23:04 ` Andrea Arcangeli
2020-12-24 1:21 ` Andy Lutomirski
2020-12-24 2:00 ` Andrea Arcangeli
2020-12-24 3:09 ` Nadav Amit
2020-12-24 3:30 ` Nadav Amit
2020-12-24 3:34 ` Yu Zhao
2020-12-24 4:01 ` Andrea Arcangeli
2020-12-24 5:18 ` Nadav Amit
2020-12-24 18:49 ` Andrea Arcangeli
2020-12-24 19:16 ` Andrea Arcangeli
2020-12-24 4:37 ` Nadav Amit
2020-12-24 3:31 ` Andrea Arcangeli
2020-12-23 23:39 ` Linus Torvalds
2020-12-24 1:01 ` Andrea Arcangeli
2020-12-22 21:14 ` Nadav Amit
2020-12-22 12:40 ` Nadav Amit [this message]
2020-12-22 18:30 ` Yu Zhao
2020-12-22 19:20 ` Nadav Amit
2020-12-23 16:23 ` Will Deacon
2020-12-23 19:04 ` Nadav Amit
2020-12-23 22:05 ` Andrea Arcangeli
2020-12-23 22:45 ` Nadav Amit
2020-12-23 23:55 ` Andrea Arcangeli
2020-12-21 21:55 ` Peter Xu
2020-12-21 23:13 ` Linus Torvalds
2020-12-21 19:53 ` Peter Xu
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=D4916F41-DE4A-42C6-8702-944382631A02@gmail.com \
--to=nadav.amit@gmail.com \
--cc=aarcange@redhat.com \
--cc=linux-kernel@vger.kernel.org \
--cc=linux-mm@kvack.org \
--cc=luto@kernel.org \
--cc=mike.kravetz@oracle.com \
--cc=minchan@kernel.org \
--cc=peterx@redhat.com \
--cc=peterz@infradead.org \
--cc=rppt@linux.vnet.ibm.com \
--cc=stable@vger.kernel.org \
--cc=torvalds@linux-foundation.org \
--cc=will@kernel.org \
--cc=xemul@openvz.org \
--cc=yuzhao@google.com \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).