Re: [PATCH] userfaultfd: fix a race between writeprotect and exit_mmap()

[Li Wang <liwang@xxxxxxxxxx>]

Hi,

I confirmed this patch (applied on 5.14) gets rid of the below userfaultfd test failure.

# ./userfaultfd anon 16 2
nr_pages: 4096, nr_pages_per_cpu: 256
bounces: 1, mode: rnd read, userfaults: 313 missing (51+34+37+26+41+28+15+20+16+12+13+7+10+2+0+1) 995 wp (121+79+96+53+90+104+48+61+56+82+56+41+49+26+11+22)
bounces: 0, mode: read, userfaults: 64 missing (15+8+10+6+5+2+4+3+3+1+4+0+0+2+0+1) 2157 wp (223+274+189+141+116+132+203+153+143+126+110+114+101+66+42+24)
testing uffd-wp with pagemap (pgsize=4096): done
testing uffd-wp with pagemap (pgsize=2097152): done
testing UFFDIO_ZEROPAGE: done.
testing signal delivery: done.
testing events (fork, remap, remove): ERROR: nr 3933 memory corruption 0 1
 (errno=0, line=963)
ERROR: faulting process failed (errno=0, line=1117)

On Wed, Sep 22, 2021 at 11:34 AM Nadav Amit <nadav.amit@xxxxxxxxx> wrote:

From: Nadav Amit <namit@xxxxxxxxxx>

A race is possible when a process exits, its VMAs are removed
by exit_mmap() and at the same time userfaultfd_writeprotect() is
called.

The race was detected by KASAN on a development kernel, but it appears
to be possible on vanilla kernels as well.

Use mmget_not_zero() to prevent the race as done in other userfaultfd
operations.

Cc: Peter Xu <peterx@xxxxxxxxxx>
Cc: Andrea Arcangeli <aarcange@xxxxxxxxxx>
Cc: stable@xxxxxxxxxxxxxxx
Fixes: 63b2d4174c4ad ("userfaultfd: wp: add the writeprotect API to userfaultfd ioctl")
Signed-off-by: Nadav Amit <namit@xxxxxxxxxx>

 

Tested-by: Li Wang <liwang@xxxxxxxxxx>

 

---
 fs/userfaultfd.c | 12 +++++++++---
 1 file changed, 9 insertions(+), 3 deletions(-)

diff --git a/fs/userfaultfd.c b/fs/userfaultfd.c
index 003f0d31743e..22bf14ab2d16 100644
--- a/fs/userfaultfd.c
+++ b/fs/userfaultfd.c
@@ -1827,9 +1827,15 @@ static int userfaultfd_writeprotect(struct userfaultfd_ctx *ctx,
        if (mode_wp && mode_dontwake)
                return -EINVAL;

-       ret = mwriteprotect_range(ctx->mm, uffdio_wp.range.start,
-                                 uffdio_wp.range.len, mode_wp,
-                                 &ctx->mmap_changing);
+       if (mmget_not_zero(ctx->mm)) {
+               ret = mwriteprotect_range(ctx->mm, uffdio_wp.range.start,
+                                         uffdio_wp.range.len, mode_wp,
+                                         &ctx->mmap_changing);
+               mmput(ctx->mm);
+       } else {
+               return -ESRCH;
+       }
+
        if (ret)
                return ret;

--
2.25.1

--

Regards,

Li Wang