From: Nadav Amit <[email protected]>
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 <[email protected]>
Cc: Andrea Arcangeli <[email protected]>
Cc: [email protected]
Fixes: 63b2d4174c4ad ("userfaultfd: wp: add the writeprotect API to userfaultfd ioctl")
Signed-off-by: Nadav Amit <[email protected]>
---
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
On Tue, Sep 21, 2021 at 01:02:47PM -0700, Nadav Amit wrote:
> From: Nadav Amit <[email protected]>
>
> 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 <[email protected]>
> Cc: Andrea Arcangeli <[email protected]>
> Cc: [email protected]
> Fixes: 63b2d4174c4ad ("userfaultfd: wp: add the writeprotect API to userfaultfd ioctl")
> Signed-off-by: Nadav Amit <[email protected]>
Reviewed-by: Peter Xu <[email protected]>
Thanks!
--
Peter Xu
On Wed, Sep 22, 2021 at 05:06:53PM +0800, Li Wang wrote:
> Hi,
Li,
>
> 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)
Just to keep a record within this thread - my understanding is above issue is a
separate issue from what Nadav has fixed. The other fix could be:
https://lore.kernel.org/lkml/[email protected]/
When verify with Nadav's patch, please check whether you have thp enabled
globally:
# echo always > /sys/kernel/mm/transparent_hugepage/enabled
Thanks,
--
Peter Xu