From: Nadav Amit <namit@xxxxxxxxxx> Subject: userfaultfd: fix a race between writeprotect and exit_mmap() 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. Link: https://lkml.kernel.org/r/20210921200247.25749-1-namit@xxxxxxxxxx Fixes: 63b2d4174c4ad ("userfaultfd: wp: add the writeprotect API to userfaultfd ioctl") Signed-off-by: Nadav Amit <namit@xxxxxxxxxx> Tested-by: Li Wang <liwang@xxxxxxxxxx> Reviewed-by: Peter Xu <peterx@xxxxxxxxxx> Cc: Andrea Arcangeli <aarcange@xxxxxxxxxx> Cc: <stable@xxxxxxxxxxxxxxx> Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> --- fs/userfaultfd.c | 12 +++++++++--- 1 file changed, 9 insertions(+), 3 deletions(-) --- a/fs/userfaultfd.c~userfaultfd-fix-a-race-between-writeprotect-and-exit_mmap +++ a/fs/userfaultfd.c @@ -1827,9 +1827,15 @@ static int userfaultfd_writeprotect(stru 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; _
