xsk: share the mmap_sem for page pinning

mainline inclusion
from mainline-v5.0-rc7
commit e451eb51
category: bugfix
bugzilla: 10768
CVE: NA

-------------------------------------------------

Holding mmap_sem exclusively for a gup() is an overkill. Lets
share the lock and replace the gup call for gup_longterm(), as
it is better suited for the lifetime of the pinning.

Fixes: c0c77d8f ("xsk: add user memory registration support sockopt")
Signed-off-by: NDavidlohr Bueso <dbueso@suse.de>
Cc: David S. Miller <davem@davemloft.net>
Cc: Bjorn Topel <bjorn.topel@intel.com>
Cc: Magnus Karlsson <magnus.karlsson@intel.com>
CC: netdev@vger.kernel.org
Acked-by: NBjörn Töpel <bjorn.topel@intel.com>
Signed-off-by: NDaniel Borkmann <daniel@iogearbox.net>
Signed-off-by: NLin Miaohe <linmiaohe@huawei.com>
Reviewed-by: NMao Wenan <maowenan@huawei.com>
Signed-off-by: NYang Yingliang <yangyingliang@huawei.com>

net/xdp/xdp_umem.c

@@ -214,10 +214,10 @@ static int xdp_umem_pin_pages(struct xdp_umem *umem)
 	if (!umem->pgs)
 		return -ENOMEM;
 
-	down_write(&current->mm->mmap_sem);
-	npgs = get_user_pages(umem->address, umem->npgs,
-			      gup_flags, &umem->pgs[0], NULL);
-	up_write(&current->mm->mmap_sem);
+	down_read(&current->mm->mmap_sem);
+	npgs = get_user_pages_longterm(umem->address, umem->npgs,
+				       gup_flags, &umem->pgs[0], NULL);
+	up_read(&current->mm->mmap_sem);
 
 	if (npgs != umem->npgs) {
 		if (npgs >= 0) {