@@ -908,6 +908,11 @@ static int loop_set_fd(struct loop_device *lo, fmode_t mode,
lo->lo_flags |= LO_FLAGS_PARTSCAN;
if (lo->lo_flags & LO_FLAGS_PARTSCAN)
ioctl_by_bdev(bdev, BLKRRPART, 0);
+
+ /* Grab the block_device to prevent its destruction after we
+ * put /dev/loopXX inode. Later in loop_clr_fd() we bdput(bdev).
+ */
+ bdgrab(bdev);
return 0;
out_clr:
@@ -1004,8 +1009,10 @@ static int loop_clr_fd(struct loop_device *lo)
memset(lo->lo_encrypt_key, 0, LO_KEY_SIZE);
memset(lo->lo_crypt_name, 0, LO_NAME_SIZE);
memset(lo->lo_file_name, 0, LO_NAME_SIZE);
- if (bdev)
+ if (bdev) {
+ bdput(bdev);
invalidate_bdev(bdev);
+ }
set_capacity(lo->lo_disk, 0);
loop_sysfs_exit(lo);
if (bdev) {
@@ -604,6 +604,7 @@ struct block_device *bdgrab(struct block_device *bdev)
ihold(bdev->bd_inode);
return bdev;
}
+EXPORT_SYMBOL(bdgrab);
long nr_blockdev_pages(void)
{
This is a note to let you know that I have just added a patch titled loop: prevent bdev freeing while device in use to the linux-3.5.y-queue branch of the 3.5.y.z extended stable tree which can be found at: http://kernel.ubuntu.com/git?p=ubuntu/linux.git;a=shortlog;h=refs/heads/linux-3.5.y-queue If you, or anyone else, feels it should not be added to this tree, please reply to this email. For more information about the 3.5.y.z tree, see https://wiki.ubuntu.com/Kernel/Dev/ExtendedStable Thanks. -Luis ------ From 3cf0f498b6aa73ecfeafdb57758e067b8c267f33 Mon Sep 17 00:00:00 2001 From: Anatol Pomozov <anatol.pomozov@gmail.com> Date: Mon, 1 Apr 2013 09:47:56 -0700 Subject: [PATCH] loop: prevent bdev freeing while device in use commit c1681bf8a7b1b98edee8b862a42c19c4e53205fd upstream. struct block_device lifecycle is defined by its inode (see fs/block_dev.c) - block_device allocated first time we access /dev/loopXX and deallocated on bdev_destroy_inode. When we create the device "losetup /dev/loopXX afile" we want that block_device stay alive until we destroy the loop device with "losetup -d". But because we do not hold /dev/loopXX inode its counter goes 0, and inode/bdev can be destroyed at any moment. Usually it happens at memory pressure or when user drops inode cache (like in the test below). When later in loop_clr_fd() we want to use bdev we have use-after-free error with following stack: BUG: unable to handle kernel NULL pointer dereference at 0000000000000280 bd_set_size+0x10/0xa0 loop_clr_fd+0x1f8/0x420 [loop] lo_ioctl+0x200/0x7e0 [loop] lo_compat_ioctl+0x47/0xe0 [loop] compat_blkdev_ioctl+0x341/0x1290 do_filp_open+0x42/0xa0 compat_sys_ioctl+0xc1/0xf20 do_sys_open+0x16e/0x1d0 sysenter_dispatch+0x7/0x1a To prevent use-after-free we need to grab the device in loop_set_fd() and put it later in loop_clr_fd(). The issue is reprodusible on current Linus head and v3.3. Here is the test: dd if=/dev/zero of=loop.file bs=1M count=1 while [ true ]; do losetup /dev/loop0 loop.file echo 2 > /proc/sys/vm/drop_caches losetup -d /dev/loop0 done [ Doing bdgrab/bput in loop_set_fd/loop_clr_fd is safe, because every time we call loop_set_fd() we check that loop_device->lo_state is Lo_unbound and set it to Lo_bound If somebody will try to set_fd again it will get EBUSY. And if we try to loop_clr_fd() on unbound loop device we'll get ENXIO. loop_set_fd/loop_clr_fd (and any other loop ioctl) is called under loop_device->lo_ctl_mutex. ] Signed-off-by: Anatol Pomozov <anatol.pomozov@gmail.com> Cc: Al Viro <viro@zeniv.linux.org.uk> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org> Signed-off-by: Luis Henriques <luis.henriques@canonical.com> --- drivers/block/loop.c | 9 ++++++++- fs/block_dev.c | 1 + 2 files changed, 9 insertions(+), 1 deletion(-) -- 1.8.1.2