[PATCH] loop: don't change loop device under exclusive opener in loop_set_status

[Raphael Pinsonneault-Thibeault <rpthibeault@gmail.com>]

loop_set_status() is allowed to change the loop device while there
are other openers of the device, even exclusive ones.

In this case, it causes a KASAN: slab-out-of-bounds Read in
ext4_search_dir(), since when looking for an entry in an inlined
directory, e_value_offs is changed underneath the filesystem by
loop_set_status().

Fix the problem by forbidding loop_set_status() from modifying the loop
device while there are exclusive openers of the device. This is similar
to the fix in loop_configure() by commit 33ec3e53e7b1 ("loop: Don't
change loop device under exclusive opener") alongside commit ecbe6bc0003b
("block: use bd_prepare_to_claim directly in the loop driver").

Reported-by: syzbot+3ee481e21fd75e14c397@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=3ee481e21fd75e14c397
Tested-by: syzbot+3ee481e21fd75e14c397@syzkaller.appspotmail.com
Signed-off-by: Raphael Pinsonneault-Thibeault <rpthibeault@gmail.com>
---
ML thread for previous, misguided patch idea:
https://lore.kernel.org/all/20251112185712.2031993-2-rpthibeault@gmail.com/t/

 drivers/block/loop.c | 41 ++++++++++++++++++++++++++++++-----------
 1 file changed, 30 insertions(+), 11 deletions(-)

diff --git a/drivers/block/loop.c b/drivers/block/loop.c
index 053a086d547e..756ee682e767 100644
--- a/drivers/block/loop.c
+++ b/drivers/block/loop.c
@@ -1222,13 +1222,24 @@ static int loop_clr_fd(struct loop_device *lo)
 }
 
 static int
-loop_set_status(struct loop_device *lo, const struct loop_info64 *info)
+loop_set_status(struct loop_device *lo, blk_mode_t mode,
+		struct block_device *bdev, const struct loop_info64 *info)
 {
 	int err;
 	bool partscan = false;
 	bool size_changed = false;
 	unsigned int memflags;
 
+	/*
+	 * If we don't hold exclusive handle for the device, upgrade to it
+	 * here to avoid changing device under exclusive owner.
+	 */
+	if (!(mode & BLK_OPEN_EXCL)) {
+		err = bd_prepare_to_claim(bdev, loop_set_status, NULL);
+		if (err)
+			goto out_reread_partitions;
+	}
+
 	err = mutex_lock_killable(&lo->lo_mutex);
 	if (err)
 		return err;
@@ -1270,6 +1281,9 @@ loop_set_status(struct loop_device *lo, const struct loop_info64 *info)
 	}
 out_unlock:
 	mutex_unlock(&lo->lo_mutex);
+	if (!(mode & BLK_OPEN_EXCL))
+		bd_abort_claiming(bdev, loop_set_status);
+out_reread_partitions:
 	if (partscan)
 		loop_reread_partitions(lo);
 
@@ -1349,7 +1363,9 @@ loop_info64_to_old(const struct loop_info64 *info64, struct loop_info *info)
 }
 
 static int
-loop_set_status_old(struct loop_device *lo, const struct loop_info __user *arg)
+loop_set_status_old(struct loop_device *lo, blk_mode_t mode,
+		    struct block_device *bdev,
+		    const struct loop_info __user *arg)
 {
 	struct loop_info info;
 	struct loop_info64 info64;
@@ -1357,17 +1373,19 @@ loop_set_status_old(struct loop_device *lo, const struct loop_info __user *arg)
 	if (copy_from_user(&info, arg, sizeof (struct loop_info)))
 		return -EFAULT;
 	loop_info64_from_old(&info, &info64);
-	return loop_set_status(lo, &info64);
+	return loop_set_status(lo, mode, bdev, &info64);
 }
 
 static int
-loop_set_status64(struct loop_device *lo, const struct loop_info64 __user *arg)
+loop_set_status64(struct loop_device *lo, blk_mode_t mode,
+		  struct block_device *bdev,
+		  const struct loop_info64 __user *arg)
 {
 	struct loop_info64 info64;
 
 	if (copy_from_user(&info64, arg, sizeof (struct loop_info64)))
 		return -EFAULT;
-	return loop_set_status(lo, &info64);
+	return loop_set_status(lo, mode, bdev, &info64);
 }
 
 static int
@@ -1546,14 +1564,14 @@ static int lo_ioctl(struct block_device *bdev, blk_mode_t mode,
 	case LOOP_SET_STATUS:
 		err = -EPERM;
 		if ((mode & BLK_OPEN_WRITE) || capable(CAP_SYS_ADMIN))
-			err = loop_set_status_old(lo, argp);
+			err = loop_set_status_old(lo, mode, bdev, argp);
 		break;
 	case LOOP_GET_STATUS:
 		return loop_get_status_old(lo, argp);
 	case LOOP_SET_STATUS64:
 		err = -EPERM;
 		if ((mode & BLK_OPEN_WRITE) || capable(CAP_SYS_ADMIN))
-			err = loop_set_status64(lo, argp);
+			err = loop_set_status64(lo, mode, bdev, argp);
 		break;
 	case LOOP_GET_STATUS64:
 		return loop_get_status64(lo, argp);
@@ -1647,8 +1665,9 @@ loop_info64_to_compat(const struct loop_info64 *info64,
 }
 
 static int
-loop_set_status_compat(struct loop_device *lo,
-		       const struct compat_loop_info __user *arg)
+loop_set_status_compat(struct loop_device *lo, blk_mode_t mode,
+		    struct block_device *bdev,
+		    const struct compat_loop_info __user *arg)
 {
 	struct loop_info64 info64;
 	int ret;
@@ -1656,7 +1675,7 @@ loop_set_status_compat(struct loop_device *lo,
 	ret = loop_info64_from_compat(arg, &info64);
 	if (ret < 0)
 		return ret;
-	return loop_set_status(lo, &info64);
+	return loop_set_status(lo, mode, bdev, &info64);
 }
 
 static int
@@ -1682,7 +1701,7 @@ static int lo_compat_ioctl(struct block_device *bdev, blk_mode_t mode,
 
 	switch(cmd) {
 	case LOOP_SET_STATUS:
-		err = loop_set_status_compat(lo,
+		err = loop_set_status_compat(lo, mode, bdev,
 			     (const struct compat_loop_info __user *)arg);
 		break;
 	case LOOP_GET_STATUS:
-- 
2.43.0