# CVE-2024-50217: btrfs: fix use-after-free of block device file in __btrfs_free_extra_devids()

CVE-2024-50217 is fixed in linux on 1 platform, first by DEBIAN-CVE-2024-50217, released November 9, 2024. It is not reported as exploited. Patch Radar priority: High.

- Web page: https://patchblox.com/security/cve/CVE-2024-50217
- Priority: High
- CVSS: 7.8 (Important)
- EPSS: 0.0028
- CISA KEV: not listed
- Exploited (vendor report): no
- Publicly disclosed: no
- Published: Nov 9, 2024
- Weakness: Use After Free
- Vendor advisory: https://msrc.microsoft.com/update-guide/vulnerability/CVE-2024-50217

## Description

In the Linux kernel, the following vulnerability has been resolved:  btrfs: fix use-after-free of block device file in __btrfs_free_extra_devids()  Mounting btrfs from two images (which have the same one fsid and two different dev_uuids) in certain executing order may trigger an UAF for variable 'device->bdev_file' in __btrfs_free_extra_devids(). And following are the details:  1. Attach image_1 to loop0, attach image_2 to loop1, and scan btrfs    devices by ioctl(BTRFS_IOC_SCAN_DEV):               /  btrfs_device_1 → loop0    fs_device              \  btrfs_device_2 → loop1 2. mount /dev/loop0 /mnt    btrfs_open_devices     btrfs_device_1->bdev_file = btrfs_get_bdev_and_sb(loop0)     btrfs_device_2->bdev_file = btrfs_get_bdev_and_sb(loop1)    btrfs_fill_super     open_ctree      fail: btrfs_close_devices // -ENOMEM 	    btrfs_close_bdev(btrfs_device_1)              fput(btrfs_device_1->bdev_file) 	      // btrfs_device_1->bdev_file is freed 	    btrfs_close_bdev(btrfs_device_2)              fput(btrfs_device_2->bdev_file)  3. mount /dev/loop1 /mnt    btrfs_open_devices     btrfs_get_bdev_and_sb(&bdev_file)      // EIO, btrfs_device_1->bdev_file is not assigned,      // which points to a freed memory area     btrfs_device_2->bdev_file = btrfs_get_bdev_and_sb(loop1)    btrfs_fill_super     open_ctree      btrfs_free_extra_devids       if (btrfs_device_1->bdev_file)        fput(btrfs_device_1->bdev_file) // UAF !  Fix it by setting 'device->bdev_file' as 'NULL' after closing the btrfs_device in btrfs_close_one_device().

## Where it's fixed

| Platform | Product | Fixed in | Fix | Released |
|---|---|---|---|---|
| Linux | linux Debian 13.5 (trixie) | 6.11.7-1 | [DEBIAN-CVE-2024-50217](https://patchblox.com/security/data/fix/linux/debian-cve-2024-50217.md) | Nov 9, 2024 |
| Linux | linux Debian 14 | 6.11.7-1 | [DEBIAN-CVE-2024-50217](https://patchblox.com/security/data/fix/linux/debian-cve-2024-50217.md) | Nov 9, 2024 |

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