BTRFS use-after-free bug at free_extent_buffer_internal

Sachi King
2024-04-22T12:34:25Z

Hi,

I've hit a bug with u-boot on my BTRFS filesystem, and I'm fairly certain
it's a bug and not a corruption issue.

A bit of history on the filesystem. It is a fairly new filesystem as it was
being used to give me access to test a wayland application on a
Raspberry Pi. The filesystem was about 3 days old when I hit the bug, and
I'm fairly certain it never had an unclean shutdown. I have checked the
filesystem with "btrfs check" which has found no errors. The filesystem
mounts on Linux and is functional.

> # btrfs check --check-data-csum /dev/sda2
> Opening filesystem to check...
> Checking filesystem on /dev/sda2
> UUID: 18db6211-ac36-42c1-a22f-5e15e1486e0d
> [1/7] checking root items
> [2/7] checking extents
> [3/7] checking free space tree
> [4/7] checking fs roots
> [5/7] checking csums against data
> [6/7] checking root refs
> [7/7] checking quota groups skipped (not enabled on this FS)
> found 5070573568 bytes used, no error found
> total csum bytes: 4451620
> total tree bytes: 370458624
> total fs tree bytes: 353124352
> total extent tree bytes: 10010624
> btree space waste bytes: 62303284
> file data blocks allocated: 6786519040
> referenced 6328619008

I've made an image of the filesystem so I could reproduce the bug in an
environment that doesn't require the physical SBC, and have reproduced
the issue using the head of the master branch with "qemu-x86_64_defconfig".

My testing qemu was produced with the following:
> # make qemu-x86_64_defconfig
> # cat << EOF >> .config
> CONFIG_AUTOBOOT=y
> CONFIG_BOOTDELAY=1
> CONFIG_USE_BOOTCOMMAND=y
> CONFIG_BOOTSTD_DEFAULTS=y
> CONFIG_BOOTSTD_FULL=y
> CONFIG_CMD_BOOTFLOW_FULL=y
> CONFIG_BOOTCOMMAND="bootflow scan -lb"
> CONFIG_ENV_IS_NOWHERE=y
> CONFIG_LZ4=y
> CONFIG_BZIP2=y
> CONFIG_ZSTD=y
> CONFIG_FS_BTRFS=y
> CONFIG_CMD_BTRFS=y
> CONFIG_GZIP=y
> CONFIG_DEVICE_TREE_INCLUDES="bootstd.dtsi"
> EOF
> # make -j24

bootstd.dtsi is placed at "arch/x86/dts/bootstd.dtsi" and contains:
> / {
> bootstd {
> compatible = "u-boot,boot-std";
> filename-prefixes = "/@boot/", "/boot/", "/";
> bootdev-order = "scsi";
> extlinux {
> compatible = "u-boot,extlinux";
> };
> };
> };

The VM was run with
> qemu-system-x86_64 -bios u-boot.rom -nographic -M q35 -action reboot=shutdown -drive file=/mnt/dbg/u-boot-debug.img

The error message I recive on boot is
> BUG at fs/btrfs/extent-io.c:629/free_extent_buffer_internal()!
> BUG!
> resetting ...

I added a print statement to free_extent_buffer_internal that prints the
start address of the extent_buffer as I'm not sure what to be looking for
here. This print statement is before the decrement.
> printf("free_extent_buffer_internal: eb->start[%llx] eb->refs[%i]\n", eb->start, eb->refs);

The last message before the crash reported eb->start to be "0", with 0 refs.
> free_extent_buffer_internal: eb->start[0] eb->refs[0]

The extent at 0 struck me as odd, so I tried commenting out the freeing, by
removing the call to free_extent_buffer_final, and this resulted in bootflow
succeeding and showing me the boot menu, which suprised me.
I expected to see the bug reproduce itself, with refs being zero, but eb->start
pointing somewhere valid, but I instead got a valid address with refs at 2.

I'm assuming that the order free_extent_buffer_internal is called is
deterministic, so by counting the print outputs the line that prior held
the extent_buffer with a 0 start was replaced with:
> free_extent_buffer_internal: eb->start[249c000] eb->refs[2]

Interestingly, as can be seen in the full logs with my included print
messages, 249c000 is being used just before this, with a ref count of
2. 249c000 does appear to reach a point where it should have been freed
in the past, before it gets used again as seen in both logs.

The failing boot log:
> U-Boot SPL 2024.04-00949-g1dd659fd62-dirty (Apr 22 2024 - 11:32:37 +1000)
> Trying to boot from SPI
> Jumping to 64-bit U-Boot: Note many features are missing
>
>
> U-Boot 2024.04-00949-g1dd659fd62-dirty (Apr 22 2024 - 11:32:37 +1000)
>
> CPU: QEMU Virtual CPU version 2.5+
> DRAM: 128 MiB
> Core: 13 devices, 13 uclasses, devicetree: separate
> Loading Environment from nowhere... OK
> Model: QEMU x86 (I440FX)
> Net: e1000: 00:00:00:00:00:00
>
> Error: e1000#0 No valid MAC address found.
> eth_initialize() No ethernet found.
>
>
> Hit any key to stop autoboot: 0
> Scanning for bootflows in all bootdevs
> Seq Method State Uclass Part Name Filename
> --- ----------- ------ -------- ---- ------------------------ ----------------
> scanning bus for devices...
> Target spinup took 0 ms.
> SATA link 1 timeout.
> Target spinup took 0 ms.
> SATA link 3 timeout.
> SATA link 4 timeout.
> SATA link 5 timeout.
> AHCI 0001.0000 32 slots 6 ports 1.5 Gbps 0x3f impl SATA mode
> flags: 64bit ncq only
> free_extent_buffer_internal: eb->start[10000] eb->refs[1]
> free_extent_buffer_internal: eb->start[1548000] eb->refs[1]
> free_extent_buffer_internal: eb->start[150c000] eb->refs[1]
> free_extent_buffer_internal: eb->start[154c000] eb->refs[1]
> free_extent_buffer_internal: eb->start[1544000] eb->refs[2]
> free_extent_buffer_internal: eb->start[28f4000] eb->refs[2]
> free_extent_buffer_internal: eb->start[28f4000] eb->refs[2]
> free_extent_buffer_internal: eb->start[28f4000] eb->refs[2]
> free_extent_buffer_internal: eb->start[2490000] eb->refs[1]
> free_extent_buffer_internal: eb->start[6de0000] eb->refs[1]
> free_extent_buffer_internal: eb->start[28f4000] eb->refs[1]
> free_extent_buffer_internal: eb->start[1544000] eb->refs[1]
> Device 0: (0:0) Vendor: ATA Prod.: QEMU HARDDISK Rev: 2.5+
> Type: Hard Disk
> Capacity: 58680.0 MB = 57.3 GB (120176640 x 512)
> timeout exit!
> Scanning bootdev 'ahci_scsi.id0lun0.bootdev':
> free_extent_buffer_internal: eb->start[10000] eb->refs[1]
> free_extent_buffer_internal: eb->start[1548000] eb->refs[1]
> free_extent_buffer_internal: eb->start[150c000] eb->refs[1]
> free_extent_buffer_internal: eb->start[154c000] eb->refs[1]
> free_extent_buffer_internal: eb->start[1544000] eb->refs[2]
> free_extent_buffer_internal: eb->start[28f4000] eb->refs[2]
> free_extent_buffer_internal: eb->start[28f4000] eb->refs[2]
> free_extent_buffer_internal: eb->start[28f4000] eb->refs[2]
> free_extent_buffer_internal: eb->start[2490000] eb->refs[2]
> free_extent_buffer_internal: eb->start[28f4000] eb->refs[2]
> free_extent_buffer_internal: eb->start[24a0000] eb->refs[1]
> free_extent_buffer_internal: eb->start[249c000] eb->refs[2]
> free_extent_buffer_internal: eb->start[24a0000] eb->refs[1]
> free_extent_buffer_internal: eb->start[249c000] eb->refs[2]
> free_extent_buffer_internal: eb->start[24a4000] eb->refs[1]
> free_extent_buffer_internal: eb->start[249c000] eb->refs[2]
> free_extent_buffer_internal: eb->start[2490000] eb->refs[1]
> free_extent_buffer_internal: eb->start[249c000] eb->refs[1]
> free_extent_buffer_internal: eb->start[6de0000] eb->refs[1]
> free_extent_buffer_internal: eb->start[28f4000] eb->refs[1]
> free_extent_buffer_internal: eb->start[1544000] eb->refs[1]
> free_extent_buffer_internal: eb->start[10000] eb->refs[1]
> free_extent_buffer_internal: eb->start[1548000] eb->refs[1]
> free_extent_buffer_internal: eb->start[150c000] eb->refs[1]
> free_extent_buffer_internal: eb->start[154c000] eb->refs[1]
> free_extent_buffer_internal: eb->start[1544000] eb->refs[2]
> free_extent_buffer_internal: eb->start[28f4000] eb->refs[2]
> free_extent_buffer_internal: eb->start[28f4000] eb->refs[2]
> free_extent_buffer_internal: eb->start[28f4000] eb->refs[2]
> free_extent_buffer_internal: eb->start[2490000] eb->refs[2]
> free_extent_buffer_internal: eb->start[28f4000] eb->refs[2]
> free_extent_buffer_internal: eb->start[24a0000] eb->refs[1]
> free_extent_buffer_internal: eb->start[249c000] eb->refs[2]
> free_extent_buffer_internal: eb->start[24a0000] eb->refs[1]
> free_extent_buffer_internal: eb->start[249c000] eb->refs[2]
> free_extent_buffer_internal: eb->start[2490000] eb->refs[2]
> free_extent_buffer_internal: eb->start[24a0000] eb->refs[1]
> free_extent_buffer_internal: eb->start[249c000] eb->refs[2]
> free_extent_buffer_internal: eb->start[24a0000] eb->refs[1]
> free_extent_buffer_internal: eb->start[249c000] eb->refs[2]
> free_extent_buffer_internal: eb->start[24a4000] eb->refs[1]
> free_extent_buffer_internal: eb->start[249c000] eb->refs[2]
> free_extent_buffer_internal: eb->start[24a4000] eb->refs[1]
> free_extent_buffer_internal: eb->start[249c000] eb->refs[2]
> free_extent_buffer_internal: eb->start[24a4000] eb->refs[1]
> free_extent_buffer_internal: eb->start[0] eb->refs[0]
> BUG at fs/btrfs/extent-io.c:626/free_extent_buffer_internal()!
> BUG!
> resetting ...

The succeeding log with the removed freeing:
> U-Boot SPL 2024.04-00949-g1dd659fd62-dirty (Apr 22 2024 - 11:38:25 +1000)
> Trying to boot from SPI
> Jumping to 64-bit U-Boot: Note many features are missing
>
>
> U-Boot 2024.04-00949-g1dd659fd62-dirty (Apr 22 2024 - 11:38:25 +1000)
>
> CPU: QEMU Virtual CPU version 2.5+
> DRAM: 128 MiB
> Core: 13 devices, 13 uclasses, devicetree: separate
> Loading Environment from nowhere... OK
> Model: QEMU x86 (I440FX)
> Net: e1000: 00:00:00:00:00:00
>
> Error: e1000#0 No valid MAC address found.
> eth_initialize() No ethernet found.
>
>
> Hit any key to stop autoboot: 0
> Scanning for bootflows in all bootdevs
> Seq Method State Uclass Part Name Filename
> --- ----------- ------ -------- ---- ------------------------ ----------------
> scanning bus for devices...
> Target spinup took 0 ms.
> SATA link 1 timeout.
> Target spinup took 0 ms.
> SATA link 3 timeout.
> SATA link 4 timeout.
> SATA link 5 timeout.
> AHCI 0001.0000 32 slots 6 ports 1.5 Gbps 0x3f impl SATA mode
> flags: 64bit ncq only
> free_extent_buffer_internal: eb->start[10000] eb->refs[1]
> free_extent_buffer_internal: eb->start[1548000] eb->refs[1]
> free_extent_buffer_internal: eb->start[150c000] eb->refs[1]
> free_extent_buffer_internal: eb->start[154c000] eb->refs[1]
> free_extent_buffer_internal: eb->start[1544000] eb->refs[2]
> free_extent_buffer_internal: eb->start[28f4000] eb->refs[2]
> free_extent_buffer_internal: eb->start[28f4000] eb->refs[2]
> free_extent_buffer_internal: eb->start[28f4000] eb->refs[2]
> free_extent_buffer_internal: eb->start[2490000] eb->refs[1]
> free_extent_buffer_internal: eb->start[6de0000] eb->refs[1]
> free_extent_buffer_internal: eb->start[28f4000] eb->refs[1]
> free_extent_buffer_internal: eb->start[1544000] eb->refs[1]
> Device 0: (0:0) Vendor: ATA Prod.: QEMU HARDDISK Rev: 2.5+
> Type: Hard Disk
> Capacity: 58680.0 MB = 57.3 GB (120176640 x 512)
> timeout exit!
> Scanning bootdev 'ahci_scsi.id0lun0.bootdev':
> free_extent_buffer_internal: eb->start[10000] eb->refs[1]
> free_extent_buffer_internal: eb->start[1548000] eb->refs[1]
> free_extent_buffer_internal: eb->start[150c000] eb->refs[1]
> free_extent_buffer_internal: eb->start[154c000] eb->refs[1]
> free_extent_buffer_internal: eb->start[1544000] eb->refs[2]
> free_extent_buffer_internal: eb->start[28f4000] eb->refs[2]
> free_extent_buffer_internal: eb->start[28f4000] eb->refs[2]
> free_extent_buffer_internal: eb->start[28f4000] eb->refs[2]
> free_extent_buffer_internal: eb->start[2490000] eb->refs[2]
> free_extent_buffer_internal: eb->start[28f4000] eb->refs[2]
> free_extent_buffer_internal: eb->start[24a0000] eb->refs[1]
> free_extent_buffer_internal: eb->start[249c000] eb->refs[2]
> free_extent_buffer_internal: eb->start[24a0000] eb->refs[1]
> free_extent_buffer_internal: eb->start[249c000] eb->refs[2]
> free_extent_buffer_internal: eb->start[24a4000] eb->refs[1]
> free_extent_buffer_internal: eb->start[249c000] eb->refs[2]
> free_extent_buffer_internal: eb->start[2490000] eb->refs[1]
> free_extent_buffer_internal: eb->start[249c000] eb->refs[1]
> free_extent_buffer_internal: eb->start[6de0000] eb->refs[1]
> free_extent_buffer_internal: eb->start[28f4000] eb->refs[1]
> free_extent_buffer_internal: eb->start[1544000] eb->refs[1]
> free_extent_buffer_internal: eb->start[10000] eb->refs[1]
> free_extent_buffer_internal: eb->start[1548000] eb->refs[1]
> free_extent_buffer_internal: eb->start[150c000] eb->refs[1]
> free_extent_buffer_internal: eb->start[154c000] eb->refs[1]
> free_extent_buffer_internal: eb->start[1544000] eb->refs[2]
> free_extent_buffer_internal: eb->start[28f4000] eb->refs[2]
> free_extent_buffer_internal: eb->start[28f4000] eb->refs[2]
> free_extent_buffer_internal: eb->start[28f4000] eb->refs[2]
> free_extent_buffer_internal: eb->start[2490000] eb->refs[2]
> free_extent_buffer_internal: eb->start[28f4000] eb->refs[2]
> free_extent_buffer_internal: eb->start[24a0000] eb->refs[1]
> free_extent_buffer_internal: eb->start[249c000] eb->refs[2]
> free_extent_buffer_internal: eb->start[24a0000] eb->refs[1]
> free_extent_buffer_internal: eb->start[249c000] eb->refs[2]
> free_extent_buffer_internal: eb->start[2490000] eb->refs[2]
> free_extent_buffer_internal: eb->start[24a0000] eb->refs[1]
> free_extent_buffer_internal: eb->start[249c000] eb->refs[2]
> free_extent_buffer_internal: eb->start[24a0000] eb->refs[1]
> free_extent_buffer_internal: eb->start[249c000] eb->refs[2]
> free_extent_buffer_internal: eb->start[24a4000] eb->refs[1]
> free_extent_buffer_internal: eb->start[249c000] eb->refs[2]
> free_extent_buffer_internal: eb->start[24a4000] eb->refs[1]
> free_extent_buffer_internal: eb->start[249c000] eb->refs[2]
> free_extent_buffer_internal: eb->start[24a4000] eb->refs[1]
> free_extent_buffer_internal: eb->start[249c000] eb->refs[2]
> free_extent_buffer_internal: eb->start[2490000] eb->refs[1]
> free_extent_buffer_internal: eb->start[6de0000] eb->refs[1]
> free_extent_buffer_internal: eb->start[28f4000] eb->refs[1]
> free_extent_buffer_internal: eb->start[1544000] eb->refs[1]
> 0 extlinux ready scsi 2 ahci_scsi.id0lun0.bootdev /@boot/extlinux/extlinux.conf
> ** Booting bootflow 'ahci_scsi.id0lun0.bootdev.part_2' with extlinux
> ------------------------------------------------------------
> 1: NixOS - Default
> 2: NixOS - Configuration 5 (2024-03-04 11:00 - 24.05.20240205.faf912b)
> 3: NixOS - Configuration 4 (2024-03-02 15:05 - 24.05.20240205.faf912b)
> 4: NixOS - Configuration 3 (2024-03-02 14:26 - 24.05.20240205.faf912b)
> 5: NixOS - Configuration 2 (2024-03-02 14:06 - 24.05.20240205.faf912b)
> 6: NixOS - Configuration 1 (1970-01-01 10:00 - 24.05.20240205.faf912b)
> Enter choice: 1: NixOS - Default
> Retrieving file: /@boot/extlinux/../nixos/gq8jsgxnhq2wvsjrni3cjw1wb5540wjw-linux-6.1.63-stable_20231123-Image
> free_extent_buffer_internal: eb->start[10000] eb->refs[1]

I'm not sure where to go from here with the bug, so I'm hoping for some
help in tracking it down so it can be fixed.

Thanks,
Sachi