In the Linux kernel, the following vulnerability has been resolved:

btrfs: fix xattr replace when multiple xattrs are packed in the same item

If we have a btrfs_dir_item item that packs multiple xattrs and then we replace the value of one of them (with the setxattr(2) family of syscalls) with another value of a different size, we end up not having a fully initialized btrfs_dir_item, resulting in a corruption that the tree checker will detect at extent buffer writeback time.

This is because in btrfs_setxattr() when we find a btrfs_dir_item with multiple xattrs (due to the crc32c hash of their name being the same) we delete one of the xattr items (btrfs_dir_item) and then insert a new one, but the deletion and insertion results in shifting existing data in the leaf and therefore when the new value of a xattr has a different size, the new btrfs_dir_item is placed in a leaf section that was not initialized and we only copy the values data and set the values length in the new btrfs_dir_item, without setting the name, the names length, the key (which must be all zeroes for xattrs), flags (BTRFS_FT_XATTR) and transaction ID.

The following script reproduces the issue:

$ cat test.sh #!/bin/bash

DEV=/dev/sdi MNT=/mnt/sdi

mkfs.btrfs -f $DEV mount $DEV $MNT

touch $MNT/testfile

Add two xattrs that, on btrfs, have the same hash (crc32c) for their

name and therefore are packed into the same btrfs_dir_item.

setfattr -n user.foobar -v 123 $MNT/testfile setfattr -n user.WvG1c1Td -v qwerty $MNT/testfile

Verify the xattrs are present.

echo xattrs before: getfattr –absolute-names –dump $MNT/testfile

Now replace the value of the foobar xattr with a significantly larger

value.

setfattr -n user.foobar -v abcdefghijklmnopqrstuvwxyz $MNT/testfile

Check the xattrs have the expected values.

echo xattrs after: getfattr –absolute-names –dump $MNT/testfile

umount $MNT

Running it:

$ ./test.sh (…) xattrs before:

file: /mnt/sdi/testfile

user.WvG1c1Td=qwerty user.foobar=123

xattrs after:

file: /mnt/sdi/testfile

user.WvG1c1Td=qwerty

So the user.foobar xattr is missing and there was a transaction abort when unmounting the fs with the following traces in dmesg:

$ dmesg [869800.159271] BTRFS warning (device sdi): access to eb bytenr 30474240 len 16384 out of range start 16015 len 25964 [869800.159293] ————[ cut here ]———— [869800.159296] WARNING: fs/btrfs/extent_io.c:4408 at report_eb_range+0x44/0x60 [btrfs], CPU#8: getfattr/2605179 [869800.168961] Modules linked in: btrfs dm_thin_pool (…) [869800.190527] CPU: 8 UID: 0 PID: 2605179 Comm: getfattr Tainted: G W 7.3.0-rc3-btrfs-next-244+ #1 PREEMPT(full) [869800.193726] Tainted: [W]=WARN [869800.194502] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.16.2-0-gea1b7a073390-prebuilt.qemu.org 04/01/2014 [869800.197576] RIP: 0010:report_eb_range+0x44/0x60 [btrfs] [869800.198985] Code: 48 8b 7b 18 (…) [869800.203531] RSP: 0018:ffffce4541937d58 EFLAGS: 00010246 [869800.204609] RAX: 0000000000000000 RBX: ffff8dde054b8738 RCX: 0000000000000000 [869800.206137] RDX: 0000000000000000 RSI: 0000000000000001 RDI: ffffffffc04c92a0 [869800.207645] RBP: 0000000000003e8f R08: 0000000000000000 R09: 3fffffffffefffff [869800.226175] R10: ffffce4541937a88 R11: 0000000000000003 R12: 000000000000656c [869800.227594] R13: ffff8dde14be800f R14: 000000000000656c R15: 0000000000000069 [869800.229097] FS: 00007f59023bb780(0000) GS:ffff8de5788ed000(0000) knlGS:0000000000000000 [869800.231137] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033 [869800.232321] CR2: 0000558dc45e6a78 CR3: 0000000765b16004 CR4: 0000000000370ef0 [869800.233783] Call Trace: [869800.234330] [869800.234791] read_extent_buffer+0x4d/0x100 [btrfs] [869800.235898] btrfs_listxattr+0x199/0x240 [btrfs] [869800.236912] vfs_listxattr+0x51/0 —truncated—

Affected Software

References

• https://git.kernel.org/stable/c/5877d7138cc4359d26fd7b010eedfd1b3162348f
• https://git.kernel.org/stable/c/7a53bb4668b319fc6c8a3ce2c21635125c7d2712
• https://git.kernel.org/stable/c/858bee1c77857aabcc2327cd32dd8266332de332
• https://git.kernel.org/stable/c/a97114b9b00ed3d05156f3b2ddbf797296260389
• https://git.kernel.org/stable/c/c1803697a60f22743aa28ad522f805e92d45e3f0