CVE-2026-98959: btrfs: fix xattr replace when multiple xattrs are packed in the same item
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 btrfsdiritem 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 btrfsdiritem, resulting in a corruption that the tree checker will detect at extent buffer writeback time.
This is because in btrfssetxattr() when we find a btrfsdiritem with multiple xattrs (due to the crc32c hash of their name being the same) we delete one of the xattr items (btrfsdiritem) 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 btrfsdiritem is placed in a leaf section that was not initialized and we only copy the value's data and set the value's length in the new btrfsdiritem, without setting the name, the name's length, the key (which must be all zeroes for xattrs), flags (BTRFSFTXATTR) 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 btrfsdiritem. 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/extentio.c:4408 at reportebrange+0x44/0x60 [btrfs], CPU#8: getfattr/2605179 [869800.168961] Modules linked in: btrfs dmthinpool (...) [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:reportebrange+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] <TASK> [869800.234791] readextentbuffer+0x4d/0x100 [btrfs] [869800.235898] btrfslistxattr+0x199/0x240 [btrfs] [869800.236912] vfslistxattr+0x51/0 ---truncated---