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 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 value's data and set the value's length
in the new btrfs_dir_item, without setting the name, the name's 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] <TASK>
[869800.234791] read_extent_buffer+0x4d/0x100 [btrfs]
[869800.235898] btrfs_listxattr+0x199/0x240 [btrfs]
[869800.236912] vfs_listxattr+0x51/0
---truncated---