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

ubifs: authentication: Fix use-after-free in ubifs_tnc_end_commit

After an insertion in TNC, the tree might split and cause a node to
change its znode->parent. A further deletion of other nodes in the
tree (which also could free the nodes), the aforementioned node’s
znode->cparent could still point to a freed node. This
znode->cparent may not be updated when getting nodes to commit in
ubifs_tnc_start_commit(). This could then trigger a use-after-free
when accessing the znode->cparent in write_index() in
ubifs_tnc_end_commit().

This can be triggered by running

rm -f /etc/test-file.bin
dd if=/dev/urandom of=/etc/test-file.bin bs=1M count=60 conv=fsync

in a loop, and with CONFIG_UBIFS_FS_AUTHENTICATION. KASAN then
reports:

BUG: KASAN: use-after-free in ubifs_tnc_end_commit+0xa5c/0x1950
Write of size 32 at addr ffffff800a3af86c by task ubifs_bgt0_20/153

Call trace:
dump_backtrace+0x0/0x340
show_stack+0x18/0x24
dump_stack_lvl+0x9c/0xbc
print_address_description.constprop.0+0x74/0x2b0
kasan_report+0x1d8/0x1f0
kasan_check_range+0xf8/0x1a0
memcpy+0x84/0xf4
ubifs_tnc_end_commit+0xa5c/0x1950
do_commit+0x4e0/0x1340
ubifs_bg_thread+0x234/0x2e0
kthread+0x36c/0x410
ret_from_fork+0x10/0x20

Allocated by task 401:
kasan_save_stack+0x38/0x70
__kasan_kmalloc+0x8c/0xd0
__kmalloc+0x34c/0x5bc
tnc_insert+0x140/0x16a4
ubifs_tnc_add+0x370/0x52c
ubifs_jnl_write_data+0x5d8/0x870
do_writepage+0x36c/0x510
ubifs_writepage+0x190/0x4dc
__writepage+0x58/0x154
write_cache_pages+0x394/0x830
do_writepages+0x1f0/0x5b0
filemap_fdatawrite_wbc+0x170/0x25c
file_write_and_wait_range+0x140/0x190
ubifs_fsync+0xe8/0x290
vfs_fsync_range+0xc0/0x1e4
do_fsync+0x40/0x90
__arm64_sys_fsync+0x34/0x50
invoke_syscall.constprop.0+0xa8/0x260
do_el0_svc+0xc8/0x1f0
el0_svc+0x34/0x70
el0t_64_sync_handler+0x108/0x114
el0t_64_sync+0x1a4/0x1a8

Freed by task 403:
kasan_save_stack+0x38/0x70
kasan_set_track+0x28/0x40
kasan_set_free_info+0x28/0x4c
__kasan_slab_free+0xd4/0x13c
kfree+0xc4/0x3a0
tnc_delete+0x3f4/0xe40
ubifs_tnc_remove_range+0x368/0x73c
ubifs_tnc_remove_ino+0x29c/0x2e0
ubifs_jnl_delete_inode+0x150/0x260
ubifs_evict_inode+0x1d4/0x2e4
evict+0x1c8/0x450
iput+0x2a0/0x3c4
do_unlinkat+0x2cc/0x490
__arm64_sys_unlinkat+0x90/0x100
invoke_syscall.constprop.0+0xa8/0x260
do_el0_svc+0xc8/0x1f0
el0_svc+0x34/0x70
el0t_64_sync_handler+0x108/0x114
el0t_64_sync+0x1a4/0x1a8

The offending memcpy() in ubifs_copy_hash() has a use-after-free
when a node becomes root in TNC but still has a cparent to an already
freed node. More specifically, consider the following TNC:

     zroot
     /
    /
  zp1
  /
 /
zn

Inserting a new node zn_new with a key smaller then zn will trigger
a split in tnc_insert() if zp1 is full:

     zroot
     /   \
    /     \
  zp1     zp2
  /         \
 /           \

zn_new zn

zn->parent has now been moved to zp2, but zn->cparent still
points to zp1.

Now, consider a removal of all the nodes except zn. Just when
tnc_delete() is about to delete zroot and zp2:

     zroot
         \
          \
          zp2
            \
             \
             zn

zroot and zp2 get freed and the tree collapses:

       zn

zn now becomes the new zroot.

get_znodes_to_commit() will now only find zn, the new zroot, and
write_index() will check its znode->cparent that wrongly points to
the already freed zp1. ubifs_copy_hash() thus gets wrongly called
with znode->cparent->zbranch[znode->iip].hash that triggers the
use-after-free!

Fix this by explicitly setting znode->cparent to NULL in
get_znodes_to_commit() for the root node. The search for the dirty
nodes
—-truncated—-