CVE-2021-47656

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

jffs2: fix use-after-free in jffs2_clear_xattr_subsystem

When we mount a jffs2 image, assume that the first few blocks of

the image are normal and contain at least one xattr-related inode,

but the next block is abnormal. As a result, an error is returned

in jffs2_scan_eraseblock(). jffs2_clear_xattr_subsystem() is then

called in jffs2_build_filesystem() and then again in

jffs2_do_fill_super().

Finally we can observe the following report:

==================================================================

BUG: KASAN: use-after-free in jffs2_clear_xattr_subsystem+0x95/0x6ac

Read of size 8 at addr ffff8881243384e0 by task mount/719

Call Trace:

dump_stack+0x115/0x16b

jffs2_clear_xattr_subsystem+0x95/0x6ac

jffs2_do_fill_super+0x84f/0xc30

jffs2_fill_super+0x2ea/0x4c0

mtd_get_sb+0x254/0x400

mtd_get_sb_by_nr+0x4f/0xd0

get_tree_mtd+0x498/0x840

jffs2_get_tree+0x25/0x30

vfs_get_tree+0x8d/0x2e0

path_mount+0x50f/0x1e50

do_mount+0x107/0x130

__se_sys_mount+0x1c5/0x2f0

__x64_sys_mount+0xc7/0x160

do_syscall_64+0x45/0x70

entry_SYSCALL_64_after_hwframe+0x44/0xa9

Allocated by task 719:

kasan_save_stack+0x23/0x60

__kasan_kmalloc.constprop.0+0x10b/0x120

kasan_slab_alloc+0x12/0x20

kmem_cache_alloc+0x1c0/0x870

jffs2_alloc_xattr_ref+0x2f/0xa0

jffs2_scan_medium.cold+0x3713/0x4794

jffs2_do_mount_fs.cold+0xa7/0x2253

jffs2_do_fill_super+0x383/0xc30

jffs2_fill_super+0x2ea/0x4c0

[...]

Freed by task 719:

kmem_cache_free+0xcc/0x7b0

jffs2_free_xattr_ref+0x78/0x98

jffs2_clear_xattr_subsystem+0xa1/0x6ac

jffs2_do_mount_fs.cold+0x5e6/0x2253

jffs2_do_fill_super+0x383/0xc30

jffs2_fill_super+0x2ea/0x4c0

[...]

The buggy address belongs to the object at ffff8881243384b8

which belongs to the cache jffs2_xattr_ref of size 48

The buggy address is located 40 bytes inside of

48-byte region [ffff8881243384b8, ffff8881243384e8)

[...]

==================================================================

The triggering of the BUG is shown in the following stack:

-----------------------------------------------------------

jffs2_fill_super

jffs2_do_fill_super

jffs2_do_mount_fs

jffs2_build_filesystem

jffs2_scan_medium

jffs2_scan_eraseblock <--- ERROR

jffs2_clear_xattr_subsystem <--- free

jffs2_clear_xattr_subsystem <--- free again

-----------------------------------------------------------

An error is returned in jffs2_do_mount_fs(). If the error is returned

by jffs2_sum_init(), the jffs2_clear_xattr_subsystem() does not need to

be executed. If the error is returned by jffs2_build_filesystem(), the

jffs2_clear_xattr_subsystem() also does not need to be executed again.

So move jffs2_clear_xattr_subsystem() from 'out_inohash' to 'out_root'

to fix this UAF problem.