Re: [PATCH RFC] ubifs: Fix use-after-free in ubifs_tnc_end_commit

Zhihao Cheng
2024-10-12T12:30:51Z

在 2024/10/9 22:46, Waqar Hameed 写道:
> 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, with `CONFIG_UBIFS_FS_AUTHENTICATION`, KASAN 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` is in `ubifs_copy_hash()`. Fix this by checking
> if the `znode` is obsolete before accessing the hash (just like we do
> for `znode->parent`).

Do you mean that the UAF occurs in following path:
do_commit -> ubifs_tnc_end_commit -> write_index:
while (1) {
...
znode = cnext;
...
if (znode->cparent)
ubifs_copy_hash(c, hash,
znode->cparent->zbranch[znode->ciip].hash); // znode->cparent has been
freed!
}

If so, according to the current implementation(lastest linux kernel is
v6.12), I cannot understand that how the znode->cparent is freed before
write_index() finished, it looks impossible.
All dirty znodes are gathered by function get_znodes_to_commit() which
is protected by c->tnc_mutex, and the 'cparent' member in all dirty
znodes is assigned with non-NULL. Then I think the znode memory freeing
path 'tnc_delete->kfree(znode)' cannot happen, because:
1) If a znode is dirtied, all its' ancestor znodes(a path from znode to
root znode) must be dirtied, which is guaranteed by UBIFS. See
dirty_cow_bottom_up/lookup_level0_dirty.
2) A dirty znode waiting for commit cannot be freed before write_index()
finished, which is guaranteed by tnc_delete:
if (znode->cnext) {
__set_bit(OBSOLETE_ZNODE, &znode->flags);
...
} else {
kfree(znode);
}
>
> Fixes: 16a26b20d2af ("ubifs: authentication: Add hashes to index nodes")
> Signed-off-by: Waqar Hameed <waqar.hameed@axis.com>
> ---
> I'm not entirely sure if this is the _correct_ way to fix this. However,
> testing shows that the problem indeed disappears.
>
> My understanding is that the `znode` could concurrently be deleted (with
> a reference in an unprotected code section without `tnc_mutex`). The
> assumption is that in this case it would be sufficient to check
> `ubifs_zn_obsolete(znode)`, like as in the if-statement for
> `znode->parent` just below.

I'm analyzing tnc-related code these days, however I can't find places
that may concurrently operate the same znode. And I cannot reproduce the
problem with your reproducer:
while true; do
rm -f /UBIFS_MNT/test-file.bin
dd if=/dev/urandom of=/UBIFS_MNT/test-file.bin bs=1M count=60 conv=fsync
done

Can you dig more deeper by adding more debug message, so that we can
figure out what is really happening.
>
> I'll be happy to get any helpful feedback!
>
> fs/ubifs/tnc_commit.c | 6 ++++--
> 1 file changed, 4 insertions(+), 2 deletions(-)
>
> diff --git a/fs/ubifs/tnc_commit.c b/fs/ubifs/tnc_commit.c
> index a55e04822d16..0b358254272b 100644
> --- a/fs/ubifs/tnc_commit.c
> +++ b/fs/ubifs/tnc_commit.c
> @@ -891,8 +891,10 @@ static int write_index(struct ubifs_info *c)
> mutex_lock(&c->tnc_mutex);
>
> if (znode->cparent)
> - ubifs_copy_hash(c, hash,
> - znode->cparent->zbranch[znode->ciip].hash);
> + if (!ubifs_zn_obsolete(znode))
> + ubifs_copy_hash(c, hash,
> + znode->cparent->zbranch[znode->ciip]
> + .hash);
>
> if (znode->parent) {
> if (!ubifs_zn_obsolete(znode))