Re: [PATCH v4] cifs: fix use-after-free caused by invalid pointer `hostname`
From: Zeng Heng
Date: Fri Oct 28 2022 - 23:09:34 EST
Make sure `cifsd` terminated to avoid race condition, it has to call
function like kthread_stop.
Then the whole `server` struct would be released by `cifsd` and another
UAF appears.
On 2022/10/28 13:41, Shyam Prasad N wrote:
On Thu, Oct 27, 2022 at 6:19 PM Zeng Heng <zengheng4@xxxxxxxxxx> wrote:
`hostname` needs to be set as null-pointer after free in
`cifs_put_tcp_session` function, or when `cifsd` thread attempts
to resolve hostname and reconnect the host, the thread would deref
the invalid pointer.
Here is one of practical backtrace examples as reference:
Task 477
---------------------------
do_mount
path_mount
do_new_mount
vfs_get_tree
smb3_get_tree
smb3_get_tree_common
cifs_smb3_do_mount
cifs_mount
mount_put_conns
cifs_put_tcp_session
--> kfree(server->hostname)
cifsd
---------------------------
kthread
cifs_demultiplex_thread
cifs_reconnect
reconn_set_ipaddr_from_hostname
--> if (!server->hostname)
--> if (server->hostname[0] == '\0') // !! UAF fault here
CIFS: VFS: cifs_mount failed w/return code = -112
mount error(112): Host is down
BUG: KASAN: use-after-free in reconn_set_ipaddr_from_hostname+0x2ba/0x310
Read of size 1 at addr ffff888108f35380 by task cifsd/480
CPU: 2 PID: 480 Comm: cifsd Not tainted 6.1.0-rc2-00106-gf705792f89dd-dirty #25
Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.13.0-1ubuntu1.1 04/01/2014
Call Trace:
<TASK>
dump_stack_lvl+0x68/0x85
print_report+0x16c/0x4a3
kasan_report+0x95/0x190
reconn_set_ipaddr_from_hostname+0x2ba/0x310
__cifs_reconnect.part.0+0x241/0x800
cifs_reconnect+0x65f/0xb60
cifs_demultiplex_thread+0x1570/0x2570
kthread+0x2c5/0x380
ret_from_fork+0x22/0x30
</TASK>
Allocated by task 477:
kasan_save_stack+0x1e/0x40
kasan_set_track+0x21/0x30
__kasan_kmalloc+0x7e/0x90
__kmalloc_node_track_caller+0x52/0x1b0
kstrdup+0x3b/0x70
cifs_get_tcp_session+0xbc/0x19b0
mount_get_conns+0xa9/0x10c0
cifs_mount+0xdf/0x1970
cifs_smb3_do_mount+0x295/0x1660
smb3_get_tree+0x352/0x5e0
vfs_get_tree+0x8e/0x2e0
path_mount+0xf8c/0x1990
do_mount+0xee/0x110
__x64_sys_mount+0x14b/0x1f0
do_syscall_64+0x3b/0x90
entry_SYSCALL_64_after_hwframe+0x63/0xcd
Freed by task 477:
kasan_save_stack+0x1e/0x40
kasan_set_track+0x21/0x30
kasan_save_free_info+0x2a/0x50
__kasan_slab_free+0x10a/0x190
__kmem_cache_free+0xca/0x3f0
cifs_put_tcp_session+0x30c/0x450
cifs_mount+0xf95/0x1970
cifs_smb3_do_mount+0x295/0x1660
smb3_get_tree+0x352/0x5e0
vfs_get_tree+0x8e/0x2e0
path_mount+0xf8c/0x1990
do_mount+0xee/0x110
__x64_sys_mount+0x14b/0x1f0
do_syscall_64+0x3b/0x90
entry_SYSCALL_64_after_hwframe+0x63/0xcd
The buggy address belongs to the object at ffff888108f35380
which belongs to the cache kmalloc-16 of size 16
The buggy address is located 0 bytes inside of
16-byte region [ffff888108f35380, ffff888108f35390)
The buggy address belongs to the physical page:
page:00000000333f8e58 refcount:1 mapcount:0 mapping:0000000000000000 index:0xffff888108f350e0 pfn:0x108f35
flags: 0x200000000000200(slab|node=0|zone=2)
raw: 0200000000000200 0000000000000000 dead000000000122 ffff8881000423c0
raw: ffff888108f350e0 000000008080007a 00000001ffffffff 0000000000000000
page dumped because: kasan: bad access detected
Memory state around the buggy address:
ffff888108f35280: fa fb fc fc fa fb fc fc fa fb fc fc fa fb fc fc
ffff888108f35300: fa fb fc fc fa fb fc fc fa fb fc fc fa fb fc fc
ffff888108f35380: fa fb fc fc fa fb fc fc fa fb fc fc fa fb fc fc
^
ffff888108f35400: fa fb fc fc fc fc fc fc fc fc fc fc fc fc fc fc
ffff888108f35480: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
Fixes: 7be3248f3139 ("cifs: To match file servers, make sure the server hostname matches")
Signed-off-by: Zeng Heng <zengheng4@xxxxxxxxxx>
Reviewed-by: Paulo Alcantara (SUSE) <pc@xxxxxx>
---
changes in v4:
- correct fix tag
- add reviewed-by
---
fs/cifs/connect.c | 1 +
1 file changed, 1 insertion(+)
diff --git a/fs/cifs/connect.c b/fs/cifs/connect.c
index ffb291579bb9..1cc47dd3b4d6 100644
--- a/fs/cifs/connect.c
+++ b/fs/cifs/connect.c
@@ -1584,6 +1584,7 @@ cifs_put_tcp_session(struct TCP_Server_Info *server, int from_reconnect)
server->session_key.response = NULL;
server->session_key.len = 0;
kfree(server->hostname);
+ server->hostname = NULL;
task = xchg(&server->tsk, NULL);
if (task)
--
2.25.1
Good catch. But I think there can be a better fix.
How about moving the lines that follow i.e. cifsd thread kill to
before setting tcpStatus? That way, we don't leave scope for things to
race.