Re: [PATCH v4] cifs: fix use-after-free caused by invalid pointer `hostname`

[Shyam Prasad N]

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.

-- 
Regards,
Shyam