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

[Shyam Prasad N]

On Sat, Oct 29, 2022 at 8:25 AM Zeng Heng <zengheng4@xxxxxxxxxx> wrote:
>
> 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.
> >

Good point. We need to ensure that.

-- 
Regards,
Shyam