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
