[PATCH] security, lsm: dentry_init_security() Handle multi LSM registration

Thu Jan 27 09:15:15 UTC 2022

On Wed, Jan 26, 2022 at 03:35:14PM -0500, Vivek Goyal wrote:
> A ceph user has reported that ceph is crashing with kernel NULL pointer
> dereference. Following is the backtrace.
> 
> /proc/version: Linux version 5.16.2-arch1-1 (linux at archlinux) (gcc (GCC)
> 11.1.0, GNU ld (GNU Binutils) 2.36.1) #1 SMP PREEMPT Thu, 20 Jan 2022
> 16:18:29 +0000
> distro / arch: Arch Linux / x86_64
> SELinux is not enabled
> ceph cluster version: 16.2.7 (dd0603118f56ab514f133c8d2e3adfc983942503)
> 
> relevant dmesg output:
> [   30.947129] BUG: kernel NULL pointer dereference, address:
> 0000000000000000
> [   30.947206] #PF: supervisor read access in kernel mode
> [   30.947258] #PF: error_code(0x0000) - not-present page
> [   30.947310] PGD 0 P4D 0
> [   30.947342] Oops: 0000 [#1] PREEMPT SMP PTI
> [   30.947388] CPU: 5 PID: 778 Comm: touch Not tainted 5.16.2-arch1-1 #1
> 86fbf2c313cc37a553d65deb81d98e9dcc2a3659
> [   30.947486] Hardware name: Gigabyte Technology Co., Ltd. B365M
> DS3H/B365M DS3H, BIOS F5 08/13/2019
> [   30.947569] RIP: 0010:strlen+0x0/0x20
> [   30.947616] Code: b6 07 38 d0 74 16 48 83 c7 01 84 c0 74 05 48 39 f7 75
> ec 31 c0 31 d2 89 d6 89 d7 c3 48 89 f8 31 d2 89 d6 89 d7 c3 0
> f 1f 40 00 <80> 3f 00 74 12 48 89 f8 48 83 c0 01 80 38 00 75 f7 48 29 f8 31
> ff
> [   30.947782] RSP: 0018:ffffa4ed80ffbbb8 EFLAGS: 00010246
> [   30.947836] RAX: 0000000000000000 RBX: ffffa4ed80ffbc60 RCX:
> 0000000000000000
> [   30.947904] RDX: 0000000000000000 RSI: 0000000000000000 RDI:
> 0000000000000000
> [   30.947971] RBP: ffff94b0d15c0ae0 R08: 0000000000000000 R09:
> 0000000000000000
> [   30.948040] R10: 0000000000000000 R11: 0000000000000000 R12:
> 0000000000000000
> [   30.948106] R13: 0000000000000001 R14: ffffa4ed80ffbc60 R15:
> 0000000000000000
> [   30.948174] FS:  00007fc7520f0740(0000) GS:ffff94b7ced40000(0000)
> knlGS:0000000000000000
> [   30.948252] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> [   30.948308] CR2: 0000000000000000 CR3: 0000000104a40001 CR4:
> 00000000003706e0
> [   30.948376] Call Trace:
> [   30.948404]  <TASK>
> [   30.948431]  ceph_security_init_secctx+0x7b/0x240 [ceph
> 49f9c4b9bf5be8760f19f1747e26da33920bce4b]
> [   30.948582]  ceph_atomic_open+0x51e/0x8a0 [ceph
> 49f9c4b9bf5be8760f19f1747e26da33920bce4b]
> [   30.948708]  ? get_cached_acl+0x4d/0xa0
> [   30.948759]  path_openat+0x60d/0x1030
> [   30.948809]  do_filp_open+0xa5/0x150
> [   30.948859]  do_sys_openat2+0xc4/0x190
> [   30.948904]  __x64_sys_openat+0x53/0xa0
> [   30.948948]  do_syscall_64+0x5c/0x90
> [   30.948989]  ? exc_page_fault+0x72/0x180
> [   30.949034]  entry_SYSCALL_64_after_hwframe+0x44/0xae
> [   30.949091] RIP: 0033:0x7fc7521e25bb
> [   30.950849] Code: 25 00 00 41 00 3d 00 00 41 00 74 4b 64 8b 04 25 18 00
> 00 00 85 c0 75 67 44 89 e2 48 89 ee bf 9c ff ff ff b8 01 01 0
> 0 00 0f 05 <48> 3d 00 f0 ff ff 0f 87 91 00 00 00 48 8b 54 24 28 64 48 2b 14
> 25
> 
> Core of the problem is that ceph checks for return code from
> security_dentry_init_security() and if return code is 0, it assumes
> everything is fine and continues to call strlen(name), which crashes.
> 
> Typically SELinux LSM returns 0 and sets name to "security.selinux" and
> it is not a problem. Or if selinux is not compiled in or disabled, it
> returns -EOPNOTSUP and ceph deals with it.
> 
> But somehow in this configuration, 0 is being returned and "name" is
> not being initialized and that's creating the problem.
> 
> Our suspicion is that BPF LSM is registering a hook for
> dentry_init_security() and returns hook default of 0.
> 
> LSM_HOOK(int, 0, dentry_init_security, struct dentry *dentry,...)
> 
> I have not been able to reproduce it just by doing CONFIG_BPF_LSM=y.
> Stephen has tested the patch though and confirms it solves the problem
> for him.
> 
> dentry_init_security() is written in such a way that it expects only one
> LSM to register the hook. Atleast that's the expectation with current code.
> 
> If another LSM returns a hook and returns default, it will simply return
> 0 as of now and that will break ceph. 
> 
> Hence, suggestion is that change semantics of this hook a bit. If there
> are no LSMs or no LSM is taking ownership and initializing security context,
> then return -EOPNOTSUP. Also allow at max one LSM to initialize security
> context. This hook can't deal with multiple LSMs trying to init security
> context. This patch implements this new behavior.
> 
> Reported-by: Stephen Muth <smuth4 at gmail.com>
> Tested-by: Stephen Muth <smuth4 at gmail.com>
> Suggested-by: Casey Schaufler <casey at schaufler-ca.com>
> Acked-by: Casey Schaufler <casey at schaufler-ca.com>
> Reviewed-by: Serge Hallyn <serge at hallyn.com>
> Cc: Jeff Layton <jlayton at kernel.org>
> Cc: Christian Brauner <brauner at kernel.org>
> Cc: Paul Moore <paul at paul-moore.com>
> Cc: <stable at vger.kernel.org> # 5.16.0
> Signed-off-by: Vivek Goyal <vgoyal at redhat.com>
> ---

Looks good,
Acked-by: Christian Brauner <brauner at kernel.org>