Re: "kernel NULL pointer dereference" crash when attempting a write

["Serge E. Hallyn" <serge@hallyn.com>]

On Tue, Jan 25, 2022 at 07:27:38PM -0500, Vivek Goyal wrote:
> On Tue, Jan 25, 2022 at 02:57:46PM -0500, Paul Moore wrote:
> > 
> > > > Looks like dentry_init_security() can't handle multiple LSMs. We probably
> > > > should disallow all other LSMs to register a hook for this and only
> > > > allow SELinux to register a hook.
> > >
> > > Not acceptable. The fix to dentry_init_security() is easy.
> > 
> > Sounds good to me, Vivek did you want to put together a patch for
> > this?  If not, let me know and I'll put one together.
> 
> Ok, I have put together this test patch. Stephen Muth, can you please
> test it and let us know if it solves your problem.
> 
> I enabled CONFIG_BPF_LSM=y but that itself does not seem to be sufficient
> for BPF to register a hook for dentry_init_security. So I don't see
> it being called in my testing. IOW, I have not been able to reproduce
> the issue and will rely on testing from Stephen to know if it it indeed
> solved the problem for him or not.
> 
> -------------------8<--------------------
> 
> Subject: lsm: dentry_init_security(): Deal with multiple LSMs registering hook
> 
> A ceph user has reported that ceph is crashing with kernel NULL pointer
> dereference. Following is backtrace.
> 
> /proc/version: Linux version 5.16.2-arch1-1 (linux@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. I have not been
> able to configure it that way so I am not 100% sure.
> 
> LSM_HOOK(int, 0, dentry_init_security, struct dentry *dentry,...)
> 
> 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. 
> 
> Anyway, 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@gmail.com>
> Suggested-by: Casey Schaufler <casey@schaufler-ca.com>
> Cc: Jeff Layton <jlayton@kernel.org>
> Cc: Christian Brauner <brauner@kernel.org>
> Cc: Paul Moore <paul@paul-moore.com>
> Yet-to-by-Signed-off-by: Vivek Goyal <vgoyal@redhat.com>

Reviewed-by: Serge Hallyn <serge@hallyn.com>

Seems good, and indeed we can't have >1 such LSMS writing to the
xattr.  Thanks.

> ---
>  include/linux/lsm_hook_defs.h |    2 +-
>  security/security.c           |   15 +++++++++++++--
>  2 files changed, 14 insertions(+), 3 deletions(-)
> 
> Index: redhat-linux/include/linux/lsm_hook_defs.h
> ===================================================================
> --- redhat-linux.orig/include/linux/lsm_hook_defs.h	2022-01-24 14:56:14.338030140 -0500
> +++ redhat-linux/include/linux/lsm_hook_defs.h	2022-01-25 18:48:46.917496696 -0500
> @@ -80,7 +80,7 @@ LSM_HOOK(int, 0, sb_clone_mnt_opts, cons
>  	 unsigned long *set_kern_flags)
>  LSM_HOOK(int, 0, move_mount, const struct path *from_path,
>  	 const struct path *to_path)
> -LSM_HOOK(int, 0, dentry_init_security, struct dentry *dentry,
> +LSM_HOOK(int, -EOPNOTSUPP, dentry_init_security, struct dentry *dentry,
>  	 int mode, const struct qstr *name, const char **xattr_name,
>  	 void **ctx, u32 *ctxlen)
>  LSM_HOOK(int, 0, dentry_create_files_as, struct dentry *dentry, int mode,
> Index: redhat-linux/security/security.c
> ===================================================================
> --- redhat-linux.orig/security/security.c	2022-01-25 18:46:59.166496696 -0500
> +++ redhat-linux/security/security.c	2022-01-25 18:56:25.251496696 -0500
> @@ -1048,8 +1048,19 @@ int security_dentry_init_security(struct
>  				  const char **xattr_name, void **ctx,
>  				  u32 *ctxlen)
>  {
> -	return call_int_hook(dentry_init_security, -EOPNOTSUPP, dentry, mode,
> -				name, xattr_name, ctx, ctxlen);
> +	struct security_hook_list *hp;
> +	int rc;
> +
> +	/*
> +	 * Only one module will provide a security context.
> +	 */
> +	hlist_for_each_entry(hp, &security_hook_heads.dentry_init_security, list) {
> +		rc = hp->hook.dentry_init_security(dentry, mode, name,
> +						   xattr_name, ctx, ctxlen);
> +		if (rc != LSM_RET_DEFAULT(dentry_init_security))
> +			return rc;
> +	}
> +	return LSM_RET_DEFAULT(dentry_init_security);
>  }
>  EXPORT_SYMBOL(security_dentry_init_security);
>