From: Vivek Goyal vgoyal@redhat.com
commit 7f5056b9e7b71149bf11073f00a57fa1ac2921a9 upstream.
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@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@gmail.com Tested-by: Stephen Muth smuth4@gmail.com Suggested-by: Casey Schaufler casey@schaufler-ca.com Acked-by: Casey Schaufler casey@schaufler-ca.com Reviewed-by: Serge Hallyn serge@hallyn.com Cc: Jeff Layton jlayton@kernel.org Cc: Christian Brauner brauner@kernel.org Cc: Paul Moore paul@paul-moore.com Cc: stable@vger.kernel.org # 5.16.0 Signed-off-by: Vivek Goyal vgoyal@redhat.com Reviewed-by: Jeff Layton jlayton@kernel.org Acked-by: Paul Moore paul@paul-moore.com Acked-by: Christian Brauner brauner@kernel.org Signed-off-by: James Morris jmorris@namei.org Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org --- include/linux/lsm_hook_defs.h | 2 +- security/security.c | 15 +++++++++++++-- 2 files changed, 14 insertions(+), 3 deletions(-)
--- a/include/linux/lsm_hook_defs.h +++ b/include/linux/lsm_hook_defs.h @@ -82,7 +82,7 @@ LSM_HOOK(int, 0, sb_add_mnt_opt, const c int len, void **mnt_opts) 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, --- a/security/security.c +++ b/security/security.c @@ -1056,8 +1056,19 @@ 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);