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SubjectRE: [External] Re: [PATCH] LSM: Delete hooks in reverse order for avoiding race
Date

> -----Original Message-----
> From: Ondrej Mosnacek <omosnace@redhat.com>
> Sent: Wednesday, January 8, 2020 7:43 PM
>
> Hi,
>
> On Wed, Jan 8, 2020 at 9:51 AM Huaisheng Ye <yehs2007@zoho.com> wrote:
> > From: Huaisheng Ye <yehs1@lenovo.com>
> >
> > There is small possibility as race condition when selinux_disable has
> > been triggered. security_delete_hooks deletes all selinux hooks from
> > security_hook_heads, but there are some selinux functions which are
> > being called at the same time.
> >
> > Here is a panic accident scene from 4.18 based kernel,
> >
> > [ 26.654494] SELinux: Disabled at runtime.
> > [ 26.654507] BUG: unable to handle kernel NULL pointer dereference
> > at 0000000000000020
> > [ 26.654508] PGD 0 P4D 0
> > [ 26.654510] Oops: 0002 [#1] SMP NOPTI
> > [ 26.654512] CPU: 53 PID: 2614 Comm: systemd-cgroups Tainted: G
> > OE --------- - - 4.18.0-80.el8.x86_64 #1
> > [ 26.654512] Hardware name: Lenovo ThinkSystem SR850P
> > -[7D2H]-/-[7D2H]-, BIOS -[TEE145P-1.10]- 12/06/2019
> > [ 26.654519] RIP: 0010:selinux_socket_post_create+0x80/0x390
> > [ 26.654520] Code: e9 95 6a 89 00 bd 16 00 00 00 c7 44 24 04 01
> > 00 00 00 45 85 c0 0f 85 f6 00 00 00 8b 56 14 85 d2 0f 84 26 01 00
> > 00 89 54 24 04 <66> 41 89 6c 24 20 31 c0 41 89 54 24 1c 41 c6 44
> > 24 22 01 49 8b 4d
> > [ 26.654521] RSP: 0018:ffffbf515cc63e48 EFLAGS: 00010246
> > [ 26.654522] RAX: 0000000000000000 RBX: 0000000000000001 RCX:
> 0000000000000019
> > [ 26.654522] RDX: 0000000000000001 RSI: 0000000000000001 RDI:
> ffffffffab46f680
> > [ 26.654523] RBP: 0000000000000019 R08: 0000000000000000 R09:
> ffffbf515cc63e4c
> > [ 26.654523] R10: 0000000000000000 R11: 0000000000000000 R12:
> 0000000000000000
> > [ 26.654524] R13: ffff97d7bb6cbc80 R14: 0000000000000001 R15:
> ffff97d7bb6cbc80
> > [ 26.654525] FS: 00007f5c608ea380(0000) GS:ffff97d7bf140000(0000)
> knlGS:0000000000000000
> > [ 26.654525] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> > [ 26.654526] CR2: 0000000000000020 CR3: 0000011ebc934004 CR4:
> 00000000007606e0
> > [ 26.654527] DR0: 0000000000000000 DR1: 0000000000000000 DR2:
> 0000000000000000
> > [ 26.654528] DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7:
> 0000000000000400
> > [ 26.654528] PKRU: 55555554
> > [ 26.654528] Call Trace:
> > [ 26.654535] security_socket_post_create+0x42/0x60
> > [ 26.654537] SELinux: Unregistering netfilter hooks
> > [ 26.654542] __sock_create+0x106/0x1a0
> > [ 26.654545] __sys_socket+0x57/0xe0
> > [ 26.654547] __x64_sys_socket+0x16/0x20
> > [ 26.654551] do_syscall_64+0x5b/0x1b0
> > [ 26.654554] entry_SYSCALL_64_after_hwframe+0x65/0xca
> >
> > The root cause is that, selinux_inode_alloc_security has been deleted
> > firstly from security_hook_heads, so security_inode_alloc directly
> > return 0, that means the value of pointer inode->i_security equalling
> > to NULL.
> >
> > But selinux_socket_post_create hasn't been deleted at that moment, so
> > which would involked by mistake. Inside the function, pointer isec
> > needs to point to inode->i_security, then a NULL pointer defect happens.
> >
> > For current upstream kernel, because of commit
> > afb1cbe37440c7f38b9cf46fc331cc9dfd5cce21
> > the inode security has been moved out to LSM infrastructure from
> > individual security modules like selinux.
> >
> > But this patch still can be applied for solving similar issue when
> > security_delete_hooks has been used. Also for stable branch v4.19, the
> > inode security still need to be created in individual modules.
>
> Thank you for the patch, however there are already existing proposed patches to
> fix this issue, see [1], [2], and [3]. At the moment it looks like the SELinux
> hooks reorder approach ([1]) will be accepted as a temporary solution (the SELinux
> runtime disable is being deprecated [4] in favor of properly disabling SELinux
> by setting
> selinux=0 on the kernel command line).

Hi Ondrej,

Thanks for introduction, and yes "selinux=0" is the simplest and effective workaround
for this defect.
I used it before as workaround and the panic issue couldn't be reproduced again.

>
> Your approach unfortunately isn't robust (depends on assumptions about how hooks
> are ordered by LSMs) nor complete (even the inverse order still has some race
> conditions that may lead to a crash - e.g.
> selinux_bpf_map() vs. selinux_bpf_map_alloc()).
>
> Also, please, don't forget to Cc the LSM/SELinux mailing lists
> (linux-security-module@vger.kernel.org/selinux@vger.kernel.org,
> respectively) for patches related to the LSM framework/SELinux.

Got it, I am new to selinux module.

Cheers,
Huaisheng Ye
Lenovo

>
> [1]
> https://lore.kernel.org/selinux/20191209075756.123157-1-omosnace@redhat.com/
> T/
> [2]
> https://lore.kernel.org/selinux/20191211140833.939845-1-omosnace@redhat.com/
> T/
> [3]
> https://lore.kernel.org/selinux/20200107133154.588958-1-omosnace@redhat.com/
> T/
> [4]
> https://git.kernel.org/pub/scm/linux/kernel/git/pcmoore/selinux.git/commit/?
> h=next&id=89b223bfb8a89731bea4c84982b5d2ad7ba460e3
>
> >
> > The patch has been verified by Lenovo SR850P server through overnight
> > reboot cycles.
> >
> > Signed-off-by: Huaisheng Ye <yehs1@lenovo.com>
> > ---
> > include/linux/lsm_hooks.h | 2 +-
> > 1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/include/linux/lsm_hooks.h b/include/linux/lsm_hooks.h
> > index 20d8cf1..57cb2ac 100644
> > --- a/include/linux/lsm_hooks.h
> > +++ b/include/linux/lsm_hooks.h
> > @@ -2164,7 +2164,7 @@ static inline void security_delete_hooks(struct
> security_hook_list *hooks,
> > int i;
> >
> > for (i = 0; i < count; i++)
> > - hlist_del_rcu(&hooks[i].list);
> > + hlist_del_rcu(&hooks[count - 1 - i].list);
> > }
> > #endif /* CONFIG_SECURITY_SELINUX_DISABLE */
> >
> > --
> > 1.8.3.1
> >
> >
>
> --
> Ondrej Mosnacek <omosnace at redhat dot com> Software Engineer, Security
> Technologies Red Hat, Inc.

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