[PATCH v3] ima: Avoid blocking in RCU read-side critical section

[Paul Moore]

On Fri, Jun 14, 2024 at 5:50 AM Roberto Sassu
<roberto.sassu at huaweicloud.com> wrote:
> On Thu, 2024-06-13 at 14:26 -0400, Paul Moore wrote:
> > On Wed, Jun 12, 2024 at 5:43 PM Paul Moore <paul at paul-moore.com> wrote:
> > > On May  6, 2024 GUO Zihua <guozihua at huawei.com> wrote:
> > > >
> > > > A panic happens in ima_match_policy:
> > > >
> > > > BUG: unable to handle kernel NULL pointer dereference at 0000000000000010
> > > > PGD 42f873067 P4D 0
> > > > Oops: 0000 [#1] SMP NOPTI
> > > > CPU: 5 PID: 1286325 Comm: kubeletmonit.sh Kdump: loaded Tainted: P
> > > > Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 0.0.0 02/06/2015
> > > > RIP: 0010:ima_match_policy+0x84/0x450
> > > > Code: 49 89 fc 41 89 cf 31 ed 89 44 24 14 eb 1c 44 39 7b 18 74 26 41 83 ff 05 74 20 48 8b 1b 48 3b 1d f2 b9 f4 00 0f 84 9c 01 00 00 <44> 85 73 10 74 ea 44 8b 6b 14 41 f6 c5 01 75 d4 41 f6 c5 02 74 0f
> > > > RSP: 0018:ff71570009e07a80 EFLAGS: 00010207
> > > > RAX: 0000000000000000 RBX: 0000000000000000 RCX: 0000000000000200
> > > > RDX: ffffffffad8dc7c0 RSI: 0000000024924925 RDI: ff3e27850dea2000
> > > > RBP: 0000000000000000 R08: 0000000000000000 R09: ffffffffabfce739
> > > > R10: ff3e27810cc42400 R11: 0000000000000000 R12: ff3e2781825ef970
> > > > R13: 00000000ff3e2785 R14: 000000000000000c R15: 0000000000000001
> > > > FS:  00007f5195b51740(0000) GS:ff3e278b12d40000(0000) knlGS:0000000000000000
> > > > CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> > > > CR2: 0000000000000010 CR3: 0000000626d24002 CR4: 0000000000361ee0
> > > > DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
> > > > DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
> > > > Call Trace:
> > > >  ima_get_action+0x22/0x30
> > > >  process_measurement+0xb0/0x830
> > > >  ? page_add_file_rmap+0x15/0x170
> > > >  ? alloc_set_pte+0x269/0x4c0
> > > >  ? prep_new_page+0x81/0x140
> > > >  ? simple_xattr_get+0x75/0xa0
> > > >  ? selinux_file_open+0x9d/0xf0
> > > >  ima_file_check+0x64/0x90
> > > >  path_openat+0x571/0x1720
> > > >  do_filp_open+0x9b/0x110
> > > >  ? page_counter_try_charge+0x57/0xc0
> > > >  ? files_cgroup_alloc_fd+0x38/0x60
> > > >  ? __alloc_fd+0xd4/0x250
> > > >  ? do_sys_open+0x1bd/0x250
> > > >  do_sys_open+0x1bd/0x250
> > > >  do_syscall_64+0x5d/0x1d0
> > > >  entry_SYSCALL_64_after_hwframe+0x65/0xca
> > > >
> > > > Commit c7423dbdbc9e ("ima: Handle -ESTALE returned by
> > > > ima_filter_rule_match()") introduced call to ima_lsm_copy_rule within a
> > > > RCU read-side critical section which contains kmalloc with GFP_KERNEL.
> > > > This implies a possible sleep and violates limitations of RCU read-side
> > > > critical sections on non-PREEMPT systems.
> > > >
> > > > Sleeping within RCU read-side critical section might cause
> > > > synchronize_rcu() returning early and break RCU protection, allowing a
> > > > UAF to happen.
> > > >
> > > > The root cause of this issue could be described as follows:
> > > > >     Thread A        |       Thread B        |
> > > > >                     |ima_match_policy       |
> > > > >                     |  rcu_read_lock        |
> > > > > ima_lsm_update_rule  |                       |
> > > > >  synchronize_rcu    |                       |
> > > > >                     |    kmalloc(GFP_KERNEL)|
> > > > >                     |      sleep            |
> > > > ==> synchronize_rcu returns early
> > > > >  kfree(entry)               |                       |
> > > > >                     |    entry = entry->next|
> > > > ==> UAF happens and entry now becomes NULL (or could be anything).
> > > > >                     |    entry->action      |
> > > > ==> Accessing entry might cause panic.
> > > >
> > > > To fix this issue, we are converting all kmalloc that is called within
> > > > RCU read-side critical section to use GFP_ATOMIC.
> > > >
> > > > Fixes: c7423dbdbc9e ("ima: Handle -ESTALE returned by ima_filter_rule_match()")
> > > > Cc: stable at vger.kernel.org
> > > > Signed-off-by: GUO Zihua <guozihua at huawei.com>
> > > > Acked-by: John Johansen <john.johansen at canonical.com>
> > > > Reviewed-by: Mimi Zohar <zohar at linux.ibm.com>
> > > > Reviewed-by: Casey Schaufler <casey at schaufler-ca.com>
> > > > ---
> > > > v3:
> > > >   ima_lsm_copy_rule takes a GFP flag as input as well.
> > > > v2:
> > > >   Changed the audit_rule_init security hook to accept a new GFP flag, as
> > > > per Stephen's suggestion.
> > > >
> > > > ---
> > > >  include/linux/lsm_hook_defs.h       |  2 +-
> > > >  include/linux/security.h            |  5 +++--
> > > >  kernel/auditfilter.c                |  5 +++--
> > > >  security/apparmor/audit.c           |  6 +++---
> > > >  security/apparmor/include/audit.h   |  2 +-
> > > >  security/integrity/ima/ima_policy.c | 15 +++++++++------
> > > >  security/security.c                 |  6 ++++--
> > > >  security/selinux/include/audit.h    |  4 +++-
> > > >  security/selinux/ss/services.c      |  5 +++--
> > > >  security/smack/smack_lsm.c          |  3 ++-
> > > >  10 files changed, 32 insertions(+), 21 deletions(-)
> > >
> > > With the exception of one small gotcha (see below), this looks okay to
> > > me.  At Mimi's request I'm going to merge this into the LSM tree, via
> > > lsm/stable-6.10, where I'll give it a few days in linux-next before
> > > sending it up to Linus.
> >
> > I also had to apply the following fix to this patch to resolve the
> > !CONFIG_IMA_LSM_RULES ca
> > se ... grrrrr.
>
> Argh, sorry...

Not fun, but it happens.  Anyway, I was going to send this to Linus
today but I think I'm going to hold off until Monday as we likely lost
a day of linux-next testing due to the compilation bug.

-- 
paul-moore.com