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

[Mimi Zohar]

On Fri, 2024-05-10 at 09:03 -0700, Casey Schaufler wrote:
> On 5/6/2024 6:25 PM, GUO Zihua 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>
> 
> Looks fine from the Smack perspective.
> Reviewed-by: Casey Schaufler <casey at schaufler-ca.com>

Thanks, Casey. JJ? Paul?