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

[Mimi Zohar]

Hi Scott,

On Tue, 2024-05-07 at 01:25 +0000, 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.

I would move the problem description before the traceback.

> 
> 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>

Reviewed-by: Mimi Zohar <zohar at linux.ibm.com>

thanks,

Mimi