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

[Mimi Zohar <zohar@linux.ibm.com>]

On Wed, 2024-05-08 at 10:32 +0200, Roberto Sassu wrote:
> On Fri, 2024-04-19 at 14:41 -0700, Stephen Smalley wrote:
> > On Wed, Apr 17, 2024 at 12:49 AM GUO Zihua <guozihua@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
> > >  ima_file_check+0x64/0x90
> > >  path_openat+0x571/0x1720
> > >  do_filp_open+0x9b/0x110
> > >  do_sys_open+0x1bd/0x250
> > >  do_syscall_64+0x5d/0x1d0
> > >  entry_SYSCALL_64_after_hwframe+0x65/0xca
> > > 
> > > (stack trace marked with ? is deleted)
> > > 
> > > 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@vger.kernel.org
> > > Signed-off-by: GUO Zihua <guozihua@huawei.com>
> > > ---
> > >  security/integrity/ima/ima_policy.c | 2 +-
> > >  security/selinux/ss/services.c      | 2 +-
> > >  2 files changed, 2 insertions(+), 2 deletions(-)
> > > 
> > > diff --git a/security/integrity/ima/ima_policy.c b/security/integrity/ima/ima_policy.c
> > > index c0556907c2e6..c0cf9b6a01f0 100644
> > > --- a/security/integrity/ima/ima_policy.c
> > > +++ b/security/integrity/ima/ima_policy.c
> > > @@ -410,7 +410,7 @@ static struct ima_rule_entry *ima_lsm_copy_rule(struct ima_rule_entry *entry)
> > >          * Immutable elements are copied over as pointers and data; only
> > >          * lsm rules can change
> > >          */
> > > -       nentry = kmemdup(entry, sizeof(*nentry), GFP_KERNEL);
> > > +       nentry = kmemdup(entry, sizeof(*nentry), GFP_ATOMIC);
> > >         if (!nentry)
> > >                 return NULL;
> > > 
> > > diff --git a/security/selinux/ss/services.c b/security/selinux/ss/services.c
> > > index e88b1b6c4adb..b7cfad1a2964 100644
> > > --- a/security/selinux/ss/services.c
> > > +++ b/security/selinux/ss/services.c
> > > @@ -3549,7 +3549,7 @@ int selinux_audit_rule_init(u32 field, u32 op, char *rulestr, void **vrule)
> > >                 return -EINVAL;
> > >         }
> > > 
> > > -       tmprule = kzalloc(sizeof(struct selinux_audit_rule), GFP_KERNEL);
> > > +       tmprule = kzalloc(sizeof(struct selinux_audit_rule), GFP_ATOMIC);
> > 
> > I would suggest passing in gfp flags from the callers and only using
> > GFP_ATOMIC for the particular call chain that requires atomic
> > allocations, or re-factoring the caller to perform the allocating
> > operations outside of the critical section.
> > Sidebar: the refactoring of the SELinux policy loading logic may have
> > made it possible to revisit the approaches here to permit holding a
> > reference to the policy from which the rule was derived so that we
> > don't have to return -ESTALE in this scenario.
> 
> That would be really nice, would simplify the code for us.
> 
> I was wondering if the label comparison would be still reliable after a
> policy update. I was thinking for example the case where a type is
> removed from the newer policy, and that type was used in an IMA rule.

Yes, I was concerned too.  We followed Stephen's suggestion to ignore the IMA
policy rules containing an unknown SELinux label.  The original discussion can
be found here: 
https://lore.kernel.org/linux-integrity/1432f617-424e-044c-4f78-47f1100262ae@tycho.nsa.gov/

thanks,

Mimi