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

[Stephen Smalley <stephen.smalley.work@gmail.com>]

On Sun, Apr 21, 2024 at 9:26 PM Guozihua (Scott) <guozihua@huawei.com> wrote:
>
> On 2024/4/20 5:41, 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.
>
> Well unfortunately the SELinux involved in this patch is called via
> security hooks. Adding new flags would mean define macros as well as all
> the callers and implementations would have to be changed.

That is ok if it is what is required to fix this correctly. However,
if you can re-factor the caller to move the allocation outside of the
RCU read lock, or eliminate the need for an allocation altogether,
that would be better.

> > 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.
>
> This would be nice if we are able to make this re-init happen in-place
> which would spare IMA from initializing a temporary rule and the list
> replace behavior.

Yes, that would be best if possible.