Re: [PATCH] ima: fix deadlock within RCU list of ima_rules

Hi Simon,

Thanks for you help, i may got it, here is the new commit message:


The current IMA ruleset is identified by the variable "ima_rules"
that default to "&ima_default_rules". When loading a custom policy
for the first time, the variable is updated to "&ima_policy_rules"
instead. That update isn't RCU-safe, and deadlocks are possible.
Because some functions like ima_match_policy() may loop indefinitely
over traversing the "ima_default_rules" as list_for_each_entry_rcu().

When iterating over the default ruleset back to head, value of
"&entry->list" is "&ima_default_rules", and "ima_rules" may have been
updated to "&ima_policy_rules", the loop condition (&entry->list != ima_rules)
stay alway true, traversing doesn't terminate, cause soft lockup and
RCU stalls.

Introduce a temporary value for "ima_rules" when iterating over
the ruleset to avoid the deadlocks.


Signed-off-by: liqiong <redacted>
---
 security/integrity/ima/ima_policy.c | 17 ++++++++++++-----
 1 file changed, 12 insertions(+), 5 deletions(-)

diff --git a/security/integrity/ima/ima_policy.c b/security/integrity/ima/ima_policy.c
index fd5d46e511f1..e92b197bfd3c 100644
--- a/security/integrity/ima/ima_policy.c
+++ b/security/integrity/ima/ima_policy.c

Thanks

liqiong

在 2021年08月26日 17:01, THOBY Simon 写道:

Hi liqiong,

On 8/26/21 10:15 AM, liqiong wrote:

quoted

Hi Simon,

Thanks for your help, your advice is clear, can i just use it,
how about this:


The current IMA ruleset is identified by the variable "ima_rules",
and the pointer starts pointing at the list "ima_default_rules".

After reading it again, maybe
"The current IMA ruleset is identified by the variable "ima_rules",
that defaults to "&ima_default_rules".'?

quoted

When loading a custom policy for the first time, the variable is
updated to point to the list "ima_policy_rules" instead. That update
isn't RCU-safe, and deadlocks are possible.

I think what Mimi was trying to say is that you could add the high-level
overview above, but keep the details. Sorry if I wasn't clearer in my
earlier messages.

Consider re-adding your previous paragraph
"""
As a consequence, when traversing the ruleset, some functions like ima_match_policy()
may loop indefinitely over "ima_default_rules" as list_for_each_entry_rcu() doesn't
terminate (after the update, "ima_rules" no longer points to the list head, so the
loop condition stays always true), causing RCU stalls.
"""
(note: I tweaked it above, feel free to fix it)

quoted

Introduce a temporary value for "ima_rules" when iterating over
the ruleset to avoid the deadlocks.

... while keeping this a separate paragraph.

quoted

Signed-off-by: liqiong <redacted>
---
 security/integrity/ima/ima_policy.c | 17 ++++++++++++-----
 1 file changed, 12 insertions(+), 5 deletions(-)

diff --git a/security/integrity/ima/ima_policy.c b/security/integrity/ima/ima_policy.c
index fd5d46e511f1..e92b197bfd3c 100644
--- a/security/integrity/ima/ima_policy.c
+++ b/security/integrity/ima/ima_policy.c

Thanks

liqiong