On 8/6/26 08:55, Jann Horn wrote:
AppArmor's begin_current_label_crit_section() is a scary function called
from lots of LSM hooks (in particular VFS/socket-related ones) that checks
if the label referenced by the current creds is marked FLAG_STALE, and if
so, attempts to use aa_replace_current_label() to replace the creds with an
updated version that uses a new label.
The first problem with this is that it would directly lead to UAF of
`struct cred` if anything in the kernel takes a pointer to the current
creds and accesses these past a security hook invocation that replaces
creds, like so:
```
const struct cred *cred = current_cred();
alloc_file_pseudo(...);
uid_t uid = cred->euid;
```
I don't know if anything in the kernel actually does this, but I think it
is very surprising that this pattern could lead to UAF.
The second problem is that things go wrong when aa_replace_current_label()
runs with overridden credentials. aa_replace_current_label() bails out if
`current_cred() != current_real_cred()` (mirroring the check in
proc_pid_attr_write()), but this check can't actually reliably detect
overridden credentials because the overridden creds can be the same as the
objective creds.
So in approximately the following scenario, things go wrong:
1. task begins with <creds A> (as both objective and subjective creds),
with refcount=2
2. task grabs an extra reference on <creds A> for overriding
3. task calls override_creds(<creds A>), which returns a pointer to the old
subjective creds (<creds A>)
4. task enters AppArmor LSM hook
5. AppArmor checks that objective/subjective creds are equal
6. AppArmor replaces both cred pointers with <creds B> and drops 2 refs on
<creds A>
7. task leaves AppArmor LSM hook
8. task calls revert_creds(<creds A>)
9. now task->cred is <creds A> while task->real_cred is <creds B>, but the
task_struct logically holds two references to <creds B>
10. another task drops the extra reference on <creds A> that was used for
overriding, refcount drops to 0
11. now task->real_cred points to freed creds
At this point, any access to current_cred() will be UAF.
I have a test case where I run aa-disable on a profile while a process
using that profile is blocked on splice() from a FUSE passthrough file into
a full pipe; after the profile update, the pipe becomes empty, splice()
resumes, the credentials go out of sync, and a subsequent getuid() syscall
results in a KASAN UAF splat.
To fix this, instead of directly replacing creds, do it via task_work that
will run at the end of the current syscall. (The point in time at which the
cred replacement happens should have no correctness impact; it is just a
performance optimization to avoid unnecessarily touching the refcount of
the new label.)
Note that AppArmor still performs direct cred replacements in the
sb_pivotroot LSM hook after this change, and that direct cred replacements
can still happen in VFS ->write() callbacks via proc_pid_attr_write().
There are two options for what to do with aa_dup_task_ctx(): Either
explicitly reset new->label_replacement_pending after the entire
aa_task_ctx has been copied, or switch to manually copying members over.
I am switching to manually copying members over because that should make
bugs more obvious.
Cc: [email protected]
Fixes: c75afcd153f6 ("AppArmor: contexts used in attaching policy to system
objects")
Signed-off-by: Jann Horn <[email protected]>
lgtm, and I have done some light testing. I have pushed this to apparmor-next
and will kick a more thorough set of testing
Acked-by: John Johansen <[email protected]>
---
Changes in v2:
- store task work in task security blob to simplify things
- remove cc to peterz, I'm no longer changing task_work implementation
- Link to v1:
https://patch.msgid.link/[email protected]
---
security/apparmor/include/cred.h | 6 +-----
security/apparmor/include/task.h | 15 +++++++++++----
security/apparmor/task.c | 27 +++++++++++++++++++++++++++
3 files changed, 39 insertions(+), 9 deletions(-)
diff --git a/security/apparmor/include/cred.h b/security/apparmor/include/cred.h
index 2b6098149b15..0e8b67159f56 100644
--- a/security/apparmor/include/cred.h
+++ b/security/apparmor/include/cred.h
@@ -222,13 +222,9 @@ static inline struct aa_label
*begin_current_label_crit_section(void)
{
struct aa_label *label = aa_current_raw_label();
- might_sleep();
-
if (label_is_stale(label)) {
label = aa_get_newest_label(label);
- if (aa_replace_current_label(label) == 0)
- /* task cred will keep the reference */
- aa_put_label(label);
+ aa_schedule_stale_label_replacement();
}
return label;
diff --git a/security/apparmor/include/task.h b/security/apparmor/include/task.h
index b1aaaf60fa8b..6f26758ca10f 100644
--- a/security/apparmor/include/task.h
+++ b/security/apparmor/include/task.h
@@ -21,15 +21,22 @@ static inline struct aa_task_ctx *task_ctx(struct
task_struct *task)
* @onexec: profile to transition to on next exec (MAY BE NULL)
* @previous: profile the task may return to (MAY BE NULL)
* @token: magic value the task must know for returning to @previous_profile
+ * @label_replacement_tw: for aa_schedule_stale_label_replacement()
+ * @label_replacement_pending: is @label_replacement_tw pending?
+ *
+ * When changing this, check if aa_dup_task_ctx() needs to be updated.
*/
struct aa_task_ctx {
struct aa_label *nnp;
struct aa_label *onexec;
struct aa_label *previous;
u64 token;
+ struct callback_head label_replacement_tw;
+ bool label_replacement_pending;
};
int aa_replace_current_label(struct aa_label *label);
+void aa_schedule_stale_label_replacement(void);
void aa_set_current_onexec(struct aa_label *label, bool stack);
int aa_set_current_hat(struct aa_label *label, u64 token);
int aa_restore_previous_label(u64 cookie);
@@ -56,10 +63,10 @@ static inline void aa_free_task_ctx(struct aa_task_ctx *ctx)
static inline void aa_dup_task_ctx(struct aa_task_ctx *new,
const struct aa_task_ctx *old)
{
- *new = *old;
- aa_get_label(new->nnp);
- aa_get_label(new->previous);
- aa_get_label(new->onexec);
+ new->nnp = aa_get_label(old->nnp);
+ new->onexec = aa_get_label(old->onexec);
+ new->previous = aa_get_label(old->previous);
+ new->token = old->token;
}
/**
diff --git a/security/apparmor/task.c b/security/apparmor/task.c
index b9fb3738124e..e16ff4130bc2 100644
--- a/security/apparmor/task.c
+++ b/security/apparmor/task.c
@@ -14,6 +14,7 @@
#include <linux/gfp.h>
#include <linux/ptrace.h>
+#include <linux/task_work.h>
#include "include/path.h"
#include "include/audit.h"
@@ -89,6 +90,32 @@ int aa_replace_current_label(struct aa_label *label)
return 0;
}
+static void aa_replace_stale_label_tw_func(struct callback_head *tw)
+{
+ struct aa_task_ctx *ctx = task_ctx(current);
+ struct aa_label *label;
+
+ ctx->label_replacement_pending = false;
+ label = aa_current_raw_label();
+ if (!label_is_stale(label))
+ return;
+ label = aa_get_newest_label(label);
+ aa_replace_current_label(label);
+ aa_put_label(label);
+}
+
+/* replace the current task's stale label on syscall return */
+void aa_schedule_stale_label_replacement(void)
+{
+ struct aa_task_ctx *ctx = task_ctx(current);
+
+ if (ctx->label_replacement_pending)
+ return;
+ init_task_work(&ctx->label_replacement_tw,
aa_replace_stale_label_tw_func);
+ if (task_work_add(current, &ctx->label_replacement_tw, TWA_RESUME) == 0)
+ ctx->label_replacement_pending = true;
+}
+
/**
* aa_set_current_onexec - set the tasks change_profile to happen onexec
---
base-commit: 0d839570765118029aa8bf4a95444c6a11aacf85
change-id: 20260714-fix-apparmor-cred-uaf-cc38ec2b38b7
Best regards,
--
Jann Horn <[email protected]>
