Re: [PATCH v2] apparmor: fix cred UAF caused by begin_current_label_crit_section()

[John Johansen <john.johansen@canonical.com>]

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: stable@vger.kernel.org
> Fixes: c75afcd153f6 ("AppArmor: contexts used in attaching policy to system objects")
> Signed-off-by: Jann Horn <jannh@google.com>

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 <john.johansen@canonical.com>



> ---
> 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/20260714-fix-apparmor-cred-uaf-v1-1-be40e8c83b90@google.com
> ---
>   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 <jannh@google.com>
>