On Sat, Jan 18, 2020 at 02:19:08AM +0100, Christian Brauner wrote:
> Commit 69f594a38967 ("ptrace: do not audit capability check when outputing /proc/pid/stat")
> introduced the ability to opt out of audit messages for accesses to various
> proc files since they are not violations of policy. While doing so it
> somehow switched the check from ns_capable() to
> has_ns_capability{_noaudit}(). That means it switched from checking the
> subjective credentials of the task to using the objective credentials. This
> is wrong since. ptrace_has_cap() is currently only used in
> ptrace_may_access() And is used to check whether the calling task (subject)
> has the CAP_SYS_PTRACE capability in the provided user namespace to operate
> on the target task (object). According to the cred.h comments this would
> mean the subjective credentials of the calling task need to be used.
> This switches ptrace_has_cap() to use security_capable(). Because we only
> call ptrace_has_cap() in ptrace_may_access() and in there we already have a
> stable reference to the calling task's creds under rcu_read_lock() there's
> no need to go through another series of dereferences and rcu locking done
> in ns_capable{_noaudit}().
>
> As one example where this might be particularly problematic, Jann pointed
> out that in combination with the upcoming IORING_OP_OPENAT feature, this
> bug might allow unprivileged users to bypass the capability checks while
> asynchronously opening files like /proc/*/mem, because the capability
> checks for this would be performed against kernel credentials.
>
> To illustrate on the former point about this being exploitable: When
> io_uring creates a new context it records the subjective credentials of the
> caller. Later on, when it starts to do work it creates a kernel thread and
> registers a callback. The callback runs with kernel creds for
> ktask->real_cred and ktask->cred. To prevent this from becoming a
> full-blown 0-day io_uring will call override_cred() and override
> ktask->cred with the subjective credentials of the creator of the io_uring
> instance. With ptrace_has_cap() currently looking at ktask->real_cred this
> override will be ineffective and the caller will be able to open arbitray
> proc files as mentioned above.
> Luckily, this is currently not exploitable but will turn into a 0-day once
> IORING_OP_OPENAT{2} land in v5.6. Fix it now!
>
> Cc: Oleg Nesterov <oleg@xxxxxxxxxx>
> Cc: Eric Paris <eparis@xxxxxxxxxx>
> Cc: stable@xxxxxxxxxxxxxxx
> Reviewed-by: Kees Cook <keescook@xxxxxxxxxxxx>
Looks good! Just to close the loop, yes, my Reviewed-by can stand. :)
-Kees
> Reviewed-by: Serge Hallyn <serge@xxxxxxxxxx>
> Reviewed-by: Jann Horn <jannh@xxxxxxxxxx>
> Fixes: 69f594a38967 ("ptrace: do not audit capability check when outputing /proc/pid/stat")
> Signed-off-by: Christian Brauner <christian.brauner@xxxxxxxxxx>
> ---
> /* v1 */
> Link: https://lore.kernel.org/r/20200115171736.16994-1-christian.brauner@xxxxxxxxxx
>
> /* v2 */
> Link: https://lore.kernel.org/r/20200116224518.30598-1-christian.brauner@xxxxxxxxxx
> - Christian Brauner <christian.brauner@xxxxxxxxxx>:
> - fix incorrect CAP_OPT_NOAUDIT, CAPT_OPT_NONE order
>
> /* v3 */
> Link: https://lore.kernel.org/r/20200117105717.29803-1-christian.brauner@xxxxxxxxxx
> - Kees Cook <keescook@xxxxxxxxxxxx>:
> - remove misleading reference to cread guard mutex from commit message
> - replace if-branches with ternary ?: operator
>
> /* v4 */
> - Kees Cook <keescook@xxxxxxxxxxxx>:
> - use security_capable() == 0 on return
> - Christian Brauner <christian.brauner@xxxxxxxxxx>:
> - replace ?: operator with if-branches since we need to check against 0.
> This makes it more legible.
> ---
> kernel/ptrace.c | 15 ++++++++++-----
> 1 file changed, 10 insertions(+), 5 deletions(-)
>
> diff --git a/kernel/ptrace.c b/kernel/ptrace.c
> index cb9ddcc08119..43d6179508d6 100644
> --- a/kernel/ptrace.c
> +++ b/kernel/ptrace.c
> @@ -264,12 +264,17 @@ static int ptrace_check_attach(struct task_struct *child, bool ignore_state)
> return ret;
> }
>
> -static int ptrace_has_cap(struct user_namespace *ns, unsigned int mode)
> +static bool ptrace_has_cap(const struct cred *cred, struct user_namespace *ns,
> + unsigned int mode)
> {
> + int ret;
> +
> if (mode & PTRACE_MODE_NOAUDIT)
> - return has_ns_capability_noaudit(current, ns, CAP_SYS_PTRACE);
> + ret = security_capable(cred, ns, CAP_SYS_PTRACE, CAP_OPT_NOAUDIT);
> else
> - return has_ns_capability(current, ns, CAP_SYS_PTRACE);
> + ret = security_capable(cred, ns, CAP_SYS_PTRACE, CAP_OPT_NONE);
> +
> + return ret == 0;
> }
>
> /* Returns 0 on success, -errno on denial. */
> @@ -321,7 +326,7 @@ static int __ptrace_may_access(struct task_struct *task, unsigned int mode)
> gid_eq(caller_gid, tcred->sgid) &&
> gid_eq(caller_gid, tcred->gid))
> goto ok;
> - if (ptrace_has_cap(tcred->user_ns, mode))
> + if (ptrace_has_cap(cred, tcred->user_ns, mode))
> goto ok;
> rcu_read_unlock();
> return -EPERM;
> @@ -340,7 +345,7 @@ static int __ptrace_may_access(struct task_struct *task, unsigned int mode)
> mm = task->mm;
> if (mm &&
> ((get_dumpable(mm) != SUID_DUMP_USER) &&
> - !ptrace_has_cap(mm->user_ns, mode)))
> + !ptrace_has_cap(cred, mm->user_ns, mode)))
> return -EPERM;
>
> return security_ptrace_access_check(task, mode);
>
> base-commit: b3a987b0264d3ddbb24293ebff10eddfc472f653
> --
> 2.25.0
>
--
Kees Cook
