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authorLinus Torvalds <torvalds@linux-foundation.org>2020-01-18 12:23:31 -0800
committerLinus Torvalds <torvalds@linux-foundation.org>2020-01-18 12:23:31 -0800
commit8cac89909a30807eb4aba56a0e29f55e3b6df42f (patch)
tree584499ed6c0bd64fca6d47281f3c949be0e3f104
parent2324de6fab2223287da7628ba92dc6cfed4f46ca (diff)
parent6b3ad6649a4c75504edeba242d3fd36b3096a57f (diff)
Merge tag 'for-linus-2020-01-18' of git://git.kernel.org/pub/scm/linux/kernel/git/brauner/linux
Pull thread fixes from Christian Brauner: "Here is an urgent fix for ptrace_may_access() permission checking. 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 switched the check from ns_capable() to has_ns_capability{_noaudit}(). That means it switched from checking the subjective credentials (ktask->cred) of the task to using the objective credentials (ktask->real_cred). This is appears to be wrong. 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 means the subjective credentials of the calling task need to be used. With this fix we switch ptrace_has_cap() to use security_capable() and thus back to using the subjective credentials. As one example where this might be particularly problematic, Jann pointed out that in combination with the upcoming IORING_OP_OPENAT{2} 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 would be so once IORING_OP_OPENAT{2} land in v5.6. Let's fix it now. To minimize potential regressions I successfully ran the criu testsuite. criu makes heavy use of ptrace() and extensively hits ptrace_may_access() codepaths and has a good change of detecting any regressions. Additionally, I succesfully ran the ptrace and seccomp kernel tests" * tag 'for-linus-2020-01-18' of git://git.kernel.org/pub/scm/linux/kernel/git/brauner/linux: ptrace: reintroduce usage of subjective credentials in ptrace_has_cap()
-rw-r--r--kernel/ptrace.c15
1 files 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 @@ ok:
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);