On Thu, Jan 16, 2020 at 11:45:18PM +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. I couldn't find the original lkml thread and so I don't know why this switch was done. But it seems wrong since ptrace_has_cap() is currently only used in ptrace_may_access(). And it's 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.
I don't follow this description. As far as I can see, both the current code and your patch end up using current's cred, yes? I'm not following the subjective/objective change mentioned here.
Before: bool has_ns_capability(struct task_struct *t, struct user_namespace *ns, int cap) { int ret;
rcu_read_lock(); ret = security_capable(__task_cred(t), ns, cap, CAP_OPT_NONE); rcu_read_unlock();
return (ret == 0); } ... return has_ns_capability(current, ns, CAP_SYS_PTRACE)
After: const struct cred *cred = current_cred(), ... ... return security_capable(cred, ns, CAP_SYS_PTRACE, CAP_OPT_NOAUDIT);
The cred passed to security_capable() is the subject before and after.
This switches it 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 tasks creds under cred_guard_mutex so there's no need to go through another series of dereferences and rcu locking done in ns_capable{_noaudit}().
This makes sense to me -- now there's no possible race on the cred changing between the two ptrace_has_cap() checks, yes?
However, I'm still trying to see where cred_guard_mutex() comes into play for callers of ptrace_may_access(). I see it for the object ("task" arg in ptrace_may_access()), but if this is dealing with the cred on current, it's just the RCU read lock protecting it (which I think is fine here), but seems confusing in the commit log.
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.
As in, winning a race between the two ptrace_has_cap() calls across a cred transition?
Cc: Oleg Nesterov oleg@redhat.com Cc: Eric Paris eparis@redhat.com Cc: stable@vger.kernel.org Reviewed-by: Serge Hallyn serge@hallyn.com Reviewed-by: Jann Horn jannh@google.com Fixes: 69f594a38967 ("ptrace: do not audit capability check when outputing /proc/pid/stat") Signed-off-by: Christian Brauner christian.brauner@ubuntu.com
kernel/ptrace.c | 11 ++++++----- 1 file changed, 6 insertions(+), 5 deletions(-)
diff --git a/kernel/ptrace.c b/kernel/ptrace.c index cb9ddcc08119..d146133e97f1 100644 --- a/kernel/ptrace.c +++ b/kernel/ptrace.c @@ -264,12 +264,13 @@ 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 int ptrace_has_cap(const struct cred *cred, struct user_namespace *ns,
unsigned int mode){ if (mode & PTRACE_MODE_NOAUDIT)
return has_ns_capability_noaudit(current, ns, CAP_SYS_PTRACE);
return security_capable(cred, ns, CAP_SYS_PTRACE, CAP_OPT_NOAUDIT);
return has_ns_capability(current, ns, CAP_SYS_PTRACE);
return security_capable(cred, ns, CAP_SYS_PTRACE, CAP_OPT_NONE);}
Style nit -- can we just make this a single invocation of security_capable(), something like:
return security_capable(cred, ns, CAP_SYS_PTRACE, mode & PTRACE_MODE_NOAUDIT ? CAP_OPT_NOAUDIT, : CAP_OPT_NONE) == 0;
Obviously not required, but the longer if hurts my eyes. ;)
/* Returns 0 on success, -errno on denial. */ @@ -321,7 +322,7 @@ 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 +341,7 @@ 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
So, I think this change looks correct, but I find the commit subject and log confusing (perhaps because I am dense) and misleading (again, perhaps because I am dense).