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From: Jann Horn <jannh@google.com>
To: Oleg Nesterov <oleg@redhat.com>,
	"Eric W . Biederman" <ebiederm@xmission.com>,
	jannh@google.com
Cc: Andrew Morton <akpm@linux-foundation.org>,
	Kees Cook <keescook@chromium.org>,
	David Howells <dhowells@redhat.com>,
	linux-kernel@vger.kernel.org
Subject: [PATCH] ptrace: restore smp_rmb() in __ptrace_may_access()
Date: Wed, 29 May 2019 13:31:57 +0200	[thread overview]
Message-ID: <20190529113157.227380-1-jannh@google.com> (raw)

Restore the read memory barrier in __ptrace_may_access() that was deleted
a couple years ago. Also add comments on this barrier and the one it pairs
with to explain why they're there (as far as I understand).

Fixes: bfedb589252c ("mm: Add a user_ns owner to mm_struct and fix ptrace permission checks")
Cc: stable@vger.kernel.org
Signed-off-by: Jann Horn <jannh@google.com>
---
(I have no clue whatsoever what the relevant tree for this is, but I
guess Oleg is the relevant maintainer?)

 kernel/cred.c   |  9 +++++++++
 kernel/ptrace.c | 10 ++++++++++
 2 files changed, 19 insertions(+)

diff --git a/kernel/cred.c b/kernel/cred.c
index 45d77284aed0..07e069d00696 100644
--- a/kernel/cred.c
+++ b/kernel/cred.c
@@ -450,6 +450,15 @@ int commit_creds(struct cred *new)
 		if (task->mm)
 			set_dumpable(task->mm, suid_dumpable);
 		task->pdeath_signal = 0;
+		/*
+		 * If a task drops privileges and becomes nondumpable,
+		 * the dumpability change must become visible before
+		 * the credential change; otherwise, a __ptrace_may_access()
+		 * racing with this change may be able to attach to a task it
+		 * shouldn't be able to attach to (as if the task had dropped
+		 * privileges without becoming nondumpable).
+		 * Pairs with a read barrier in __ptrace_may_access().
+		 */
 		smp_wmb();
 	}
 
diff --git a/kernel/ptrace.c b/kernel/ptrace.c
index 5710d07e67cf..e54452c2954b 100644
--- a/kernel/ptrace.c
+++ b/kernel/ptrace.c
@@ -324,6 +324,16 @@ static int __ptrace_may_access(struct task_struct *task, unsigned int mode)
 	return -EPERM;
 ok:
 	rcu_read_unlock();
+	/*
+	 * If a task drops privileges and becomes nondumpable (through a syscall
+	 * like setresuid()) while we are trying to access it, we must ensure
+	 * that the dumpability is read after the credentials; otherwise,
+	 * we may be able to attach to a task that we shouldn't be able to
+	 * attach to (as if the task had dropped privileges without becoming
+	 * nondumpable).
+	 * Pairs with a write barrier in commit_creds().
+	 */
+	smp_rmb();
 	mm = task->mm;
 	if (mm &&
 	    ((get_dumpable(mm) != SUID_DUMP_USER) &&
-- 
2.22.0.rc1.257.g3120a18244-goog


             reply	other threads:[~2019-05-29 11:33 UTC|newest]

Thread overview: 18+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-05-29 11:31 Jann Horn [this message]
2019-05-29 15:59 ` Eric W. Biederman
2019-05-29 16:01   ` Jann Horn
2019-05-29 16:21 ` Oleg Nesterov
2019-05-29 17:38   ` Jann Horn
2019-05-30  1:41     ` Eric W. Biederman
2019-05-31 15:04       ` Jann Horn
2019-05-30 10:34     ` Andrea Parri
2019-05-31  9:08       ` Peter Zijlstra
2019-05-30 12:05     ` Oleg Nesterov
2019-05-31  9:12       ` Peter Zijlstra
2019-05-31  9:55         ` Oleg Nesterov
2019-05-29 21:02   ` Jann Horn
2019-05-29 18:55 ` Kees Cook
2019-05-30 12:34 ` Oleg Nesterov
2019-05-31 11:56   ` Jann Horn
2019-05-31 13:35     ` Oleg Nesterov
2019-05-31 19:37       ` Jann Horn

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