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* [PATCH] proc: restrict kernel stack dumps to root
@ 2018-09-11 18:39 Jann Horn
  2018-09-12 15:29 ` Jann Horn
  0 siblings, 1 reply; 8+ messages in thread
From: Jann Horn @ 2018-09-11 18:39 UTC (permalink / raw)
  To: Alexey Dobriyan, jannh
  Cc: Ken Chen, linux-kernel, linux-fsdevel, Will Deacon, Laura Abbott,
	Andy Lutomirski, security, Catalin Marinas, Josh Poimboeuf,
	Thomas Gleixner, Ingo Molnar, H. Peter Anvin

Restrict the ability to inspect kernel stacks of arbitrary tasks to root
in order to prevent a local attacker from exploiting racy stack unwinding
to leak kernel task stack contents.
See the added comment for a longer rationale.

There don't seem to be any users of this userspace API that can't
gracefully bail out if reading from the file fails. Therefore, I believe
that this change is unlikely to break things.
In the case that this patch does end up needing a revert, the next-best
solution might be to fake a single-entry stack based on wchan.

Fixes: 2ec220e27f50 ("proc: add /proc/*/stack")
Cc: stable@vger.kernel.org
Signed-off-by: Jann Horn <jannh@google.com>
---
 fs/proc/base.c | 14 ++++++++++++++
 1 file changed, 14 insertions(+)

diff --git a/fs/proc/base.c b/fs/proc/base.c
index ccf86f16d9f0..7e9f07bf260d 100644
--- a/fs/proc/base.c
+++ b/fs/proc/base.c
@@ -407,6 +407,20 @@ static int proc_pid_stack(struct seq_file *m, struct pid_namespace *ns,
 	unsigned long *entries;
 	int err;
 
+	/*
+	 * The ability to racily run the kernel stack unwinder on a running task
+	 * and then observe the unwinder output is scary; while it is useful for
+	 * debugging kernel issues, it can also allow an attacker to leak kernel
+	 * stack contents.
+	 * Doing this in a manner that is at least safe from races would require
+	 * some work to ensure that the remote task can not be scheduled; and
+	 * even then, this would still expose the unwinder as local attack
+	 * surface.
+	 * Therefore, this interface is restricted to root.
+	 */
+	if (!file_ns_capable(m->file, &init_user_ns, CAP_SYS_ADMIN))
+		return -EACCES;
+
 	entries = kmalloc_array(MAX_STACK_TRACE_DEPTH, sizeof(*entries),
 				GFP_KERNEL);
 	if (!entries)
-- 
2.19.0.rc2.392.g5ba43deb5a-goog

^ permalink raw reply	[flat|nested] 8+ messages in thread

end of thread, other threads:[~2018-09-27  8:19 UTC | newest]

Thread overview: 8+ messages (download: mbox.gz / follow: Atom feed)
-- links below jump to the message on this page --
2018-09-11 18:39 [PATCH] proc: restrict kernel stack dumps to root Jann Horn
2018-09-12 15:29 ` Jann Horn
2018-09-12 22:27   ` Kees Cook
2018-09-12 22:47     ` Laura Abbott
2018-09-13 11:55     ` Jann Horn
2018-09-13 14:39       ` Kees Cook
2018-09-27  1:19         ` Jann Horn
2018-09-27  2:03           ` Kees Cook

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