archive mirror
 help / color / mirror / Atom feed
* [PATCH resend] proc: restrict kernel stack dumps to root
@ 2018-09-27 15:33 Jann Horn
  2018-09-27 18:29 ` Kees Cook
  2018-09-27 22:29 ` Andrew Morton
  0 siblings, 2 replies; 4+ messages in thread
From: Jann Horn @ 2018-09-27 15:33 UTC (permalink / raw)
  To: Andrew Morton, jannh
  Cc: Kees Cook, Alexey Dobriyan, Ken Chen, kernel list, linux-fsdevel,
	Will Deacon, Laura Abbott, Andy Lutomirski, Security Officers,
	Catalin Marinas, Josh Poimboeuf, Thomas Gleixner, Ingo Molnar,
	H . Peter Anvin, Linux API

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")
Signed-off-by: Jann Horn <>
Resending because I forgot to send this to akpm the first time.

 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),
 	if (!entries)

^ permalink raw reply related	[flat|nested] 4+ messages in thread

end of thread, other threads:[~2018-09-28  4:49 UTC | newest]

Thread overview: 4+ messages (download: mbox.gz / follow: Atom feed)
-- links below jump to the message on this page --
2018-09-27 15:33 [PATCH resend] proc: restrict kernel stack dumps to root Jann Horn
2018-09-27 18:29 ` Kees Cook
2018-09-27 22:29 ` Andrew Morton
2018-09-27 22:39   ` Jann Horn

This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).