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From: Jann Horn <>
To: Alexey Dobriyan <>,
Cc: Ken Chen <>,,,
	Will Deacon <>,
	Laura Abbott <>,
	Andy Lutomirski <>,, Catalin Marinas <>,
	Josh Poimboeuf <>,
	Thomas Gleixner <>,
	Ingo Molnar <>, "H. Peter Anvin" <>
Subject: [PATCH] proc: restrict kernel stack dumps to root
Date: Tue, 11 Sep 2018 20:39:09 +0200	[thread overview]
Message-ID: <> (raw)

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 <>
 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)

             reply	other threads:[~2018-09-11 23:39 UTC|newest]

Thread overview: 8+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2018-09-11 18:39 Jann Horn [this message]
2018-09-12 15:29 ` [PATCH] proc: restrict kernel stack dumps to root 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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