Re: [PATCH resend] proc: restrict kernel stack dumps to root
From: Kees Cook <hidden>
Date: 2018-09-27 18:29:53
Also in:
linux-fsdevel, lkml
On Thu, Sep 27, 2018 at 8:33 AM, Jann Horn [off-list ref] wrote:
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>Acked-by: Kees Cook <redacted> -Kees
quoted hunk ↗ jump to hunk
--- 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), GFP_KERNEL); if (!entries) --2.19.0.rc2.392.g5ba43deb5a-goog
-- Kees Cook Pixel Security