Re: [PATCH] proc: restrict kernel stack dumps to root

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On 09/12/2018 03:27 PM, Kees Cook wrote:
On Wed, Sep 12, 2018 at 8:29 AM, Jann Horn <jannh@xxxxxxxxxx> wrote:
+linux-api, I guess

On Tue, Sep 11, 2018 at 8:39 PM Jann Horn <jannh@xxxxxxxxxx> 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@xxxxxxxxxxxxxxx
Signed-off-by: Jann Horn <jannh@xxxxxxxxxx>
---
  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;

In the past, we've avoided hard errors like this in favor of just
censoring the output. Do we want to be more cautious here? (i.e.
return 0 or a fuller seq_printf(m, "[<0>] privileged\n"); return 0;)


The -EACCES is a strong hint to run with root privileges which is
nice from an end user perspective. If we don't want to return an
actual error, I think the "privileged" message would be okay.

Laura

+
         entries = kmalloc_array(MAX_STACK_TRACE_DEPTH, sizeof(*entries),
                                 GFP_KERNEL);
         if (!entries)
--
2.19.0.rc2.392.g5ba43deb5a-goog


-Kees





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