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
