From: Jann Horn jannh@google.com Subject: proc: restrict kernel stack dumps to root
Currently, you can use /proc/self/task/*/stack to cause a stack walk on a task you control while it is running on another CPU. That means that the stack can change under the stack walker. The stack walker does have guards against going completely off the rails and into random kernel memory, but it can interpret random data from your kernel stack as instruction pointers and stack pointers. This can cause exposure of kernel stack contents to userspace.
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.
Link: http://lkml.kernel.org/r/20180927153316.200286-1-jannh@google.com Fixes: 2ec220e27f50 ("proc: add /proc/*/stack") Signed-off-by: Jann Horn jannh@google.com Acked-by: Kees Cook keescook@chromium.org Cc: Alexey Dobriyan adobriyan@gmail.com Cc: Ken Chen kenchen@google.com Cc: Will Deacon will.deacon@arm.com Cc: Laura Abbott labbott@redhat.com Cc: Andy Lutomirski luto@amacapital.net Cc: Catalin Marinas catalin.marinas@arm.com Cc: Josh Poimboeuf jpoimboe@redhat.com Cc: Thomas Gleixner tglx@linutronix.de Cc: Ingo Molnar mingo@redhat.com Cc: "H . Peter Anvin" hpa@zytor.com Cc: stable@vger.kernel.org Signed-off-by: Andrew Morton akpm@linux-foundation.org ---
fs/proc/base.c | 14 ++++++++++++++ 1 file changed, 14 insertions(+)
--- a/fs/proc/base.c~proc-restrict-kernel-stack-dumps-to-root +++ a/fs/proc/base.c @@ -407,6 +407,20 @@ static int proc_pid_stack(struct seq_fil 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) _