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