Re: [PATCH v4 1/4] seccomp: add a return code to trap to userspace

Jann Horn
2018-06-22T22:28:01Z

On Fri, Jun 22, 2018 at 11:51 PM Kees Cook <keescook@chromium.org> wrote:
>
> On Fri, Jun 22, 2018 at 11:09 AM, Andy Lutomirski <luto@amacapital.net> wrote:
> > One possible extra issue: IIRC /proc/.../mem uses FOLL_FORCE, which is not what we want here.

Uuugh, I forgot about that.

> > How about just adding an explicit “read/write the seccomp-trapped task’s memory” primitive? That should be easier than a “open mem fd” primitive.
>
> Uuugh. Can we avoid adding another "read/write remote process memory"
> interface? The point of this series was to provide a lightweight
> approach to what should normally be possible via the existing
> seccomp+ptrace interface. I do like Jann's context idea, but I agree
> with Andy: it can't be a handle to /proc/$pid/mem, since it's
> FOLL_FORCE. Is there any other kind of process context id we can use
> for this instead of pid? There was once an idea of pid-fd but it never
> landed... This would let us get rid of the "id" in the structure too.
> And if that existed, we could make process_vm_*v() safer too (taking a
> pid-fd instead of a pid).

Or make a duplicate of /proc/$pid/mem that only differs in whether it
sets FOLL_FORCE? The code is basically already there... something like
this:

diff --git a/fs/proc/base.c b/fs/proc/base.c
index 80aa42506b8b..e8a6a63046da 100644
--- a/fs/proc/base.c
+++ b/fs/proc/base.c
@@ -762,6 +762,8 @@ static int mem_open(struct inode *inode, struct file *file)
return ret;
}

+static const struct file_operations proc_mem_operations;
+
static ssize_t mem_rw(struct file *file, char __user *buf,
size_t count, loff_t *ppos, int write)
{
@@ -782,7 +784,10 @@ static ssize_t mem_rw(struct file *file, char __user *buf,
if (!mmget_not_zero(mm))
goto free;

- flags = FOLL_FORCE | (write ? FOLL_WRITE : 0);
+ flags = (write ? FOLL_WRITE : 0);
+ if (file->f_op == &proc_mem_operations) {
+ flags |= FOLL_FORCE;
+ }

while (count > 0) {
int this_len = min_t(int, count, PAGE_SIZE);
@@ -861,6 +866,14 @@ static const struct file_operations proc_mem_operations = {
.release = mem_release,
};

+static const struct file_operations proc_mem_noforce_operations = {
+ .llseek = mem_lseek,
+ .read = mem_read,
+ .write = mem_write,
+ .open = mem_open,
+ .release = mem_release,
+};
+
static int environ_open(struct inode *inode, struct file *file)
{
return __mem_open(inode, file, PTRACE_MODE_READ);
@@ -2916,6 +2929,7 @@ static const struct pid_entry tgid_base_stuff[] = {
REG("numa_maps", S_IRUGO, proc_pid_numa_maps_operations),
#endif
REG("mem", S_IRUSR|S_IWUSR, proc_mem_operations),
+ REG("mem_noforce", S_IRUSR|S_IWUSR, proc_mem_noforce_operations),
LNK("cwd", proc_cwd_link),
LNK("root", proc_root_link),
LNK("exe", proc_exe_link),
@@ -3302,6 +3316,7 @@ static const struct pid_entry tid_base_stuff[] = {
REG("numa_maps", S_IRUGO, proc_tid_numa_maps_operations),
#endif
REG("mem", S_IRUSR|S_IWUSR, proc_mem_operations),
+ REG("mem_noforce",S_IRUSR|S_IWUSR, proc_mem_noforce_operations),
LNK("cwd", proc_cwd_link),
LNK("root", proc_root_link),
LNK("exe", proc_exe_link),