On Tue, Jan 29, 2019 at 2:07 AM Jann Horn <[email protected]> wrote:
> On Mon, Jan 28, 2019 at 10:35 PM Jens Axboe <[email protected]> wrote:
> > The submission queue (SQ) and completion queue (CQ) rings are shared
> > between the application and the kernel. This eliminates the need to
> > copy data back and forth to submit and complete IO.
> [...]
> > +static bool io_get_sqring(struct io_ring_ctx *ctx, struct sqe_submit *s)
> > +{
> > + struct io_sq_ring *ring = ctx->sq_ring;
> > + unsigned head;
> > +
> > + /*
> > + * The cached sq head (or cq tail) serves two purposes:
> > + *
> > + * 1) allows us to batch the cost of updating the user visible
> > + * head updates.
> > + * 2) allows the kernel side to track the head on its own, even
> > + * though the application is the one updating it.
> > + */
> > + head = ctx->cached_sq_head;
> > + smp_rmb();
> > + if (head == READ_ONCE(ring->r.tail))
> > + return false;
> > +
> > + head = ring->array[head & ctx->sq_mask];
> > + if (head < ctx->sq_entries) {
> > + s->index = head;
> > + s->sqe = &ctx->sq_sqes[head];
>
> ring->array can be mapped writable into userspace, right? If so: This
> looks like a double-read issue; the compiler might assume that
> ring->array is not modified concurrently and perform separate memory
> accesses for the "if (head < ctx->sq_entries)" check and the
> "&ctx->sq_sqes[head]" computation. Please use READ_ONCE()/WRITE_ONCE()
> for all accesses to memory that userspace could concurrently modify in
> a malicious way.
>
> There have been some pretty severe security bugs caused by missing
> READ_ONCE() annotations around accesses to shared memory; see, for
> example,
> https://www.blackhat.com/docs/us-16/materials/us-16-Wilhelm-Xenpwn-Breaking-Paravirtualized-Devices.pdf
> . Slides 35-48 show how the code "switch (op->cmd)", where "op" is a
> pointer to shared memory, allowed an attacker to break out of a Xen
> virtual machine because the compiler generated multiple memory
> accesses.
Oh, actually, it's even worse (comments with "//" added by me):
io_sq_thread() does this:
do {
// sqes[i].sqe is pointer to shared memory, result of
// io_sqe_needs_user() is unreliable
if (all_fixed && io_sqe_needs_user(sqes[i].sqe))
all_fixed = false;
i++;
if (i == ARRAY_SIZE(sqes))
break;
} while (io_get_sqring(ctx, &sqes[i]));
// sqes[...].sqe are pointers to shared memory
io_commit_sqring(ctx);
/* Unless all new commands are FIXED regions, grab mm */
if (!all_fixed && !cur_mm) {
mm_fault = !mmget_not_zero(ctx->sqo_mm);
if (!mm_fault) {
use_mm(ctx->sqo_mm);
cur_mm = ctx->sqo_mm;
}
}
inflight += io_submit_sqes(ctx, sqes, i, mm_fault);
Then the shared memory pointers go into io_submit_sqes():
static int io_submit_sqes(struct io_ring_ctx *ctx, struct sqe_submit *sqes,
unsigned int nr, bool mm_fault)
{
struct io_submit_state state, *statep = NULL;
int ret, i, submitted = 0;
// sqes[...].sqe are pointers to shared memory
[...]
for (i = 0; i < nr; i++) {
if (unlikely(mm_fault))
ret = -EFAULT;
else
ret = io_submit_sqe(ctx, &sqes[i], statep);
[...]
}
[...]
}
And on into io_submit_sqe():
static int io_submit_sqe(struct io_ring_ctx *ctx, struct sqe_submit *s,
struct io_submit_state *state)
{
[...]
ret = __io_submit_sqe(ctx, req, s, true, state);
[...]
}
And there it gets interesting:
static int __io_submit_sqe(struct io_ring_ctx *ctx, struct io_kiocb *req,
struct sqe_submit *s, bool force_nonblock,
struct io_submit_state *state)
{
// s->sqe is a pointer to shared memory
const struct io_uring_sqe *sqe = s->sqe;
// sqe is a pointer to shared memory
ssize_t ret;
if (unlikely(s->index >= ctx->sq_entries))
return -EINVAL;
req->user_data = sqe->user_data;
ret = -EINVAL;
// switch() on read from shared memory, potential instruction pointer
// control
switch (sqe->opcode) {
[...]
case IORING_OP_READV:
if (unlikely(sqe->buf_index))
return -EINVAL;
ret = io_read(req, sqe, force_nonblock, state);
break;
[...]
case IORING_OP_READ_FIXED:
ret = io_read(req, sqe, force_nonblock, state);
break;
[...]
}
[...]
}
On into io_read():
static ssize_t io_read(struct io_kiocb *req, const struct io_uring_sqe *sqe,
bool force_nonblock, struct io_submit_state *state)
{
[...]
// sqe is a pointer to shared memory
ret = io_prep_rw(req, sqe, force_nonblock, state);
[...]
}
And then io_prep_rw() does multiple reads even in the source code:
static int io_prep_rw(struct io_kiocb *req, const struct io_uring_sqe *sqe,
bool force_nonblock, struct io_submit_state *state)
{
struct io_ring_ctx *ctx = req->ctx;
struct kiocb *kiocb = &req->rw;
int ret;
// sqe is a pointer to shared memory
// double-read of sqe->flags, see end of function
if (sqe->flags & IOSQE_FIXED_FILE) {
// double-read of sqe->fd for the bounds check and the
array access, potential OOB pointer read
if (unlikely(!ctx->user_files || sqe->fd >= ctx->nr_user_files))
return -EBADF;
kiocb->ki_filp = ctx->user_files[sqe->fd];
req->flags |= REQ_F_FIXED_FILE;
} else {
kiocb->ki_filp = io_file_get(state, sqe->fd);
}
if (unlikely(!kiocb->ki_filp))
return -EBADF;
kiocb->ki_pos = sqe->off;
kiocb->ki_flags = iocb_flags(kiocb->ki_filp);
kiocb->ki_hint = ki_hint_validate(file_write_hint(kiocb->ki_filp));
// three reads of sqe->ioprio, bypassable capability check
if (sqe->ioprio) {
ret = ioprio_check_cap(sqe->ioprio);
if (ret)
goto out_fput;
kiocb->ki_ioprio = sqe->ioprio;
} else
kiocb->ki_ioprio = get_current_ioprio();
[...]
return 0;
out_fput:
// double-read of sqe->flags, changed value can lead to
unbalanced refcount
if (!(sqe->flags & IOSQE_FIXED_FILE))
io_file_put(state, kiocb->ki_filp);
return ret;
}
Please create a local copy of the request before parsing it to keep
the data from changing under you. Additionally, it might make sense to
annotate every pointer to shared memory with a comment, or something
like that, to ensure that anyone looking at the code can immediately
see for which pointers special caution is required on access.
