On Mon, Oct 28, 2019 at 12:57 PM Kees Cook <keescook@xxxxxxxxxxxx> wrote: > On Mon, Oct 28, 2019 at 04:35:33PM +0000, Mark Rutland wrote: > > On Fri, Oct 25, 2019 at 01:49:21PM -0700, Sami Tolvanen wrote: > > > To keep the address of the currently active shadow stack out of > > > memory, the arm64 implementation clears this field when it loads x18 > > > and saves the current value before a context switch. The generic code > > > doesn't expect the arch code to necessarily do so, but does allow it. > > > This requires us to use __scs_base() when accessing the base pointer > > > and to reset it in idle tasks before they're reused, hence > > > scs_task_reset(). > > > > Ok. That'd be worth a comment somewhere, since it adds a number of > > things which would otherwise be unnecessary. > > > > IIUC this assumes an adversary who knows the address of a task's > > thread_info, and has an arbitrary-read (to extract the SCS base from > > thead_info) and an arbitrary-write (to modify the SCS area). > > > > Assuming that's the case, I don't think this buys much. If said > > adversary controls two userspace threads A and B, they only need to wait > > until A is context-switched out or in userspace, and read A's SCS base > > using B. > > > > Given that, I'd rather always store the SCS base in the thread_info, and > > simplify the rest of the code manipulating it. > > I'd like to keep this as-is since it provides a temporal protection. > Having arbitrary kernel read and write at arbitrary time is a very > powerful attack primitive, and is, IMO, not very common. Many attacks > tend to be chains of bugs that give attackers narrow visibility in to the > kernel at specific moments. I would say this design is more about stopping > "current" from dumping thread_info (as there are many more opportunities > for current to see its own thread_info compared to arbitrary addresses > or another task's thread_info). As such, I think it's a reasonable > precaution to take. I'm not sure if always storing the base address in thread_info would simplify the code that much. We could remove __scs_base() and scs_task_reset(), which are both trivial, and drop a few instructions in the arch-specific code that clear the field. I do agree that a comment or two would help understand what's going on here though. Sami
