[kernel-hardening] Re: [PATCH v2 4/4] arm64/syscalls: Specific usage of verify_pre_usermode_state

[Russell King - ARM Linux <linux@armlinux.org.uk>]

On Thu, Mar 09, 2017 at 07:56:49AM -0800, Thomas Garnier wrote:
> On Thu, Mar 9, 2017 at 4:23 AM, Mark Rutland <mark.rutland@arm.com> wrote:
> > We generally stick to lower case for the arm64 assembly macros. If we
> > need this, we should stick to the existing convention.
> >
> >> +/* Similar to set_fs(USER_DS) in verify_pre_usermode_state without a warning. */
> >> +.macro VERIFY_PRE_USERMODE_STATE
> >> +     mov     x1, #TASK_SIZE_64
> >> +     str     x1, [tsk, #TSK_TI_ADDR_LIMIT]
> >> +.endm
> >
> > We need arm64's set_fs() to configure UAO, too, so this is much weaker
> > than set_fs(), and will leave __{get,put}_user and
> > __copy_{to,from}_user() able to access kernel memory.
> >
> > We don't currently have an asm helper to clear UAO, and unconditionally
> > poking that on exception return is liable to be somewhat expensive.
> >
> > Also, given we're only trying to catch this in syscalls, I'm afraid I
> > don't see what we gain by doing this in the entry assembly.
> >
> 
> I optimized all architectures from the arm (32-bit) discussion. I will
> come back to a simple bl to the verify function. Thanks!

I wouldn't call what you've done on ARM an "optimisation", because my
comment about making the fast path worthless still stands.

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