Re: [PATCH] x86/asm: Pin sensitive CR4 bits

Jann Horn
2019-02-20T02:37:51Z

On Wed, Feb 20, 2019 at 1:55 AM Kees Cook <keescook@chromium.org> wrote:
> Several recent exploits have used direct calls to the native_write_cr4()
> function to disable SMEP and SMAP before then continuing their exploits
> using userspace memory access. This pins bits of cr4 so that they cannot
> be changed through a common function. This is not intended to be general
> ROP protection (which would require CFI to defend against properly), but
> rather a way to avoid trivial direct function calling (or CFI bypassing
> via a matching function prototype) as seen in:
>
> https://googleprojectzero.blogspot.com/2017/05/exploiting-linux-kernel-via-packet.html
> (https://github.com/xairy/kernel-exploits/tree/master/CVE-2017-7308)
>
> The goals of this change:
> - pin specific bits (SMEP, SMAP, and UMIP) when writing cr4.
> - avoid setting the bits too early (they must become pinned only after
> first being used).
> - pinning mask needs to be read-only during normal runtime.
> - pinning needs to be rechecked after set to avoid jumps into the middle
> of the function.
>
> Using __ro_after_init on the mask is done so it can't be first disabled
> with a malicious write. And since it becomes read-only, we must avoid
> writing to it later (hence the check for bits already having been set
> instead of unconditionally writing to the mask).
>
> The use of volatile is done to force the compiler to perform a full reload
> of the mask after setting cr4 (to protect against just jumping into the
> function past where the masking happens; we must check that the mask was
> applied after we do the set). Due to how this function can be built by the
> compiler (especially due to the removal of frame pointers), jumping into
> the middle of the function frequently doesn't require stack manipulation
> to construct a stack frame (there may only a retq without pops, which is
> sufficient for use with exploits like timer overwrites mentioned above).
>
> For example, without the recheck, the function may appear as:
>
> native_write_cr4:
> mov [pin], %rbx
> or %rbx, %rdi
> 1: mov %rdi, %cr4
> retq
>
> The masking "or" could be trivially bypassed by just calling to label "1"
> instead of "native_write_cr4". (CFI will force calls to only be able to
> call into native_write_cr4, but CFI and CET are uncommon currently.)
>
> Signed-off-by: Kees Cook <keescook@chromium.org>
> ---
[...]
> static inline void native_write_cr4(unsigned long val)
> {
> +again:
> + val |= cr4_pin;
> asm volatile("mov %0,%%cr4": : "r" (val), "m" (__force_order));
> + /*
> + * If the MOV above was used directly as a ROP gadget we can
> + * notice the lack of pinned bits in "val" and start the function
> + * from the beginning to gain the cr4_pin bits for sure.
> + */
> + if (WARN_ONCE(cr4_pin && (val & cr4_pin) == 0,

Don't you mean `cr4_pin && (val & cr4_pin) != cr4_pin)`?

> + "cr4 pin bypass attempt?!\n"))
> + goto again;
> }