Re: [PATCH] x86/asm/entry/64: better check for canonical address

[Brian Gerst]

On Thu, Mar 26, 2015 at 8:42 AM, Denys Vlasenko <dvlasenk@xxxxxxxxxx> wrote:
> This change makes the check exact (no more false positives
> on kernel addresses).
>
> It isn't really important to be fully correct here -
> almost all addresses we'll ever see will be userspace ones,
> but OTOH it looks to be cheap enough:
> the new code uses two more ALU ops but preserves %rcx,
> allowing to not reload it from pt_regs->cx again.
> On disassembly level, the changes are:
>
> cmp %rcx,0x80(%rsp) -> mov 0x80(%rsp),%r11; cmp %rcx,%r11
> shr $0x2f,%rcx      -> shl $0x10,%rcx; sar $0x10,%rcx; cmp %rcx,%r11
> mov 0x58(%rsp),%rcx -> (eliminated)
>
> Signed-off-by: Denys Vlasenko <dvlasenk@xxxxxxxxxx>
> CC: Borislav Petkov <bp@xxxxxxxxx>
> CC: x86@xxxxxxxxxx
> CC: linux-kernel@xxxxxxxxxxxxxxx
> ---
>
> Andy, I'd undecided myself on the merits of doing this.
> If you like it, feel free to take it in your tree.
> I trimmed CC list to not bother too many people with this trivial
> and quite possibly "useless churn"-class change.
>
>  arch/x86/kernel/entry_64.S | 23 ++++++++++++-----------
>  1 file changed, 12 insertions(+), 11 deletions(-)
>
> diff --git a/arch/x86/kernel/entry_64.S b/arch/x86/kernel/entry_64.S
> index bf9afad..a36d04d 100644
> --- a/arch/x86/kernel/entry_64.S
> +++ b/arch/x86/kernel/entry_64.S
> @@ -688,26 +688,27 @@ retint_swapgs:            /* return to user-space */
>          * a completely clean 64-bit userspace context.
>          */
>         movq RCX(%rsp),%rcx
> -       cmpq %rcx,RIP(%rsp)             /* RCX == RIP */
> +       movq RIP(%rsp),%r11
> +       cmpq %rcx,%r11                  /* RCX == RIP */
>         jne opportunistic_sysret_failed
>
>         /*
>          * On Intel CPUs, sysret with non-canonical RCX/RIP will #GP
>          * in kernel space.  This essentially lets the user take over
> -        * the kernel, since userspace controls RSP.  It's not worth
> -        * testing for canonicalness exactly -- this check detects any
> -        * of the 17 high bits set, which is true for non-canonical
> -        * or kernel addresses.  (This will pessimize vsyscall=native.
> -        * Big deal.)
> +        * the kernel, since userspace controls RSP.
>          *
> -        * If virtual addresses ever become wider, this will need
> +        * If width of "canonical tail" ever become variable, this will need
>          * to be updated to remain correct on both old and new CPUs.
>          */
>         .ifne __VIRTUAL_MASK_SHIFT - 47
>         .error "virtual address width changed -- sysret checks need update"
>         .endif
> -       shr $__VIRTUAL_MASK_SHIFT, %rcx
> -       jnz opportunistic_sysret_failed
> +       /* Change top 16 bits to be a sign-extension of the rest */
> +       shl     $(64 - (__VIRTUAL_MASK_SHIFT+1)), %rcx
> +       sar     $(64 - (__VIRTUAL_MASK_SHIFT+1)), %rcx
> +       /* If this changed %rcx, it was not canonical */
> +       cmpq    %rcx, %r11
> +       jne     opportunistic_sysret_failed
>
>         cmpq $__USER_CS,CS(%rsp)        /* CS must match SYSRET */
>         jne opportunistic_sysret_failed

Would it be possible to to skip this check entirely on AMD processors?
 It's my understanding that AMD correctly issues the #GP from CPL3,
causing a stack switch.

Looking at the AMD docs, sysret doesn't even check for a canonical
address.  The #GP is probably from the instruction fetch at the
non-canonical address instead of from sysret itself.

--
Brian Gerst
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