Re: [PATCH v2 3/3] x86/pti/64: Remove the SYSCALL64 entry trampoline

[Peter Zijlstra <peterz@infradead.org>]

On Mon, Sep 03, 2018 at 03:59:44PM -0700, Andy Lutomirski wrote:
> The SYSCALL64 trampoline has a couple of nice properties:
> 
>  - The usual sequence of SWAPGS followed by two GS-relative accesses to
>    set up RSP is somewhat slow because the GS-relative accesses need
>    to wait for SWAPGS to finish.  The trampoline approach allows
>    RIP-relative accesses to set up RSP, which avoids the stall.
> 
>  - The trampoline avoids any percpu access before CR3 is set up,
>    which means that no percpu memory needs to be mapped in the user
>    page tables.  This prevents using Meltdown to read any percpu memory
>    outside the cpu_entry_area and prevents using timing leaks
>    to directly locate the percpu areas.
> 
> The downsides of using a trampoline may outweigh the upsides, however.
> It adds an extra non-contiguous I$ cache line to system calls, and it
> forces an indirect jump to transfer control back to the normal kernel
> text after CR3 is set up.  The latter is because x86 lacks a 64-bit
> direct jump instruction that could jump from the trampoline to the entry
> text.  With retpolines enabled, the indirect jump is extremely slow.
> 
> This patch changes the code to map the percpu TSS into the user page
> tables to allow the non-trampoline SYSCALL64 path to work under PTI.
> This does not add a new direct information leak, since the TSS is
> readable by Meltdown from the cpu_entry_area alias regardless.  It
> does allow a timing attack to locate the percpu area, but KASLR is
> more or less a lost cause against local attack on CPUs vulnerable to
> Meltdown regardless.  As far as I'm concerned, on current hardware,
> KASLR is only useful to mitigate remote attacks that try to attack
> the kernel without first gaining RCE against a vulnerable user
> process.
> 
> On Skylake, with CONFIG_RETPOLINE=y and KPTI on, this reduces
> syscall overhead from ~237ns to ~228ns.
> 
> There is a possible alternative approach: we could instead move the
> trampoline within 2G of the entry text and make a separate copy for
> each CPU.  Then we could use a direct jump to rejoin the normal
> entry path.

Can we have a few words on why this solution and not this alternative? I
mean, you raise the possibility, but then surely you chose not to
implement that. Might as well share that with us.