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

[Ingo Molnar <mingo@kernel.org>]

* Brian Gerst <brgerst@gmail.com> wrote:

> On Thu, Mar 26, 2015 at 8:42 AM, Denys Vlasenko <dvlasenk@redhat.com> 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@redhat.com>
> > CC: Borislav Petkov <bp@alien8.de>
> > CC: x86@kernel.org
> > CC: linux-kernel@vger.kernel.org
> > ---
> >
> > 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.

This needs a testcase I suspect.

> 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.

I suspect it's similar to what would happen if we tried a RET to a 
non-canonical address: the fetch fails and the JMP gets the #GP?

In that sense it's the fault of the return instruction.

Thanks,

	Ingo