* Denys Vlasenko <[email protected]> wrote:
> On 03/26/2015 01:42 PM, Denys Vlasenko 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)
>
>
>
> > .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
>
>
> Another thing we can do here is to just canonicalize the address.
> IOW: same code as above but without last two insns.
>
> The difference would be that if userspace gives us bogus,
> noncanonical return address, we would return to a different address
> instead of SIGSEGV.
So in general it's better to be proactive with such things and
generate an error as early as possible, making it easier to debug
user-space bugs.
> There is no security implications in doing this as far as I can see,
> and no sane program uses noncanonical addresses.
> Apart from not having any legitimate need to do so, it's also quite
> complicated to achieve.
>
> So it should not break any real-world cases.
It would make user-space debugging harder though, so it's a quality of
implmentation issue.
Thanks,
Ingo
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