Re: [RFC] Circumventing FineIBT Via Entrypoints

[Jennifer Miller]

On Fri, Feb 14, 2025 at 11:06:50PM +0000, Andrew Cooper wrote:
> On 13/02/2025 11:24 pm, Jennifer Miller wrote:
> > On Thu, Feb 13, 2025 at 09:24:18PM +0000, Andrew Cooper wrote:
> >>>> ; swap stacks as normal
> >>>>     mov    QWORD PTR gs:[rip+0x7f005f85],rsp       # 0x6014 <cpu_tss_rw+20>
> >>>>     mov    rsp,QWORD PTR gs:[rip+0x7f02c56d]       # 0x2c618 <pcpu_hot+24>
> >> ... these are memory accesses using the user %gs.  As you note a few
> >> lines lower, %gs isn't safe at this point.
> >>
> >> A cunning attacker can make gs:[rip+0x7f02c56d] be a read-only mapping,
> >> at point we'll have loaded an attacker controlled %rsp, then take #PF
> >> trying to spill %rsp into pcpu_hot, and now we're running the pagefault
> >> handler on an attacker controlled stack and gsbase.
> >>
> > I don't follow, the spill of %rsp into pcpu_hot occurs first, before we
> > would move to the attacker controlled stack. This is Intel asm syntax,
> > sorry if that was unclear.
> 
> No, sorry.  It's clearly written; I simply wasn't paying enough attention.
> 
> > Still, I hadn't considered misusing readonly/unmapped pages on the GPR
> > register spill that follows. Could we enforce that the stack pointer we get
> > be page aligned to prevent this vector? So that if one were to attempt to
> > point the stack to readonly or unmapped memory they should be guaranteed to
> > double fault?
> 
> Hmm.
> 
> Espfix64 does involve #DF recovering from a write to a read-only stack. 
> (This broken corner of x86 is also fixed in FRED.   We fixed a *lot* of
> thing.)

Interesting, I haven't gotten around to reading into how FRED works, it
sounds neat.

> 
> As long the #DF handler can be updated to safely distinguish espfix64
> from this entrypoint attack, this seems like it might mitigate the
> read-only case.
> > I think we can do the overwrite at any point before actually calling into 
> > the individual syscall handlers, really anywhere before potentially 
> > hijacked indirect control flow can occur and then restore it just after 
> > those return e.g., for the 64-bit path I am currently overwriting it at the
> > start of do_syscall_64 and then restoring it just before 
> > syscall_exit_to_user_mode. I'm not sure if there is any reason to do it
> > sooner while we'd still be register constrained.
> 
> I don't follow.  If any "bad" execution is found in an entrypoint, Linux
> needs to panic().  Detecting the malice involves clobbering an in-use
> stack, and there's no ability to safely recover.

Sorry, this was in response to Jann's question about the mitigation
strategy proposed in my initial email.

> 
> ~Andrew

~Jennifer