[PATCH 07/11] signal/arm64: Document conflicts with SI_USER and SIGFPE, SIGTRAP, SIGBUS

[Dave Martin]

On Mon, Jan 15, 2018 at 11:23:03AM -0600, Eric W. Biederman wrote:
> Dave Martin <Dave.Martin at arm.com> writes:
> 
> > On Thu, Jan 11, 2018 at 06:59:36PM -0600, Eric W. Biederman wrote:
> >> Setting si_code to 0 results in a userspace seeing an si_code of 0.
> >> This is the same si_code as SI_USER.  Posix and common sense requires
> >> that SI_USER not be a signal specific si_code.  As such this use of 0
> >> for the si_code is a pretty horribly broken ABI.
> >
> > I think this situation may have come about because 0 is used as a
> > padding value for "impossible" cases -- i.e., things that can't happen
> > unless the kernel is broken, or things that are too unrecoverable for
> > clean error reporting to be helpful.
> >
> > In general, I think these values are not expected to reach userspace in
> > practice.
> >
> > This is not an excuse though -- and not 100% true -- so it's certainly
> > worthy of cleanup.
> >
> >
> > It would be good to approach this similarly for arm and arm64, since
> > the arm64 fault code is derived from arm.
> 
> In this case the fault_info is something I have only seen on arm64.
> I have been approaching all architectures the same way.

Bad guess on my part; this table-driven approach seems to be new for
arm64.

> If there is insufficient information without architecture expertise
> to fix this class of error I have been ading FPE_FIXME to them.

Fair enough.

> >> Further use of si_code == 0 guaranteed that copy_siginfo_to_user saw a
> >> value of __SI_KILL and now sees a value of SIL_KILL with the result
> >> that uid and pid fields are copied and which might copying the si_addr
> >> field by accident but certainly not by design.  Making this a very
> >> flakey implementation.
> >> 
> >> Utilizing FPE_FIXME, BUS_FIXME, TRAP_FIXME siginfo_layout will now return
> >> SIL_FAULT and the appropriate fields will be reliably copied.
> >> 
> >> But folks this is a new and unique kind of bad.  This is massively
> >> untested code bad.  This is inventing new and unique was to get
> >> siginfo wrong bad.  This is don't even think about Posix or what
> >> siginfo means bad.  This is lots of eyeballs all missing the fact
> >> that the code does the wrong thing bad.  This is getting stuck
> >> and keep making the same mistake bad.
> >> 
> >> I really hope we can find a non userspace breaking fix for this on a
> >> port as new as arm64.
> >
> >> Possible ABI fixes include:
> >> - Send the signal without siginfo
> >> - Don't generate a signal
> >
> > The above two sould like ABI breaks?
> 
> They are ways I have seen code on other platforms deal with
> not information to generate siginfo.  Sending the signal without siginfo
> is roughly equivalent to your send SIGKILL suggestion below.
> 
> A good example of that is code that calls force_sigsegv.
> 
> Calling "force_sig(SIGBUS, current);" is perfectly valid.
> And then the parent when it reaped the process would have
> a little more information to go on when guessing what happened
> to the process.
> 
> >> - Possibly assign and use an appropriate si_code
> >> - Don't handle cases which can't happen
> >
> > I think a mixture of these two is the best approach.
> >
> > In any case, si_code == 0 here doesn't seem to have any explicit meaning.
> > I think we can translate all of the arm64 faults to proper si_codes --
> > see my sketch below.  Probably means a bit more thought though.
> 
> Yes I would be very happy to see that.
> 
> > The only counterargument would be if there is software relying on
> > these bogus signal cases getting si_code == 0 for a useful purpose.
> >
> > The main reason I see to check for SI_USER is to allow a process to
> > filter out spurious signals (say, an asynchronous I/O signal for
> > which si_value would be garbage), and to print out diagnostics
> > before (in the case of a well-behaved program) resetting the signal
> > to SIG_DFL and killing itself to report the signal to the waiter.
> >
> > Daemons may be more discerning about who is allowed to signal them,
> > but overloading SIGBUS (say) as an IPC channel sounds like a very odd
> > thing to do.  The same probably applies to any signal that has
> > nontrivial metadata.
> 
> Agreed.  Although I have seen ltp test cases that do crazy things like
> that.
> 
> > Have you found software that is impacted by this in practice?
> 
> No.

Searching for si_code on codesearch.debian.net, I looked at a few
random hits.  Although this is far from exhaustive, I saw no instance
of code that assumes some arch-specific meaning for SI_USER (or 0).

Most code seems to check for signal-specific si_code values before
assuming that signal-specific signifo fields are valid; or for
SI_USER (or si_code <= 0) before assuming that si_uid and si_pid
are valid.

Returning proper values for si_code values in place of "0" would fix
rather than break such cases.


> I don't expect many userspace applications look at siginfo and
> everything I have found is some rare hard to trigger non-x86 case which
> limits the exposure to userspace applications tremendously.
> 
> The angle I am coming at all of this from is that the linux kernel code
> that filled out out struct siginfo was not comprehensible or correct.

I think "not comprehensible or correct" is a pretty good summary of
all signal-related code...

> Internal to the kernel it was using a magic value (not exportable to
> userspace) in the upper bits of si_code.  That was causing problems for
> signal injection and converting signals from 32bit to 64bit, and from
> 64bit to 32bit.
> 
> So I wrote kernel/signal.c:siginfo_layout() to figure out which fields
> of struct siginfo should be sent to userspace.  In doing so I discovered
> that using 0 in si_code (aka SI_USER) is ambiguous, and problematic.
> 
> Unfortuantely in most of the cases I have spotted using 0 in the si_code
> requires architectural knowledge that I don't currently have to sort
> out.  So the best I can do is change si_code from 0 to
> FPE_FIXME/BUS_FIXME/TRAP_FIXME and bring the architecture maintainers
> attention to this area.
> 
> One of the problems that results from all of this is that we copy
> unitialized data to userspace.   I am slowly unifying and cleaning the
> code up so that the code is simple enough we can be certain we are
> not copying unitialized data to userspace.
> 
> With si_coes of FPE_FIXME/BUS_FIXME/TRAP_FIXME I can at least attempt to
> keep the craziness from happening.
> 
> My next step is to unify struct siginfo and struct compat_siginfo
> and the functions that copy them to userspace because there are very
> siginficant problems there.

All of which sounds valuable.

> All of that said I like the way you are thinking about fixing these
> issues.

Is it feasible to have a different internal constant for SI_USER?
Then the generic could warn if it sees si_code == 0.  If the
special nonzero KERNEL_SI_USER is seen instead, it is silently
translated to SI_USER (0) for userspace.  This might help us
track down cases where 0 is passed by accident.

It may not be worth it though: if the affected cases are ones
that happen never or almost never, a runtime BUG_ON() may not be
helpful for tracking them down.

Also, I'm making an assumption that si_code always flows through
some generic code before reaching userspace (maybe untrue).


> >> +++ b/arch/arm64/kernel/fpsimd.c
> >> @@ -867,7 +867,7 @@ asmlinkage void do_fpsimd_acc(unsigned int esr, struct pt_regs *regs)
> >>  asmlinkage void do_fpsimd_exc(unsigned int esr, struct pt_regs *regs)
> >>  {
> >>  	siginfo_t info;
> >> -	unsigned int si_code = 0;
> >> +	unsigned int si_code = FPE_FIXME;
> >>  
> >>  	if (esr & FPEXC_IOF)
> >>  		si_code = FPE_FLTINV;
> >
> > This 0 can happen for vector operations where the implementation may
> > not be able to report exactly what happened, for example where
> > the implementer didn't want to pay the cost of tracking exactly
> > what went wrong in each lane.
> >
> > However, the FPEXC_* bits can be garbage in such a case rather
> > than being all zero: we should be checking the TFV bit in the ESR here.
> > This may be a bug.
> >
> > Perhaps FPE_FLTINV should be returned in si_code for such cases:  it's
> > not otherwise used on arm64 -- invalid instructions would be reported as
> > SIGILL/ILL_ILLOPC instead).
> >
> > Otherwise, we might want to define a new code or arbitrarily pick
> > one of the existing FLT_* since this is really a more benign condition
> > than executing an illegal instruction.  Alternatively, treat the
> > fault as spurious and suppress it, but that doesn't feel right either.
> 
> I would love to see this sorted out.  There is a very similar pattern
> on several different architectures.  I suspect if we have a clean
> solution on one architecture the other architectures will be able to use
> that solution as well.

Since user code that relies on checking si_code for fp exceptions will
probably already break in these cases, we can probably fudge things a
bit here.

I'll have a think.  IEEE may also define some rules that are relevant
here...

For the proposed conversion of the si_code==0 cases for arm64, I'll
draft an RFC for discussion (hopefully sometime this week).

[...]

Cheers
---Dave