Date: Mon, 17 Sep 2018 17:24:15 +0200 From: Markus Wichmann <nullplan@....net> To: musl@...ts.openwall.com Subject: Re: Replacing a_crash() ? On Sun, Sep 16, 2018 at 11:23:17PM -0400, Rich Felker wrote: > Now that we have an abort() that reliably terminates with uncatchable > SIGABRT, I've been thinking about replacing the a_crash() calls in > musl (which are usually an instruction generating SIGILL or SIGSEGV) > with calls to the uncatchable tail of abort(), which I would factor > off as a __forced_abort() function. > > In case it's not clear, the reason for not just calling abort() is > that too many programs catch it, and catching it is even encouraged. > Catchability is a problem with the current approach too, since > a_crash() is used in places where process state is known to be > dangerously corrupt and likely under attacker control; eliminating it > is one of the potential goals of switching to __forced_abort(). > > Are there any objections to making such a change? So far I've gotten > mostly positive feedback -- SIGABRT is more telling of what's happened > than SIGSEGV/SIGILL. It would also get rid of the ugly misplacement of > a_crash() (no longer needed) in "atomic.h" and the inclusion of > "atomic.h" in some files where it makes no sense without knowing it's > where a_crash() is defined. > > For i386, some nontrivial work would be needed to make abort's tail > perform syscalls with int $128 rather than the vdso, which is unsafe > since the pointer to it may have been subverted. On other archs, > inline syscalls are fully inline. I'd probably add a > NEED_FAILSAFE_SYSCALL macro to define before including "syscall.h" and > have arch/i386/syscall_arch.h adjust the asm string based on it; this > is more maintainable than writing an asm version of the function. > > Rich Simple checklist for whether to perform a change or not: 1. Does the change fix problems? Check (namely, maintainability, legibility, understandability of problems). 2. Does the change introduce problems? Unlikely. Someone might subvert __forced_abort(), but then, someone might catch SIGILL, so we haven't gone anywhere. 3. Is the change compatible with old programs? No, but a_crash() was never a defined interface, so any program catching it was walking on thin ice, anyway. So that's two green lights and a don't care, so please go ahead. Ciao, Markus
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