Date: Thu, 30 Jan 2020 08:37:40 -0500 From: Rich Felker <dalias@...c.org> To: Segher Boessenkool <segher@...nel.crashing.org> Cc: Sergei Trofimovich <slyfox@...too.org>, musl@...ts.openwall.com, libc-alpha@...rceware.org, gcc@....gnu.org, toolchain@...too.org Subject: Re: musl, glibc and ideal place for __stack_chk_fail_local On Thu, Jan 30, 2020 at 06:33:51AM -0600, Segher Boessenkool wrote: > On Sat, Jan 25, 2020 at 10:54:24AM -0500, Rich Felker wrote: > > > To support smash stack protection gcc emits __stack_chk_fail > > > calls on all targets. On top of that gcc emits __stack_chk_fail_local > > > calls at least on i386 and powerpc: > > (Only on 32-bit -fPIC -msecure-plt, for Power). Right, but musl only supports the secure-plt ABI. > > There is a half-serious proposal to put it in crti.o which is always > > linked too, but that seems like an ugly hack to me... > > Not *very* ugly, but it would be very effective, and no real downsides > to it (or do you see something?) Well either the thunk has to be written in asm per-arch, or some ld -r magic (which is fragile and something I don't want musl to depend on since I know users will someday hit breakage and rightfully blame us for using ld -r) to merge an asm source and C source. Or perhaps the existing crti.s content could be moved to file-scope __asm__ included in the C source file...that might be ok? > > > My understanding of requirements for libc that exposes ssp support: > > > - __stack_chk_fail is implemented as a default symbol > > > - __stack_chk_fail_local is implemented as a local symbol to avoid PLT. > > > (Why is it important? To avoid use of potentially already broken stack?) > > > > Because performance cost of -fstack-protector would go from 1-2% up to > > 5-10% on i386 and other archs where PLT contract requires a GOT > > register, since loading the GOT register is expensive > > (__x86.get_pc_thunk.* thunk itself is somewhat costly, and you throw > > away one of only a small number of available registers, increasing > > register pressure and hurting codegen). > > On Power it is just the setting up itself that is costly (in the config > where we have this _local thing). I think it'd be the same. If a function otherwise has no reason to access global data or calls though PLT, it can avoid the cost of finding the GOT and spending a fixed register on it. But possibility of having to call __stack_chk_fail makes *every* (stack-protected) function need to be able to make calls thru PLT, and thus introduces this cost to every function. > > Absolutely not. libssp is unsafe and creates new vulns/attack surface > > by doing introspective stuff after the process is already *known to > > be* in a compromised state. It should never be used. musl's > > __stack_chk_fail is safe and terminates immediately. > > Some implementations even print strings from the stack, it can be worse ;-) :-) > > Ideally, though, GCC would just emit the termination inline (or at > > least have an option to do so) rather than calling __stack_chk_fail or > > the local version. This would additionally harden against the case > > where the GOT is compromised. > > Yeah, but how to terminate is system-specific, it's much easier to punt > this job to the libc to do ;-) My ideas was __builtin_trap, although a slightly more hardened version (that might make users unhappy? :) is inlining a syscall to sigprocmask to mask SIGILL/SIGSEGV before the trapping instruction so that termination occurs regardless of whether there's a signal handler installed. > Open a GCC PR for this please? Filed as https://gcc.gnu.org/bugzilla/show_bug.cgi?id=93509 Rich
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