Date: Fri, 26 Apr 2019 15:02:43 +0000 From: Jason Gunthorpe <jgg@...lanox.com> To: Ingo Molnar <mingo@...nel.org> CC: Kees Cook <keescook@...omium.org>, Andrew Morton <akpm@...ux-foundation.org>, Hector Marco-Gisbert <hecmargi@....es>, Marc Gonzalez <marc.w.gonzalez@...e.fr>, Will Deacon <will.deacon@....com>, X86 ML <x86@...nel.org>, Thomas Gleixner <tglx@...utronix.de>, Andy Lutomirski <luto@...capital.net>, Stephen Rothwell <sfr@...b.auug.org.au>, Catalin Marinas <catalin.marinas@....com>, Mark Rutland <mark.rutland@....com>, Arnd Bergmann <arnd@...db.de>, Linux ARM <linux-arm-kernel@...ts.infradead.org>, Kernel Hardening <kernel-hardening@...ts.openwall.com>, LKML <linux-kernel@...r.kernel.org>, Linus Torvalds <torvalds@...ux-foundation.org>, Borislav Petkov <bp@...en8.de>, Peter Zijlstra <a.p.zijlstra@...llo.nl> Subject: Re: [PATCH v2] binfmt_elf: Update READ_IMPLIES_EXEC logic for modern CPUs On Thu, Apr 25, 2019 at 10:07:25PM +0200, Ingo Molnar wrote: > > But yes, your above diff for "has NX" is roughly correct. I'll walk > > through each piece I'm thinking about. Here is the current state: > > > > CPU: | lacks NX* | has NX, ia32 | has NX, x86_64 | > > ELF: | | | | > > missing GNU_STACK | exec-all | exec-all | exec-all | > > GNU_STACK == RWX | exec-all | exec-all | exec-all | > > GNU_STACK == RW | exec-none | exec-none | exec-none | > > > > *this column has no architecture effect: NX markings are ignored by > > hardware, but may have behavioral effects when "wants X" collides with > > "cannot be X" constraints in memory permission flags, as in . > > So  appears to be device driver mapping a BAR that isn't intended to > be excutable: > > https://lore.kernel.org/netdev/20190418055759.GA3155@mellanox.com/ > > and the question is, do we reject this at the device driver mmap() level > already, right? No, we wanted to reject it at the driver mmap() level, but if an executable is marked with GNU_STACK=RWX then the core mm code always calls the driver with VM_EXEC (even though the mmap isn't a stack) and the driver becomes incompatible with userspace using GNU_STACK=RWX (ie some Fortran programs, apparently) > I suspect the best behavior is to reject as early as possible, so I agree > with your change here - even though !NX systems tend to become less and > less relevant these days. I suggested the idea of adding a flag in either the struct file or the file_operations flag that says mmap is never to be executable for this file with the idea that most/all cdev users would set it. Does that seem reasonable? Jason
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