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Message-ID: <61ae9398a03d4fe7868b68c9026d5998@AcuMS.aculab.com>
Date: Thu, 1 Apr 2021 11:15:43 +0000
From: David Laight <David.Laight@...LAB.COM>
To: 'Will Deacon' <will@...nel.org>, Kees Cook <keescook@...omium.org>
CC: Thomas Gleixner <tglx@...utronix.de>, Elena Reshetova
<elena.reshetova@...el.com>, "x86@...nel.org" <x86@...nel.org>, "Andy
Lutomirski" <luto@...nel.org>, Peter Zijlstra <peterz@...radead.org>,
"Catalin Marinas" <catalin.marinas@....com>, Mark Rutland
<mark.rutland@....com>, Alexander Potapenko <glider@...gle.com>, Alexander
Popov <alex.popov@...ux.com>, Ard Biesheuvel <ard.biesheuvel@...aro.org>,
Jann Horn <jannh@...gle.com>, Vlastimil Babka <vbabka@...e.cz>, David
Hildenbrand <david@...hat.com>, Mike Rapoport <rppt@...ux.ibm.com>, Andrew
Morton <akpm@...ux-foundation.org>, Jonathan Corbet <corbet@....net>, Randy
Dunlap <rdunlap@...radead.org>, "kernel-hardening@...ts.openwall.com"
<kernel-hardening@...ts.openwall.com>, "linux-hardening@...r.kernel.org"
<linux-hardening@...r.kernel.org>, "linux-arm-kernel@...ts.infradead.org"
<linux-arm-kernel@...ts.infradead.org>, "linux-mm@...ck.org"
<linux-mm@...ck.org>, "linux-kernel@...r.kernel.org"
<linux-kernel@...r.kernel.org>
Subject: RE: [PATCH v8 3/6] stack: Optionally randomize kernel stack offset
each syscall
From: Will Deacon
> Sent: 01 April 2021 09:31
...
> > +/*
> > + * These macros must be used during syscall entry when interrupts and
> > + * preempt are disabled, and after user registers have been stored to
> > + * the stack.
> > + */
> > +#define add_random_kstack_offset() do { \
> > + if (static_branch_maybe(CONFIG_RANDOMIZE_KSTACK_OFFSET_DEFAULT, \
> > + &randomize_kstack_offset)) { \
> > + u32 offset = __this_cpu_read(kstack_offset); \
> > + u8 *ptr = __builtin_alloca(KSTACK_OFFSET_MAX(offset)); \
> > + asm volatile("" : "=m"(*ptr) :: "memory"); \
>
> Using the "m" constraint here is dangerous if you don't actually evaluate it
> inside the asm. For example, if the compiler decides to generate an
> addressing mode relative to the stack but with writeback (autodecrement), then
> the stack pointer will be off by 8 bytes. Can you use "o" instead?
Is it allowed to use such a mode?
It would have to know that the "m" was substituted exactly once.
I think there are quite a few examples with 'strange' uses of memory
asm arguments.
However, in this case, isn't it enough to ensure the address is 'saved'?
So:
asm volatile("" : "=r"(ptr) );
should be enough.
David
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