[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]
Message-ID: <ZJSRD1xZauOW3jFO@casper.infradead.org>
Date: Thu, 22 Jun 2023 19:21:03 +0100
From: Matthew Wilcox <willy@...radead.org>
To: Rick Edgecombe <rick.p.edgecombe@...el.com>
Cc: x86@...nel.org, "H . Peter Anvin" <hpa@...or.com>,
Thomas Gleixner <tglx@...utronix.de>,
Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org,
linux-doc@...r.kernel.org, linux-mm@...ck.org,
linux-arch@...r.kernel.org, linux-api@...r.kernel.org,
Arnd Bergmann <arnd@...db.de>,
Andy Lutomirski <luto@...nel.org>,
Balbir Singh <bsingharora@...il.com>,
Borislav Petkov <bp@...en8.de>,
Cyrill Gorcunov <gorcunov@...il.com>,
Dave Hansen <dave.hansen@...ux.intel.com>,
Eugene Syromiatnikov <esyr@...hat.com>,
Florian Weimer <fweimer@...hat.com>,
"H . J . Lu" <hjl.tools@...il.com>, Jann Horn <jannh@...gle.com>,
Jonathan Corbet <corbet@....net>,
Kees Cook <keescook@...omium.org>,
Mike Kravetz <mike.kravetz@...cle.com>,
Nadav Amit <nadav.amit@...il.com>,
Oleg Nesterov <oleg@...hat.com>, Pavel Machek <pavel@....cz>,
Peter Zijlstra <peterz@...radead.org>,
Randy Dunlap <rdunlap@...radead.org>,
Weijiang Yang <weijiang.yang@...el.com>,
"Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
John Allen <john.allen@....com>, kcc@...gle.com,
eranian@...gle.com, rppt@...nel.org, jamorris@...ux.microsoft.com,
dethoma@...rosoft.com, akpm@...ux-foundation.org,
Andrew.Cooper3@...rix.com, christina.schimpe@...el.com,
david@...hat.com, debug@...osinc.com, szabolcs.nagy@....com,
torvalds@...ux-foundation.org, broonie@...nel.org,
Yu-cheng Yu <yu-cheng.yu@...el.com>,
Pengfei Xu <pengfei.xu@...el.com>
Subject: Re: [PATCH v9 16/42] mm: Add guard pages around a shadow stack.
On Mon, Jun 12, 2023 at 05:10:42PM -0700, Rick Edgecombe wrote:
> +++ b/include/linux/mm.h
> @@ -342,7 +342,36 @@ extern unsigned int kobjsize(const void *objp);
> #endif /* CONFIG_ARCH_HAS_PKEYS */
>
> #ifdef CONFIG_X86_USER_SHADOW_STACK
> -# define VM_SHADOW_STACK VM_HIGH_ARCH_5 /* Should not be set with VM_SHARED */
> +/*
> + * This flag should not be set with VM_SHARED because of lack of support
> + * core mm. It will also get a guard page. This helps userspace protect
> + * itself from attacks. The reasoning is as follows:
> + *
> + * The shadow stack pointer(SSP) is moved by CALL, RET, and INCSSPQ. The
> + * INCSSP instruction can increment the shadow stack pointer. It is the
> + * shadow stack analog of an instruction like:
> + *
> + * addq $0x80, %rsp
> + *
> + * However, there is one important difference between an ADD on %rsp
> + * and INCSSP. In addition to modifying SSP, INCSSP also reads from the
> + * memory of the first and last elements that were "popped". It can be
> + * thought of as acting like this:
> + *
> + * READ_ONCE(ssp); // read+discard top element on stack
> + * ssp += nr_to_pop * 8; // move the shadow stack
> + * READ_ONCE(ssp-8); // read+discard last popped stack element
> + *
> + * The maximum distance INCSSP can move the SSP is 2040 bytes, before
> + * it would read the memory. Therefore a single page gap will be enough
> + * to prevent any operation from shifting the SSP to an adjacent stack,
> + * since it would have to land in the gap at least once, causing a
> + * fault.
> + *
> + * Prevent using INCSSP to move the SSP between shadow stacks by
> + * having a PAGE_SIZE guard gap.
> + */
> +# define VM_SHADOW_STACK VM_HIGH_ARCH_5
> #else
> # define VM_SHADOW_STACK VM_NONE
> #endif
This is a lot of very x86-specific language in a generic header file.
I'm sure there's a better place for all this text.
Powered by blists - more mailing lists