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Message-ID: <202210031127.C6CF796@keescook> Date: Mon, 3 Oct 2022 11:30:59 -0700 From: Kees Cook <keescook@...omium.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>, 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>, "Ravi V . Shankar" <ravi.v.shankar@...el.com>, Weijiang Yang <weijiang.yang@...el.com>, "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>, joao.moreira@...el.com, John Allen <john.allen@....com>, kcc@...gle.com, eranian@...gle.com, rppt@...nel.org, jamorris@...ux.microsoft.com, dethoma@...rosoft.com, Yu-cheng Yu <yu-cheng.yu@...el.com> Subject: Re: [PATCH v2 18/39] mm: Add guard pages around a shadow stack. On Thu, Sep 29, 2022 at 03:29:15PM -0700, Rick Edgecombe wrote: > [...] > +unsigned long stack_guard_start_gap(struct vm_area_struct *vma) > +{ > + if (vma->vm_flags & VM_GROWSDOWN) > + return stack_guard_gap; > + > + /* > + * Shadow stack pointer is moved by CALL, RET, and INCSSP(Q/D). > + * INCSSPQ moves shadow stack pointer up to 255 * 8 = ~2 KB > + * (~1KB for INCSSPD) and touches the first and the last element > + * in the range, which triggers a page fault if the range is not > + * in a shadow stack. Because of this, creating 4-KB guard pages > + * around a shadow stack prevents these instructions from going > + * beyond. > + * > + * Creation of VM_SHADOW_STACK is tightly controlled, so a vma > + * can't be both VM_GROWSDOWN and VM_SHADOW_STACK > + */ Thank you for the details on how the size choice is made here! :) > diff --git a/include/linux/mm.h b/include/linux/mm.h > index fef14ab3abcb..09458e77bf52 100644 > --- a/include/linux/mm.h > +++ b/include/linux/mm.h > @@ -2775,15 +2775,16 @@ struct vm_area_struct *vma_lookup(struct mm_struct *mm, unsigned long addr) > return vma; > } > > +unsigned long stack_guard_start_gap(struct vm_area_struct *vma); > + > static inline unsigned long vm_start_gap(struct vm_area_struct *vma) > { > + unsigned long gap = stack_guard_start_gap(vma); > unsigned long vm_start = vma->vm_start; > > - if (vma->vm_flags & VM_GROWSDOWN) { > - vm_start -= stack_guard_gap; > - if (vm_start > vma->vm_start) > - vm_start = 0; > - } > + vm_start -= gap; > + if (vm_start > vma->vm_start) > + vm_start = 0; > return vm_start; > } > > diff --git a/mm/mmap.c b/mm/mmap.c > index 9d780f415be3..f0d2e9143bd0 100644 > --- a/mm/mmap.c > +++ b/mm/mmap.c > @@ -247,6 +247,13 @@ SYSCALL_DEFINE1(brk, unsigned long, brk) > return origbrk; > } > I feel like something could be done with this definitions to make them inline, instead of __weak: #ifndef stack_guard_start_gap > +unsigned long __weak stack_guard_start_gap(struct vm_area_struct *vma) > +{ > + if (vma->vm_flags & VM_GROWSDOWN) > + return stack_guard_gap; > + return 0; > +} #endif And then move the x86 stack_guard_start_gap to a header? It's not exactly fast-path, but it feels a little weird. Regardlesss: Reviewed-by: Kees Cook <keescook@...omium.org> -- Kees Cook
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