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Peter Anvin" , Thomas Gleixner , Ingo Molnar , linux-kernel@vger.kernel.org, linux-doc@vger.kernel.org, linux-mm@kvack.org, linux-arch@vger.kernel.org, linux-api@vger.kernel.org, Arnd Bergmann , Andy Lutomirski , Balbir Singh , Borislav Petkov , Cyrill Gorcunov , Dave Hansen , Eugene Syromiatnikov , Florian Weimer , "H . J . Lu" , Jann Horn , Jonathan Corbet , Kees Cook , Mike Kravetz , Nadav Amit , Oleg Nesterov , Pavel Machek , Peter Zijlstra , Randy Dunlap , "Ravi V . Shankar" , Dave Martin , Weijiang Yang , "Kirill A . Shutemov" , joao.moreira@intel.com, John Allen , kcc@google.com, eranian@google.com Cc: rick.p.edgecombe@intel.com, Yu-cheng Yu Subject: [PATCH 15/35] x86/mm: Check Shadow Stack page fault errors Date: Sun, 30 Jan 2022 13:18:18 -0800 Message-Id: <20220130211838.8382-16-rick.p.edgecombe@intel.com> X-Mailer: git-send-email 2.17.1 In-Reply-To: <20220130211838.8382-1-rick.p.edgecombe@intel.com> References: <20220130211838.8382-1-rick.p.edgecombe@intel.com> Precedence: bulk List-ID: X-Mailing-List: linux-arch@vger.kernel.org From: Yu-cheng Yu Shadow stack accesses are those that are performed by the CPU where it expects to encounter a shadow stack mapping. These accesses are performed implicitly by CALL/RET at the site of the shadow stack pointer. These accesses are made explicitly by shadow stack management instructions like WRUSSQ. Shadow stacks accesses to shadow-stack mapping can see faults in normal, valid operation just like regular accesses to regular mappings. Shadow stacks need some of the same features like delayed allocation, swap and copy-on-write. Shadow stack accesses can also result in errors, such as when a shadow stack overflows, or if a shadow stack access occurs to a non-shadow-stack mapping. In handling a shadow stack page fault, verify it occurs within a shadow stack mapping. It is always an error otherwise. For valid shadow stack accesses, set FAULT_FLAG_WRITE to effect copy-on-write. Because clearing _PAGE_DIRTY (vs. _PAGE_RW) is used to trigger the fault, shadow stack read fault and shadow stack write fault are not differentiated and both are handled as a write access. Signed-off-by: Yu-cheng Yu Reviewed-by: Kees Cook Reviewed-by: Kirill A. Shutemov Signed-off-by: Rick Edgecombe --- Yu-cheng v30: - Update Subject line and add a verb. arch/x86/include/asm/trap_pf.h | 2 ++ arch/x86/mm/fault.c | 19 +++++++++++++++++++ 2 files changed, 21 insertions(+) diff --git a/arch/x86/include/asm/trap_pf.h b/arch/x86/include/asm/trap_pf.h index 10b1de500ab1..afa524325e55 100644 --- a/arch/x86/include/asm/trap_pf.h +++ b/arch/x86/include/asm/trap_pf.h @@ -11,6 +11,7 @@ * bit 3 == 1: use of reserved bit detected * bit 4 == 1: fault was an instruction fetch * bit 5 == 1: protection keys block access + * bit 6 == 1: shadow stack access fault * bit 15 == 1: SGX MMU page-fault */ enum x86_pf_error_code { @@ -20,6 +21,7 @@ enum x86_pf_error_code { X86_PF_RSVD = 1 << 3, X86_PF_INSTR = 1 << 4, X86_PF_PK = 1 << 5, + X86_PF_SHSTK = 1 << 6, X86_PF_SGX = 1 << 15, }; diff --git a/arch/x86/mm/fault.c b/arch/x86/mm/fault.c index d0074c6ed31a..6769134986ec 100644 --- a/arch/x86/mm/fault.c +++ b/arch/x86/mm/fault.c @@ -1107,6 +1107,17 @@ access_error(unsigned long error_code, struct vm_area_struct *vma) (error_code & X86_PF_INSTR), foreign)) return 1; + /* + * Verify a shadow stack access is within a shadow stack VMA. + * It is always an error otherwise. Normal data access to a + * shadow stack area is checked in the case followed. + */ + if (error_code & X86_PF_SHSTK) { + if (!(vma->vm_flags & VM_SHADOW_STACK)) + return 1; + return 0; + } + if (error_code & X86_PF_WRITE) { /* write, present and write, not present: */ if (unlikely(!(vma->vm_flags & VM_WRITE))) @@ -1300,6 +1311,14 @@ void do_user_addr_fault(struct pt_regs *regs, perf_sw_event(PERF_COUNT_SW_PAGE_FAULTS, 1, regs, address); + /* + * Clearing _PAGE_DIRTY is used to detect shadow stack access. + * This method cannot distinguish shadow stack read vs. write. + * For valid shadow stack accesses, set FAULT_FLAG_WRITE to effect + * copy-on-write. + */ + if (error_code & X86_PF_SHSTK) + flags |= FAULT_FLAG_WRITE; if (error_code & X86_PF_WRITE) flags |= FAULT_FLAG_WRITE; if (error_code & X86_PF_INSTR) -- 2.17.1