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(p200300cbc7058300e5194218a8b55bec.dip0.t-ipconnect.de. [2003:cb:c705:8300:e519:4218:a8b5:5bec]) by smtp.gmail.com with ESMTPSA id w15-20020adfec4f000000b002c54241b4fesm5961541wrn.80.2023.02.20.04.57.08 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Mon, 20 Feb 2023 04:57:09 -0800 (PST) Message-ID: <458b3d39-ddce-c0f2-fe80-4e0cc5b101bd@redhat.com> Date: Mon, 20 Feb 2023 13:57:08 +0100 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:102.0) Gecko/20100101 Thunderbird/102.7.2 Subject: Re: [PATCH v6 19/41] x86/mm: Check shadow stack page fault errors Content-Language: en-US To: Rick Edgecombe , x86@kernel.org, "H . 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 , Weijiang Yang , "Kirill A . Shutemov" , John Allen , kcc@google.com, eranian@google.com, rppt@kernel.org, jamorris@linux.microsoft.com, dethoma@microsoft.com, akpm@linux-foundation.org, Andrew.Cooper3@citrix.com, christina.schimpe@intel.com, debug@rivosinc.com Cc: Yu-cheng Yu References: <20230218211433.26859-1-rick.p.edgecombe@intel.com> <20230218211433.26859-20-rick.p.edgecombe@intel.com> From: David Hildenbrand Organization: Red Hat In-Reply-To: <20230218211433.26859-20-rick.p.edgecombe@intel.com> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: linux-api@vger.kernel.org On 18.02.23 22:14, Rick Edgecombe wrote: > From: Yu-cheng Yu > > The CPU performs "shadow stack accesses" when it expects to encounter > shadow stack mappings. These accesses can be implicit (via CALL/RET > instructions) or explicit (instructions like WRSS). > > Shadow stack accesses to shadow-stack mappings can result in 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. The kernel needs to use faults to implement those > features. > > The architecture has concepts of both shadow stack reads and shadow stack > writes. Any shadow stack access to non-shadow stack memory will generate > a fault with the shadow stack error code bit set. > > This means that, unlike normal write protection, the fault handler needs > to create a type of memory that can be written to (with instructions that > generate shadow stack writes), even to fulfill a read access. So in the > case of COW memory, the COW needs to take place even with a shadow stack > read. Otherwise the page will be left (shadow stack) writable in > userspace. So to trigger the appropriate behavior, set FAULT_FLAG_WRITE > for shadow stack accesses, even if the access was a shadow stack read. > > For the purpose of making this clearer, consider the following example. > If a process has a shadow stack, and forks, the shadow stack PTEs will > become read-only due to COW. If the CPU in one process performs a shadow > stack read access to the shadow stack, for example executing a RET and > causing the CPU to read the shadow stack copy of the return address, then > in order for the fault to be resolved the PTE will need to be set with > shadow stack permissions. But then the memory would be changeable from > userspace (from CALL, RET, WRSS, etc). So this scenario needs to trigger > COW, otherwise the shared page would be changeable from both processes. > > 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. Also, generate the errors for invalid shadow stack accesses. > > Tested-by: Pengfei Xu > Tested-by: John Allen > Reviewed-by: Kees Cook > Signed-off-by: Yu-cheng Yu > Co-developed-by: Rick Edgecombe > Signed-off-by: Rick Edgecombe > > --- > v6: > - Update comment due to rename of Cow bit to SavedDirty > > v5: > - Add description of COW example (Boris) > - Replace "permissioned" (Boris) > - Remove capitalization of shadow stack (Boris) > > v4: > - Further improve comment talking about FAULT_FLAG_WRITE (Peterz) > > v3: > - Improve comment talking about using FAULT_FLAG_WRITE (Peterz) > --- > arch/x86/include/asm/trap_pf.h | 2 ++ > arch/x86/mm/fault.c | 38 ++++++++++++++++++++++++++++++++++ > 2 files changed, 40 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 7b0d4ab894c8..42885d8e2036 100644 > --- a/arch/x86/mm/fault.c > +++ b/arch/x86/mm/fault.c > @@ -1138,8 +1138,22 @@ access_error(unsigned long error_code, struct vm_area_struct *vma) > (error_code & X86_PF_INSTR), foreign)) > return 1; > > + /* > + * Shadow stack accesses (PF_SHSTK=1) are only permitted to > + * shadow stack VMAs. All other accesses result in an error. > + */ > + if (error_code & X86_PF_SHSTK) { > + if (unlikely(!(vma->vm_flags & VM_SHADOW_STACK))) > + return 1; > + if (unlikely(!(vma->vm_flags & VM_WRITE))) > + return 1; > + return 0; > + } > + > if (error_code & X86_PF_WRITE) { > /* write, present and write, not present: */ > + if (unlikely(vma->vm_flags & VM_SHADOW_STACK)) > + return 1; > if (unlikely(!(vma->vm_flags & VM_WRITE))) > return 1; > return 0; > @@ -1331,6 +1345,30 @@ void do_user_addr_fault(struct pt_regs *regs, > > perf_sw_event(PERF_COUNT_SW_PAGE_FAULTS, 1, regs, address); > > + /* > + * When a page becomes COW it changes from a shadow stack permission > + * page (Write=0,Dirty=1) to (Write=0,Dirty=0,SavedDirty=1), which is simply > + * read-only to the CPU. When shadow stack is enabled, a RET would > + * normally pop the shadow stack by reading it with a "shadow stack > + * read" access. However, in the COW case the shadow stack memory does > + * not have shadow stack permissions, it is read-only. So it will > + * generate a fault. > + * > + * For conventionally writable pages, a read can be serviced with a > + * read only PTE, and COW would not have to happen. But for shadow > + * stack, there isn't the concept of read-only shadow stack memory. > + * If it is shadow stack permission, it can be modified via CALL and > + * RET instructions. So COW needs to happen before any memory can be > + * mapped with shadow stack permissions. > + * > + * Shadow stack accesses (read or write) need to be serviced with > + * shadow stack permission memory, so in the case of a shadow stack > + * read access, treat it as a WRITE fault so both COW will happen and > + * the write fault path will tickle maybe_mkwrite() and map the memory > + * shadow stack. > + */ Again, I suggest dropping all details about COW from this comment and from the patch description. It's just one such case that can happen. -- Thanks, David / dhildenb