Re: [PATCH v5 21/27] x86/cet/shstk: Introduce WRUSS instruction
From: Dave Hansen <hidden>
Date: 2018-11-06 18:43:37
Also in:
linux-arch, linux-doc, linux-mm, lkml
On 10/11/18 8:15 AM, Yu-cheng Yu wrote:
quoted hunk ↗ jump to hunk
--- a/arch/x86/mm/fault.c +++ b/arch/x86/mm/fault.c@@ -1305,6 +1305,15 @@ __do_page_fault(struct pt_regs *regs, unsigned long error_code, error_code |= X86_PF_USER; flags |= FAULT_FLAG_USER; } else { + /* + * WRUSS is a kernel instruction and but writes + * to user shadow stack. When a fault occurs, + * both X86_PF_USER and X86_PF_SHSTK are set. + * Clear X86_PF_USER here. + */ + if ((error_code & (X86_PF_USER | X86_PF_SHSTK)) == + (X86_PF_USER | X86_PF_SHSTK)) + error_code &= ~X86_PF_USER;
This hunk of code basically points out that the architecture of WRUSS is broken for Linux. The setting of X86_PF_USER for a ring-0 instruction really is a mis-feature of the architecture for us and we *undo* it in software which is unfortunate. Wish I would have caught this earlier. Andy, note that this is another case where hw_error_code and sw_error_code will diverge, unfortunately. Anyway, this is going to necessitate some comment updates in the page fault code. Yu-cheng, you are going to collide with some recent changes I made to the page fault code. Please be careful with the context when you do the merge and make sure that all the new comments stay correct.