Commit-ID: 85063fac1f72419eec4349621fe829b07f9acb1e Gitweb: http://git.kernel.org/tip/85063fac1f72419eec4349621fe829b07f9acb1e Author: Andy Lutomirski <luto@xxxxxxxxxx> AuthorDate: Mon, 12 Sep 2016 15:05:51 -0700 Committer: Ingo Molnar <mingo@xxxxxxxxxx> CommitDate: Tue, 13 Sep 2016 20:34:16 +0200 x86/entry/64: Clean up and document espfix64 stack setup The espfix64 setup code was a bit inscrutible and contained an unnecessary push of RAX. Remove that push, update all the stack offsets to match, and document the whole mess. Reported-By: Borislav Petkov <bp@xxxxxxxxx> Signed-off-by: Andy Lutomirski <luto@xxxxxxxxxx> Reviewed-by: Borislav Petkov <bp@xxxxxxx> Cc: Borislav Petkov <bp@xxxxxxxxx> Cc: Brian Gerst <brgerst@xxxxxxxxx> Cc: Denys Vlasenko <dvlasenk@xxxxxxxxxx> Cc: H. Peter Anvin <hpa@xxxxxxxxx> Cc: Josh Poimboeuf <jpoimboe@xxxxxxxxxx> Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx> Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx> Link: http://lkml.kernel.org/r/e5459eb10cf1175c8b36b840bc425f210d045f35.1473717910.git.luto@xxxxxxxxxx Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx> --- arch/x86/entry/entry_64.S | 64 +++++++++++++++++++++++++++++++++++++++-------- 1 file changed, 53 insertions(+), 11 deletions(-) diff --git a/arch/x86/entry/entry_64.S b/arch/x86/entry/entry_64.S index c0373d6..e7fba58 100644 --- a/arch/x86/entry/entry_64.S +++ b/arch/x86/entry/entry_64.S @@ -586,27 +586,69 @@ native_irq_return_iret: #ifdef CONFIG_X86_ESPFIX64 native_irq_return_ldt: - pushq %rax - pushq %rdi + /* + * We are running with user GSBASE. All GPRs contain their user + * values. We have a percpu ESPFIX stack that is eight slots + * long (see ESPFIX_STACK_SIZE). espfix_waddr points to the bottom + * of the ESPFIX stack. + * + * We clobber RAX and RDI in this code. We stash RDI on the + * normal stack and RAX on the ESPFIX stack. + * + * The ESPFIX stack layout we set up looks like this: + * + * --- top of ESPFIX stack --- + * SS + * RSP + * RFLAGS + * CS + * RIP <-- RSP points here when we're done + * RAX <-- espfix_waddr points here + * --- bottom of ESPFIX stack --- + */ + + pushq %rdi /* Stash user RDI */ SWAPGS movq PER_CPU_VAR(espfix_waddr), %rdi - movq %rax, (0*8)(%rdi) /* RAX */ - movq (2*8)(%rsp), %rax /* RIP */ + movq %rax, (0*8)(%rdi) /* user RAX */ + movq (1*8)(%rsp), %rax /* user RIP */ movq %rax, (1*8)(%rdi) - movq (3*8)(%rsp), %rax /* CS */ + movq (2*8)(%rsp), %rax /* user CS */ movq %rax, (2*8)(%rdi) - movq (4*8)(%rsp), %rax /* RFLAGS */ + movq (3*8)(%rsp), %rax /* user RFLAGS */ movq %rax, (3*8)(%rdi) - movq (6*8)(%rsp), %rax /* SS */ + movq (5*8)(%rsp), %rax /* user SS */ movq %rax, (5*8)(%rdi) - movq (5*8)(%rsp), %rax /* RSP */ + movq (4*8)(%rsp), %rax /* user RSP */ movq %rax, (4*8)(%rdi) - andl $0xffff0000, %eax - popq %rdi + /* Now RAX == RSP. */ + + andl $0xffff0000, %eax /* RAX = (RSP & 0xffff0000) */ + popq %rdi /* Restore user RDI */ + + /* + * espfix_stack[31:16] == 0. The page tables are set up such that + * (espfix_stack | (X & 0xffff0000)) points to a read-only alias of + * espfix_waddr for any X. That is, there are 65536 RO aliases of + * the same page. Set up RSP so that RSP[31:16] contains the + * respective 16 bits of the /userspace/ RSP and RSP nonetheless + * still points to an RO alias of the ESPFIX stack. + */ orq PER_CPU_VAR(espfix_stack), %rax SWAPGS movq %rax, %rsp - popq %rax + + /* + * At this point, we cannot write to the stack any more, but we can + * still read. + */ + popq %rax /* Restore user RAX */ + + /* + * RSP now points to an ordinary IRET frame, except that the page + * is read-only and RSP[31:16] are preloaded with the userspace + * values. We can now IRET back to userspace. + */ jmp native_irq_return_iret #endif END(common_interrupt) -- To unsubscribe from this list: send the line "unsubscribe linux-tip-commits" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
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