From c7ca0b614513afba57824cae68447f9c32b1ee61 Mon Sep 17 00:00:00 2001 From: Andy Lutomirski Date: Mon, 15 Jul 2019 07:21:44 -0700 Subject: [PATCH] Revert "x86/ptrace: Prevent ptrace from clearing the FS/GS selector" and fix the test This reverts commit 48f5e52e916b55fb73754833efbacc7f8081a159. The ptrace ABI change was a prerequisite to the proposed design for FSGSBASE. Since FSGSBASE support has been reverted, and since I'm not convinced that the ABI was ever adequately tested, revert the ABI change as well. This also modifies the test case so that it tests the preexisting behavior. Signed-off-by: Andy Lutomirski Signed-off-by: Thomas Gleixner Link: https://lkml.kernel.org/r/fca39c478ea7fb15bc76fe8a36bd180810a067f6.1563200250.git.luto@kernel.org --- arch/x86/kernel/ptrace.c | 14 ++++++++++++-- tools/testing/selftests/x86/fsgsbase.c | 22 ++++------------------ 2 files changed, 16 insertions(+), 20 deletions(-) diff --git a/arch/x86/kernel/ptrace.c b/arch/x86/kernel/ptrace.c index 71691a8..0fdbe89 100644 --- a/arch/x86/kernel/ptrace.c +++ b/arch/x86/kernel/ptrace.c @@ -369,12 +369,22 @@ static int putreg(struct task_struct *child, case offsetof(struct user_regs_struct,fs_base): if (value >= TASK_SIZE_MAX) return -EIO; - x86_fsbase_write_task(child, value); + /* + * When changing the FS base, use do_arch_prctl_64() + * to set the index to zero and to set the base + * as requested. + */ + if (child->thread.fsbase != value) + return do_arch_prctl_64(child, ARCH_SET_FS, value); return 0; case offsetof(struct user_regs_struct,gs_base): + /* + * Exactly the same here as the %fs handling above. + */ if (value >= TASK_SIZE_MAX) return -EIO; - x86_gsbase_write_task(child, value); + if (child->thread.gsbase != value) + return do_arch_prctl_64(child, ARCH_SET_GS, value); return 0; #endif } diff --git a/tools/testing/selftests/x86/fsgsbase.c b/tools/testing/selftests/x86/fsgsbase.c index 5ab4c60..15a329d 100644 --- a/tools/testing/selftests/x86/fsgsbase.c +++ b/tools/testing/selftests/x86/fsgsbase.c @@ -489,25 +489,11 @@ static void test_ptrace_write_gsbase(void) * selector value is changed or not by the GSBASE write in * a ptracer. */ - if (gs != *shared_scratch) { - nerrs++; - printf("[FAIL]\tGS changed to %lx\n", gs); - - /* - * On older kernels, poking a nonzero value into the - * base would zero the selector. On newer kernels, - * this behavior has changed -- poking the base - * changes only the base and, if FSGSBASE is not - * available, this may have no effect. - */ - if (gs == 0) - printf("\tNote: this is expected behavior on older kernels.\n"); - } else if (have_fsgsbase && (base != 0xFF)) { - nerrs++; - printf("[FAIL]\tGSBASE changed to %lx\n", base); + if (gs == 0 && base == 0xFF) { + printf("[OK]\tGS was reset as expected\n"); } else { - printf("[OK]\tGS remained 0x%hx%s", *shared_scratch, have_fsgsbase ? " and GSBASE changed to 0xFF" : ""); - printf("\n"); + nerrs++; + printf("[FAIL]\tGS=0x%lx, GSBASE=0x%lx (should be 0, 0xFF)\n", gs, base); } } -- 2.7.4