From fddf8ba1e48860211c9639d00883833b42fcc1e0 Mon Sep 17 00:00:00 2001 From: "Chang S. Bae" Date: Thu, 28 May 2020 16:13:47 -0400 Subject: x86/ptrace: Prevent ptrace from clearing the FS/GS selector When a ptracer writes a ptracee's FS/GSBASE with a different value, the selector is also cleared. This behavior is not correct as the selector should be preserved. Update only the base value and leave the selector intact. To simplify the code further remove the conditional checking for the same value as this code is not performance critical. The only recognizable downside of this change is when the selector is already nonzero on write. The base will be reloaded according to the selector. But the case is highly unexpected in real usages. [ tglx: Massage changelog ] Suggested-by: Andy Lutomirski Signed-off-by: Chang S. Bae Signed-off-by: Thomas Gleixner Signed-off-by: Sasha Levin Signed-off-by: Thomas Gleixner Link: https://lkml.kernel.org/r/9040CFCD-74BD-4C17-9A01-B9B713CF6B10@intel.com Link: https://lkml.kernel.org/r/20200528201402.1708239-2-sashal@kernel.org --- arch/x86/kernel/ptrace.c | 17 ++--------------- 1 file changed, 2 insertions(+), 15 deletions(-) (limited to 'arch/x86/kernel/ptrace.c') diff --git a/arch/x86/kernel/ptrace.c b/arch/x86/kernel/ptrace.c index 44130588987f..1c7646c8d0fe 100644 --- a/arch/x86/kernel/ptrace.c +++ b/arch/x86/kernel/ptrace.c @@ -379,25 +379,12 @@ static int putreg(struct task_struct *child, case offsetof(struct user_regs_struct,fs_base): if (value >= TASK_SIZE_MAX) return -EIO; - /* - * When changing the FS base, use do_arch_prctl_64() - * to set the index to zero and to set the base - * as requested. - * - * NB: This behavior is nonsensical and likely needs to - * change when FSGSBASE support is added. - */ - if (child->thread.fsbase != value) - return do_arch_prctl_64(child, ARCH_SET_FS, value); + x86_fsbase_write_task(child, 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; - if (child->thread.gsbase != value) - return do_arch_prctl_64(child, ARCH_SET_GS, value); + x86_gsbase_write_task(child, value); return 0; #endif } -- cgit v1.2.3-70-g09d2