From: "Chang S. Bae" <chang.seok.bae@intel.com>
Date: Thu, 28 May 2020 16:13:47 -0400
Subject: x86/ptrace: Prevent ptrace from clearing the FS/GS selector
Git-commit: fddf8ba1e48860211c9639d00883833b42fcc1e0
Patch-mainline: v5.9-rc1
References: jsc#SLE-14846

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 <luto@kernel.org>
Signed-off-by: Chang S. Bae <chang.seok.bae@intel.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Sasha Levin <sashal@kernel.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
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

Acked-by: Borislav Petkov <bp@suse.de>
---
 arch/x86/kernel/ptrace.c |   14 ++------------
 1 file changed, 2 insertions(+), 12 deletions(-)

--- a/arch/x86/kernel/ptrace.c
+++ b/arch/x86/kernel/ptrace.c
@@ -370,22 +370,12 @@ static int putreg(struct task_struct *ch
 	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.
-		 */
-		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
 	}