1 files changed, 24 insertions, 12 deletions

diff --git a/arch/x86/kernel/ptrace.c b/arch/x86/kernel/ptrace.c
index 066e5b01a7e0..f0e1ddbc2fd7 100644
--- a/arch/x86/kernel/ptrace.c
+++ b/arch/x86/kernel/ptrace.c
@@ -182,6 +182,9 @@ static u16 get_segment_reg(struct task_struct *task, unsigned long offset)
 static int set_segment_reg(struct task_struct *task,
 			   unsigned long offset, u16 value)
 {
+	if (WARN_ON_ONCE(task == current))
+		return -EIO;
+
 	/*
 	 * The value argument was already truncated to 16 bits.
 	 */
@@ -209,10 +212,7 @@ static int set_segment_reg(struct task_struct *task,
 		break;
 
 	case offsetof(struct user_regs_struct, gs):
-		if (task == current)
-			set_user_gs(task_pt_regs(task), value);
-		else
-			task_user_gs(task) = value;
+		task_user_gs(task) = value;
 	}
 
 	return 0;
@@ -272,32 +272,41 @@ static u16 get_segment_reg(struct task_struct *task, unsigned long offset)
 static int set_segment_reg(struct task_struct *task,
 			   unsigned long offset, u16 value)
 {
+	if (WARN_ON_ONCE(task == current))
+		return -EIO;
+
 	/*
 	 * The value argument was already truncated to 16 bits.
 	 */
 	if (invalid_selector(value))
 		return -EIO;
 
+	/*
+	 * This function has some ABI oddities.
+	 *
+	 * A 32-bit ptracer probably expects that writing FS or GS will change
+	 * FSBASE or GSBASE respectively.  In the absence of FSGSBASE support,
+	 * this code indeed has that effect.  When FSGSBASE is added, this
+	 * will require a special case.
+	 *
+	 * For existing 64-bit ptracers, writing FS or GS *also* currently
+	 * changes the base if the selector is nonzero the next time the task
+	 * is run.  This behavior may not be needed, and trying to preserve it
+	 * when FSGSBASE is added would be complicated at best.
+	 */
+
 	switch (offset) {
 	case offsetof(struct user_regs_struct,fs):
 		task->thread.fsindex = value;
-		if (task == current)
-			loadsegment(fs, task->thread.fsindex);
 		break;
 	case offsetof(struct user_regs_struct,gs):
 		task->thread.gsindex = value;
-		if (task == current)
-			load_gs_index(task->thread.gsindex);
 		break;
 	case offsetof(struct user_regs_struct,ds):
 		task->thread.ds = value;
-		if (task == current)
-			loadsegment(ds, task->thread.ds);
 		break;
 	case offsetof(struct user_regs_struct,es):
 		task->thread.es = value;
-		if (task == current)
-			loadsegment(es, task->thread.es);
 		break;
 
 		/*
@@ -375,6 +384,9 @@ static int putreg(struct task_struct *child,
 		 * 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);