x86/fpu: Hook up PKRU into ptrace()

One nice thing about having PKRU be XSAVE-managed is that it gets naturally
exposed into the XSAVE-using ABIs.  Now that XSAVE will not be used to
manage PKRU, these ABIs need to be manually enabled to deal with PKRU.

ptrace() uses copy_uabi_xstate_to_kernel() to collect the tracee's
XSTATE. As PKRU is not in the task's XSTATE buffer, use task->thread.pkru
for filling in up the ptrace buffer.
Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Borislav Petkov <bp@suse.de>
Reviewed-by: Borislav Petkov <bp@suse.de>
Link: https://lkml.kernel.org/r/20210623121456.508770763@linutronix.de

arch/x86/include/asm/fpu/xstate.h

@@ -139,7 +139,7 @@ enum xstate_copy_mode {
 };
 
 struct membuf;
-void copy_xstate_to_uabi_buf(struct membuf to, struct xregs_state *xsave,
+void copy_xstate_to_uabi_buf(struct membuf to, struct task_struct *tsk,
 			     enum xstate_copy_mode mode);
 
 #endif

arch/x86/kernel/fpu/regset.c

@@ -78,7 +78,7 @@ int xfpregs_get(struct task_struct *target, const struct user_regset *regset,
 				    sizeof(fpu->state.fxsave));
 	}
 
-	copy_xstate_to_uabi_buf(to, &fpu->state.xsave, XSTATE_COPY_FX);
+	copy_xstate_to_uabi_buf(to, target, XSTATE_COPY_FX);
 	return 0;
 }
 
@@ -126,14 +126,12 @@ int xfpregs_set(struct task_struct *target, const struct user_regset *regset,
 int xstateregs_get(struct task_struct *target, const struct user_regset *regset,
 		struct membuf to)
 {
-	struct fpu *fpu = &target->thread.fpu;
-
 	if (!cpu_feature_enabled(X86_FEATURE_XSAVE))
 		return -ENODEV;
 
-	sync_fpstate(fpu);
+	sync_fpstate(&target->thread.fpu);
 
-	copy_xstate_to_uabi_buf(to, &fpu->state.xsave, XSTATE_COPY_XSAVE);
+	copy_xstate_to_uabi_buf(to, target, XSTATE_COPY_XSAVE);
 	return 0;
 }
 
@@ -336,7 +334,7 @@ int fpregs_get(struct task_struct *target, const struct user_regset *regset,
 		struct membuf mb = { .p = &fxsave, .left = sizeof(fxsave) };
 
 		/* Handle init state optimized xstate correctly */
-		copy_xstate_to_uabi_buf(mb, &fpu->state.xsave, XSTATE_COPY_FP);
+		copy_xstate_to_uabi_buf(mb, target, XSTATE_COPY_FP);
 		fx = &fxsave;
 	} else {
 		fx = &fpu->state.fxsave;

arch/x86/kernel/fpu/xstate.c

@@ -962,7 +962,7 @@ static void copy_feature(bool from_xstate, struct membuf *to, void *xstate,
 /**
  * copy_xstate_to_uabi_buf - Copy kernel saved xstate to a UABI buffer
  * @to:		membuf descriptor
- * @xsave:	The kernel xstate buffer to copy from
+ * @tsk:	The task from which to copy the saved xstate
  * @copy_mode:	The requested copy mode
  *
  * Converts from kernel XSAVE or XSAVES compacted format to UABI conforming
@@ -971,10 +971,11 @@ static void copy_feature(bool from_xstate, struct membuf *to, void *xstate,
  *
  * It supports partial copy but @to.pos always starts from zero.
  */
-void copy_xstate_to_uabi_buf(struct membuf to, struct xregs_state *xsave,
+void copy_xstate_to_uabi_buf(struct membuf to, struct task_struct *tsk,
 			     enum xstate_copy_mode copy_mode)
 {
 	const unsigned int off_mxcsr = offsetof(struct fxregs_state, mxcsr);
+	struct xregs_state *xsave = &tsk->thread.fpu.state.xsave;
 	struct xregs_state *xinit = &init_fpstate.xsave;
 	struct xstate_header header;
 	unsigned int zerofrom;
@@ -1048,11 +1049,21 @@ void copy_xstate_to_uabi_buf(struct membuf to, struct xregs_state *xsave,
 		if (zerofrom < xstate_offsets[i])
 			membuf_zero(&to, xstate_offsets[i] - zerofrom);
 
+		if (i == XFEATURE_PKRU) {
+			struct pkru_state pkru = {0};
+			/*
+			 * PKRU is not necessarily up to date in the
+			 * thread's XSAVE buffer.  Fill this part from the
+			 * per-thread storage.
+			 */
+			pkru.pkru = tsk->thread.pkru;
+			membuf_write(&to, &pkru, sizeof(pkru));
+		} else {
 			copy_feature(header.xfeatures & BIT_ULL(i), &to,
 				     __raw_xsave_addr(xsave, i),
 				     __raw_xsave_addr(xinit, i),
 				     xstate_sizes[i]);
-
+		}
 		/*
 		 * Keep track of the last copied state in the non-compacted
 		 * target buffer for gap zeroing.