[patch V2 13/52] x86/fpu: Clean up fpregs_set()

[Thomas Gleixner <tglx@linutronix.de>]

From: Andy Lutomirski <luto@kernel.org>

fpregs_set() had unnecessary complexity to support short or nonzero-offset
writes and to handle the case in which a copy from userspace overwrites
some of the target buffer and then fails.  Support for partial writes is
useless -- just require that the write have offset 0 and the correct size,
and copy into a temporary kernel buffer to avoid clobbering the state if
the user access fails.

Signed-off-by: Andy Lutomirski <luto@kernel.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
---
V2: New patch. Picked up from Andy
---
 arch/x86/kernel/fpu/regset.c |   27 ++++++++++++++-------------
 1 file changed, 14 insertions(+), 13 deletions(-)
---
--- a/arch/x86/kernel/fpu/regset.c
+++ b/arch/x86/kernel/fpu/regset.c
@@ -305,31 +305,32 @@ int fpregs_set(struct task_struct *targe
 	struct user_i387_ia32_struct env;
 	int ret;
 
-	fpu__prepare_write(fpu);
-	fpstate_sanitize_xstate(fpu);
+	/* No funny business with partial or oversized writes is permitted. */
+	if (pos != 0 || count != sizeof(struct user_i387_ia32_struct))
+		return -EINVAL;
 
 	if (!boot_cpu_has(X86_FEATURE_FPU))
 		return fpregs_soft_set(target, regset, pos, count, kbuf, ubuf);
 
-	if (!boot_cpu_has(X86_FEATURE_FXSR))
-		return user_regset_copyin(&pos, &count, &kbuf, &ubuf,
-					  &fpu->state.fsave, 0,
-					  -1);
+	ret = user_regset_copyin(&pos, &count, &kbuf, &ubuf, &env, 0, -1);
+	if (ret)
+		return ret;
 
-	if (pos > 0 || count < sizeof(env))
-		convert_from_fxsr(&env, target);
+	fpu__prepare_write(fpu);
 
-	ret = user_regset_copyin(&pos, &count, &kbuf, &ubuf, &env, 0, -1);
-	if (!ret)
+	if (static_cpu_has(X86_FEATURE_FXSR))
 		convert_to_fxsr(&target->thread.fpu.state.fxsave, &env);
+	else
+		memcpy(&target->thread.fpu.state.fsave, &env, sizeof(env));
 
 	/*
-	 * update the header bit in the xsave header, indicating the
+	 * Update the header bit in the xsave header, indicating the
 	 * presence of FP.
 	 */
-	if (boot_cpu_has(X86_FEATURE_XSAVE))
+	if (static_cpu_has(X86_FEATURE_XSAVE))
 		fpu->state.xsave.header.xfeatures |= XFEATURE_MASK_FP;
-	return ret;
+
+	return 0;
 }
 
 #endif	/* CONFIG_X86_32 || CONFIG_IA32_EMULATION */