[PATCH v3 14/21] x86/fpu/xstate: Support ptracer-induced xstate buffer expansion

["Chang S. Bae" <chang.seok.bae@intel.com>]

ptrace() may update xstate data before the target task has taken an XFD
fault and expanded the context switch buffer. Detect this case and allocate
a sufficient buffer to support the request. Also, disable the (now
unnecessary) associated first-use fault.

No functional change until the kernel supports dynamic user states.

Signed-off-by: Chang S. Bae <chang.seok.bae@intel.com>
Reviewed-by: Len Brown <len.brown@intel.com>
Cc: x86@kernel.org
Cc: linux-kernel@vger.kernel.org
---
Changes from v2:
* Updated the changelog with task->fpu removed. (Boris Petkov)
* Updated the code comments.
---
 arch/x86/kernel/fpu/regset.c | 29 +++++++++++++++++++++++++++++
 1 file changed, 29 insertions(+)

diff --git a/arch/x86/kernel/fpu/regset.c b/arch/x86/kernel/fpu/regset.c
index 8d863240b9c6..16ff8ac765c1 100644
--- a/arch/x86/kernel/fpu/regset.c
+++ b/arch/x86/kernel/fpu/regset.c
@@ -125,6 +125,35 @@ int xstateregs_set(struct task_struct *target, const struct user_regset *regset,
 
 	xsave = __xsave(fpu);
 
+	/*
+	 * When a ptracer attempts to write any state in the target buffer but not
+	 * sufficiently allocated, it dynamically expands the buffer.
+	 */
+	if (count > get_xstate_size(fpu->state_mask)) {
+		unsigned int offset, size;
+		struct xstate_header hdr;
+		u64 mask;
+
+		offset = offsetof(struct xregs_state, header);
+		size = sizeof(hdr);
+
+		/* Retrieve XSTATE_BV */
+		if (kbuf) {
+			memcpy(&hdr, kbuf + offset, size);
+		} else {
+			ret = __copy_from_user(&hdr, ubuf + offset, size);
+			if (ret)
+				return ret;
+		}
+
+		mask = hdr.xfeatures & xfeatures_mask_user_dynamic;
+		if (!mask) {
+			ret = alloc_xstate_buffer(fpu, mask);
+			if (ret)
+				return ret;
+		}
+	}
+
 	fpu__prepare_write(fpu);
 
 	if (using_compacted_format()) {
-- 
2.17.1