[PATCH v5 16/28] 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 xstate buffer. Detect this case and allocate a
sufficient buffer to support the request. Also, disable the (now
unnecessary) associated first-use fault.

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 v4:
* Improved the condition check for the expansion.
* Simplified the XSTATE_BV retrieval.
* Updated the code comment.

Changes from v3:
* Removed 'no functional changes' in the changelog. (Borislav Petkov)

Changes from v2:
* Updated the changelog with task->fpu removed. (Borislav Petkov)
* Updated the code comments.
---
 arch/x86/kernel/fpu/regset.c | 32 ++++++++++++++++++++++++++++++++
 1 file changed, 32 insertions(+)

diff --git a/arch/x86/kernel/fpu/regset.c b/arch/x86/kernel/fpu/regset.c
index ee27df4caed6..b2fac4b5e483 100644
--- a/arch/x86/kernel/fpu/regset.c
+++ b/arch/x86/kernel/fpu/regset.c
@@ -122,6 +122,38 @@ int xstateregs_set(struct task_struct *target, const struct user_regset *regset,
 
 	xsave = &fpu->state->xsave;
 
+	/*
+	 * When a ptracer attempts to write any dynamic user state in the target buffer but not
+	 * sufficiently allocated, it dynamically expands the buffer.
+	 *
+	 * Check if the expansion is possibly needed.
+	 */
+	if (xfeatures_mask_user_dynamic &&
+	    ((fpu->state_mask & xfeatures_mask_user_dynamic) != xfeatures_mask_user_dynamic)) {
+		unsigned int offset, size;
+		u64 state_mask;
+
+		offset = offsetof(struct xregs_state, header);
+		size = sizeof(u64);
+
+		/* Retrieve XSTATE_BV. */
+		if (kbuf) {
+			memcpy(&state_mask, kbuf + offset, size);
+		} else {
+			ret = __copy_from_user(&state_mask, ubuf + offset, size);
+			if (ret)
+				return ret;
+		}
+
+		/* Expand the xstate buffer based on the XSTATE_BV. */
+		state_mask &= xfeatures_mask_user_dynamic;
+		if (state_mask) {
+			ret = alloc_xstate_buffer(fpu, state_mask);
+			if (ret)
+				return ret;
+		}
+	}
+
 	fpu__prepare_write(fpu);
 
 	if (using_compacted_format()) {
-- 
2.17.1