[PATCH v6 13/26] x86/fpu/xstate: Support ptracer-induced XSTATE buffer expansion

[Chang S. Bae]

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@xxxxxxxxx>
Reviewed-by: Len Brown <len.brown@xxxxxxxxx>
Cc: x86@xxxxxxxxxx
Cc: linux-kernel@xxxxxxxxxxxxxxx
---
Changes from v5:
* Adjusted to use 'tmpbuf' for the new base code.

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 | 23 +++++++++++++++++++++++
 1 file changed, 23 insertions(+)

diff --git a/arch/x86/kernel/fpu/regset.c b/arch/x86/kernel/fpu/regset.c
index 73d7d7b489fe..0e067c6684e2 100644
--- a/arch/x86/kernel/fpu/regset.c
+++ b/arch/x86/kernel/fpu/regset.c
@@ -163,6 +163,29 @@ int xstateregs_set(struct task_struct *target, const struct user_regset *regset,
 		}
 	}
 
+	/*
+	 * 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)) {
+		u64 state_mask;
+
+		/* Retrieve XSTATE_BV. */
+		memcpy(&state_mask, (kbuf ?: tmpbuf) + offsetof(struct xregs_state, header),
+		       sizeof(u64));
+
+		/* 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)
+				goto out;
+		}
+	}
+
 	fpu_force_restore(fpu);
 	ret = copy_uabi_from_kernel_to_xstate(fpu, kbuf ?: tmpbuf);
 
-- 
2.17.1