[PATCH] rseq: Fix rseq registration with CONFIG_DEBUG_RSEQ

[Michael Jeanson]

With CONFIG_DEBUG_RSEQ at registration the read-only fields are copied
from user-space, if this copy fails the syscall returns EFAULT and the
registration should not be activated.

Move the activation of the registration after the copy of the fields to
ensure this.

Fixes: 7d5265ffcd8b ("rseq: Validate read-only fields under DEBUG_RSEQ config")
Signed-off-by: Michael Jeanson <mjeanson@xxxxxxxxxxxx>
Reviewed-by: Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxxxx>
---
 kernel/rseq.c | 11 ++++++++---
 1 file changed, 8 insertions(+), 3 deletions(-)

diff --git a/kernel/rseq.c b/kernel/rseq.c
index 442aba29bc4c..2cb16091ec0a 100644
--- a/kernel/rseq.c
+++ b/kernel/rseq.c
@@ -507,9 +507,6 @@ SYSCALL_DEFINE4(rseq, struct rseq __user *, rseq, u32, rseq_len,
 		return -EINVAL;
 	if (!access_ok(rseq, rseq_len))
 		return -EFAULT;
-	current->rseq = rseq;
-	current->rseq_len = rseq_len;
-	current->rseq_sig = sig;
 #ifdef CONFIG_DEBUG_RSEQ
 	/*
 	 * Initialize the in-kernel rseq fields copy for validation of
@@ -521,6 +518,14 @@ SYSCALL_DEFINE4(rseq, struct rseq __user *, rseq, u32, rseq_len,
 	    get_user(rseq_kernel_fields(current)->mm_cid, &rseq->mm_cid))
 		return -EFAULT;
 #endif
+	/*
+	 * Activate the registration by setting the rseq area address, length
+	 * and signature in the task struct.
+	 */
+	current->rseq = rseq;
+	current->rseq_len = rseq_len;
+	current->rseq_sig = sig;
+
 	/*
 	 * If rseq was previously inactive, and has just been
 	 * registered, ensure the cpu_id_start and cpu_id fields
-- 
2.43.0