[tip: sched/urgent] rseq: Fix rseq registration with CONFIG_DEBUG_RSEQ

[tip-bot2 for Michael Jeanson]

The following commit has been merged into the sched/urgent branch of tip:

Commit-ID:     dc0a241ceaf3b7df6f1a7658b020c92682b75bfc
Gitweb:        https://git.kernel.org/tip/dc0a241ceaf3b7df6f1a7658b020c92682b75bfc
Author:        Michael Jeanson <mjeanson@xxxxxxxxxxxx>
AuthorDate:    Wed, 19 Feb 2025 15:53:26 -05:00
Committer:     Ingo Molnar <mingo@xxxxxxxxxx>
CommitterDate: Fri, 21 Feb 2025 14:21:02 +01:00

rseq: Fix rseq registration with CONFIG_DEBUG_RSEQ

With CONFIG_DEBUG_RSEQ=y, at rseq 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 - but it erroneously is.

Move the activation of the registration after the copy of the fields to
fix this bug.

Fixes: 7d5265ffcd8b ("rseq: Validate read-only fields under DEBUG_RSEQ config")
Signed-off-by: Michael Jeanson <mjeanson@xxxxxxxxxxxx>
Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>
Reviewed-by: Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxxxx>
Link: https://lore.kernel.org/r/20250219205330.324770-1-mjeanson@xxxxxxxxxxxx
---
 kernel/rseq.c | 11 ++++++++---
 1 file changed, 8 insertions(+), 3 deletions(-)

diff --git a/kernel/rseq.c b/kernel/rseq.c
index 442aba2..2cb1609 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
@@ -522,6 +519,14 @@ SYSCALL_DEFINE4(rseq, struct rseq __user *, rseq, u32, rseq_len,
 		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
 	 * are updated before returning to user-space.