[PATCH] arm64/gcs: Fix error handling in arch_set_shadow_stack_status()

[Breno Leitao]

alloc_gcs() returns an error-encoded pointer on failure, which comes
from do_mmap(), not NULL.

The current NULL check fails to detect errors, which could lead to using
an invalid GCS address.

Use IS_ERR_VALUE() to properly detect errors, consistent with the
check in gcs_alloc_thread_stack().

Fixes: b57180c75c7eb ("arm64/gcs: Implement shadow stack prctl() interface")
Signed-off-by: Breno Leitao <leitao at debian.org>
---
PS: This was compiled-tested only, given I unfortunately don't have
a hardware to test on _yet_.
---
 arch/arm64/mm/gcs.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/arch/arm64/mm/gcs.c b/arch/arm64/mm/gcs.c
index 6e93f78de79b1..efce7642b1d7b 100644
--- a/arch/arm64/mm/gcs.c
+++ b/arch/arm64/mm/gcs.c
@@ -199,8 +199,8 @@ int arch_set_shadow_stack_status(struct task_struct *task, unsigned long arg)
 
 		size = gcs_size(0);
 		gcs = alloc_gcs(0, size);
-		if (!gcs)
-			return -ENOMEM;
+		if (IS_ERR_VALUE(gcs))
+			return PTR_ERR((void *)gcs);
 
 		task->thread.gcspr_el0 = gcs + size - sizeof(u64);
 		task->thread.gcs_base = gcs;

---
base-commit: 8dfce8991b95d8625d0a1d2896e42f93b9d7f68d
change-id: 20260129-arm64_cgs-496817025d1e

Best regards,
--  
Breno Leitao <leitao at debian.org>