[PATCH 6.15.y] s390/ptrace: Fix pointer dereferencing in regs_get_kernel_stack_nth()

[Heiko Carstens <hca@linux.ibm.com>]

The recent change which added READ_ONCE_NOCHECK() to read the nth entry
from the kernel stack incorrectly dropped dereferencing of the stack
pointer in order to read the requested entry.

In result the address of the entry is returned instead of its content.

Dereference the pointer again to fix this.

Reported-by: Will Deacon <will@kernel.org>
Closes: https://lore.kernel.org/r/20250612163331.GA13384@willie-the-truck
Fixes: d93a855c31b7 ("s390/ptrace: Avoid KASAN false positives in regs_get_kernel_stack_nth()")
Cc: stable@vger.kernel.org
Reviewed-by: Alexander Gordeev <agordeev@linux.ibm.com>
Signed-off-by: Alexander Gordeev <agordeev@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>
(cherry picked from commit 7f8073cfb04a97842fe891ca50dad60afd1e3121)
---
 arch/s390/kernel/ptrace.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/s390/kernel/ptrace.c b/arch/s390/kernel/ptrace.c
index 34b8d9e745df..1b8bc1720d60 100644
--- a/arch/s390/kernel/ptrace.c
+++ b/arch/s390/kernel/ptrace.c
@@ -1574,5 +1574,5 @@ unsigned long regs_get_kernel_stack_nth(struct pt_regs *regs, unsigned int n)
 	addr = kernel_stack_pointer(regs) + n * sizeof(long);
 	if (!regs_within_kernel_stack(regs, addr))
 		return 0;
-	return READ_ONCE_NOCHECK(addr);
+	return READ_ONCE_NOCHECK(*(unsigned long *)addr);
 }
-- 
2.48.1