[PATCH v2 3/3] mips: sched: Fix CPUMASK_OFFSTACK memory corruption

[Aaron Tomlin <atomlin@atomlin.com>]

This patch addresses a critical memory management flaw.

When CONFIG_CPUMASK_OFFSTACK is enabled, cpumask_var_t is a pointer.
Consequently, sizeof(new_mask) evaluates to the pointer size, causing
copy_from_user() to clobber the stack pointer. The subsequent
alloc_cpumask_var() overwrites this with an uninitialised heap address,
discarding the user's mask and risking data leaks. Fix this by
allocating masks first, and using cpumask_size() to copy data directly
into the allocated buffer.

Fixes: 295cbf6d63165 ("[MIPS] Move FPU affinity code into separate file.")
Cc: stable@vger.kernel.org
Signed-off-by: Aaron Tomlin <atomlin@atomlin.com>
---
 arch/mips/kernel/mips-mt-fpaff.c | 32 +++++++++++++++-----------------
 1 file changed, 15 insertions(+), 17 deletions(-)

diff --git a/arch/mips/kernel/mips-mt-fpaff.c b/arch/mips/kernel/mips-mt-fpaff.c
index 6424152d9091..7c215372c5e8 100644
--- a/arch/mips/kernel/mips-mt-fpaff.c
+++ b/arch/mips/kernel/mips-mt-fpaff.c
@@ -71,17 +71,23 @@ asmlinkage long mipsmt_sys_sched_setaffinity(pid_t pid, unsigned int len,
 	struct task_struct *p;
 	int retval;
 
+	if (len < cpumask_size())
+		return -EINVAL;
+
 	if (!alloc_cpumask_var(&new_mask, GFP_KERNEL))
 		return -ENOMEM;
-
-	if (len < sizeof(new_mask)) {
-		retval = -EINVAL;
+	if (!alloc_cpumask_var(&cpus_allowed, GFP_KERNEL)) {
+		retval = -ENOMEM;
 		goto out_free_new_mask;
 	}
+	if (!alloc_cpumask_var(&effective_mask, GFP_KERNEL)) {
+		retval = -ENOMEM;
+		goto out_free_cpus_allowed;
+	}
 
-	if (copy_from_user(&new_mask, user_mask_ptr, sizeof(new_mask))) {
+	if (copy_from_user(new_mask, user_mask_ptr, cpumask_size())) {
 		retval = -EFAULT;
-		goto out_free_new_mask;
+		goto out_free_effective_mask;
 	}
 
 	cpus_read_lock();
@@ -92,21 +98,13 @@ asmlinkage long mipsmt_sys_sched_setaffinity(pid_t pid, unsigned int len,
 		rcu_read_unlock();
 		cpus_read_unlock();
 		retval = -ESRCH;
-		goto out_free_new_mask;
+		goto out_free_effective_mask;
 	}
 
 	/* Prevent p going away */
 	get_task_struct(p);
 	rcu_read_unlock();
 
-	if (!alloc_cpumask_var(&cpus_allowed, GFP_KERNEL)) {
-		retval = -ENOMEM;
-		goto out_put_task;
-	}
-	if (!alloc_cpumask_var(&effective_mask, GFP_KERNEL)) {
-		retval = -ENOMEM;
-		goto out_free_cpus_allowed;
-	}
 	if (!check_same_owner(p) && !capable(CAP_SYS_NICE)) {
 		retval = -EPERM;
 		goto out_unlock;
@@ -145,12 +143,12 @@ asmlinkage long mipsmt_sys_sched_setaffinity(pid_t pid, unsigned int len,
 		}
 	}
 out_unlock:
+	put_task_struct(p);
+	cpus_read_unlock();
+out_free_effective_mask:
 	free_cpumask_var(effective_mask);
 out_free_cpus_allowed:
 	free_cpumask_var(cpus_allowed);
-out_put_task:
-	put_task_struct(p);
-	cpus_read_unlock();
 out_free_new_mask:
 	free_cpumask_var(new_mask);
 	return retval;
-- 
2.51.0