[PATCH v3] mips: sched: Fix CPUMASK_OFFSTACK memory corruption

[Aaron Tomlin]

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 mask pointer. Furthermore, the old
logic performed copy_from_user() before allocating the mask.

Fix this by allocating new_mask first. To handle variable-sized user
masks correctly, use cpumask_size() to truncate overly large user masks
or pad undersized masks with zeros before copying the data directly into
the allocated buffer.

Fixes: 295cbf6d63165 ("[MIPS] Move FPU affinity code into separate file.")
Cc: stable@xxxxxxxxxxxxxxx
Signed-off-by: Aaron Tomlin <atomlin@xxxxxxxxxxx>
---
Changes since v2 [1]:
 - Dropped patch 1. This is to be addressed by the cgroup cpuset
   maintainer (Waiman Long)

 - Dropped patch 3. Will be submitted as a separate patch (Paul Moore)

Changes since v1 [2]:
 - Reordered the allocation and user-copy of new_mask in the MIPS
   architecture's mipsmt_sys_sched_setaffinity() to occur before the
   LSM hook is invoked. This ensures the security modules evaluate a fully
   populated mask rather than uninitialised memory, while cleanly handling
   error unwinding

 - Updated cpuset_can_fork() to pass the destination cpuset's effective CPU
   mask instead of NULL

[1]: https://lore.kernel.org/lkml/20260509213803.968464-1-atomlin@xxxxxxxxxxx/
[2]: https://lore.kernel.org/lkml/20260509164847.939294-1-atomlin@xxxxxxxxxxx/
---
 arch/mips/kernel/mips-mt-fpaff.c | 28 +++++++++++++++-------------
 1 file changed, 15 insertions(+), 13 deletions(-)

diff --git a/arch/mips/kernel/mips-mt-fpaff.c b/arch/mips/kernel/mips-mt-fpaff.c
index 10172fc4f627..4fead87d2f43 100644
--- a/arch/mips/kernel/mips-mt-fpaff.c
+++ b/arch/mips/kernel/mips-mt-fpaff.c
@@ -71,11 +71,16 @@ asmlinkage long mipsmt_sys_sched_setaffinity(pid_t pid, unsigned int len,
 	struct task_struct *p;
 	int retval;
 
-	if (len < sizeof(new_mask))
-		return -EINVAL;
-
-	if (copy_from_user(&new_mask, user_mask_ptr, sizeof(new_mask)))
-		return -EFAULT;
+	if (!alloc_cpumask_var(&new_mask, GFP_KERNEL))
+		return -ENOMEM;
+	if (len < cpumask_size())
+		cpumask_clear(new_mask);
+	else if (len > cpumask_size())
+		len = cpumask_size();
+	if (copy_from_user(new_mask, user_mask_ptr, len)) {
+		retval = -EFAULT;
+		goto out_free_new_mask;
+	}
 
 	cpus_read_lock();
 	rcu_read_lock();
@@ -84,7 +89,8 @@ asmlinkage long mipsmt_sys_sched_setaffinity(pid_t pid, unsigned int len,
 	if (!p) {
 		rcu_read_unlock();
 		cpus_read_unlock();
-		return -ESRCH;
+		retval = -ESRCH;
+		goto out_free_new_mask;
 	}
 
 	/* Prevent p going away */
@@ -95,13 +101,9 @@ asmlinkage long mipsmt_sys_sched_setaffinity(pid_t pid, unsigned int len,
 		retval = -ENOMEM;
 		goto out_put_task;
 	}
-	if (!alloc_cpumask_var(&new_mask, GFP_KERNEL)) {
-		retval = -ENOMEM;
-		goto out_free_cpus_allowed;
-	}
 	if (!alloc_cpumask_var(&effective_mask, GFP_KERNEL)) {
 		retval = -ENOMEM;
-		goto out_free_new_mask;
+		goto out_free_cpus_allowed;
 	}
 	if (!check_same_owner(p) && !capable(CAP_SYS_NICE)) {
 		retval = -EPERM;
@@ -142,13 +144,13 @@ asmlinkage long mipsmt_sys_sched_setaffinity(pid_t pid, unsigned int len,
 	}
 out_unlock:
 	free_cpumask_var(effective_mask);
-out_free_new_mask:
-	free_cpumask_var(new_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;
 }
 

base-commit: 5200f5f493f79f14bbdc349e402a40dfb32f23c8
-- 
2.51.0