[PATCH 5.10.y v2] mips: sched: Fix CPUMASK_OFFSTACK memory corruption

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: [email protected]
Signed-off-by: Aaron Tomlin <[email protected]>
Signed-off-by: Thomas Bogendoerfer <[email protected]>
(cherry picked from commit 98e37db4a34d3af3fb2f4648295c25b5e40b20e3)

---

Changes since v1:

 - Fixed the !p error path to set retval = -ESRCH and jump to
   out_free_new_mask so new_mask is properly freed on failure (Sasha Levin)

 - Link to v1: https://lore.kernel.org/stable/[email protected]/
---
 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 6c590ef27648..5feffc8708ad 100644
--- a/arch/mips/kernel/mips-mt-fpaff.c
+++ b/arch/mips/kernel/mips-mt-fpaff.c
@@ -70,11 +70,16 @@ 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;
+	}
 
 	get_online_cpus();
 	rcu_read_lock();
@@ -83,7 +88,8 @@ asmlinkage long mipsmt_sys_sched_setaffinity(pid_t pid, unsigned int len,
 	if (!p) {
 		rcu_read_unlock();
 		put_online_cpus();
-		return -ESRCH;
+		retval = -ESRCH;
+		goto out_free_new_mask;
 	}
 
 	/* Prevent p going away */
@@ -94,13 +100,9 @@ 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;
@@ -141,13 +143,13 @@ 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);
 	put_online_cpus();
+out_free_new_mask:
+	free_cpumask_var(new_mask);
 	return retval;
 }
 
-- 
2.55.0