proc: fix the issue of proc_mem_open returning NULL

proc_mem_open() can return an errno, NULL, or mm_struct*.  If it fails to
acquire mm, it returns NULL, but the caller does not check for the case
when the return value is NULL.

The following conditions lead to failure in acquiring mm:

  - The task is a kernel thread (PF_KTHREAD)
  - The task is exiting (PF_EXITING)

Changes:

  - Add documentation comments for the return value of proc_mem_open().
  - Add checks in the caller to return -ESRCH when proc_mem_open()
    returns NULL.

Link: https://lkml.kernel.org/r/20250404063357.78891-1-superman.xpt@gmail.com
Reported-by: syzbot+f9238a0a31f9b5603fef@syzkaller.appspotmail.com
Closes: https://lore.kernel.org/all/000000000000f52642060d4e3750@google.com
Signed-off-by: Penglei Jiang <superman.xpt@gmail.com>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Adrian Ratiu <adrian.ratiu@collabora.com>
Cc: Christian Brauner <brauner@kernel.org>
Cc: Felix Moessbauer <felix.moessbauer@siemens.com>
Cc: Jeff layton <jlayton@kernel.org>
Cc: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Cc: Mateusz Guzik <mjguzik@gmail.com>
Cc: Thomas Gleinxer <tglx@linutronix.de>
Cc: xu xin <xu.xin16@zte.com.cn>
Cc: Alexey Dobriyan <adobriyan@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

3 files changed, 17 insertions, 11 deletions

diff --git a/fs/proc/base.c b/fs/proc/base.c
index b0d4e1908b22..85a3f5e253d4 100644
--- a/fs/proc/base.c
+++ b/fs/proc/base.c
@@ -827,7 +827,13 @@ static const struct file_operations proc_single_file_operations = {
 	.release	= single_release,
 };
 
-
+/*
+ * proc_mem_open() can return errno, NULL or mm_struct*.
+ *
+ *   - Returns NULL if the task has no mm (PF_KTHREAD or PF_EXITING)
+ *   - Returns mm_struct* on success
+ *   - Returns error code on failure
+ */
 struct mm_struct *proc_mem_open(struct inode *inode, unsigned int mode)
 {
 	struct task_struct *task = get_proc_task(inode);
@@ -854,8 +860,8 @@ static int __mem_open(struct inode *inode, struct file *file, unsigned int mode)
 {
 	struct mm_struct *mm = proc_mem_open(inode, mode);
 
-	if (IS_ERR(mm))
-		return PTR_ERR(mm);
+	if (IS_ERR_OR_NULL(mm))
+		return mm ? PTR_ERR(mm) : -ESRCH;
 
 	file->private_data = mm;
 	return 0;
diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c
index 994cde10e3f4..b9e3bd006346 100644
--- a/fs/proc/task_mmu.c
+++ b/fs/proc/task_mmu.c
@@ -212,8 +212,8 @@ static int proc_maps_open(struct inode *inode, struct file *file,
 
 	priv->inode = inode;
 	priv->mm = proc_mem_open(inode, PTRACE_MODE_READ);
-	if (IS_ERR(priv->mm)) {
-		int err = PTR_ERR(priv->mm);
+	if (IS_ERR_OR_NULL(priv->mm)) {
+		int err = priv->mm ? PTR_ERR(priv->mm) : -ESRCH;
 
 		seq_release_private(inode, file);
 		return err;
@@ -1325,8 +1325,8 @@ static int smaps_rollup_open(struct inode *inode, struct file *file)
 
 	priv->inode = inode;
 	priv->mm = proc_mem_open(inode, PTRACE_MODE_READ);
-	if (IS_ERR(priv->mm)) {
-		ret = PTR_ERR(priv->mm);
+	if (IS_ERR_OR_NULL(priv->mm)) {
+		ret = priv->mm ? PTR_ERR(priv->mm) : -ESRCH;
 
 		single_release(inode, file);
 		goto out_free;
@@ -2069,8 +2069,8 @@ static int pagemap_open(struct inode *inode, struct file *file)
 	struct mm_struct *mm;
 
 	mm = proc_mem_open(inode, PTRACE_MODE_READ);
-	if (IS_ERR(mm))
-		return PTR_ERR(mm);
+	if (IS_ERR_OR_NULL(mm))
+		return mm ? PTR_ERR(mm) : -ESRCH;
 	file->private_data = mm;
 	return 0;
 }
diff --git a/fs/proc/task_nommu.c b/fs/proc/task_nommu.c
index bce674533000..59bfd61d653a 100644
--- a/fs/proc/task_nommu.c
+++ b/fs/proc/task_nommu.c
@@ -260,8 +260,8 @@ static int maps_open(struct inode *inode, struct file *file,
 
 	priv->inode = inode;
 	priv->mm = proc_mem_open(inode, PTRACE_MODE_READ);
-	if (IS_ERR(priv->mm)) {
-		int err = PTR_ERR(priv->mm);
+	if (IS_ERR_OR_NULL(priv->mm)) {
+		int err = priv->mm ? PTR_ERR(priv->mm) : -ESRCH;
 
 		seq_release_private(inode, file);
 		return err;