proc: use the same treatment to check proc_lseek as ones for proc_read_iter et.al

Check pde->proc_ops->proc_lseek directly may cause UAF in rmmod scenario. 
It's a gap in proc_reg_open() after commit 654b33ada4ab("proc: fix UAF in
proc_get_inode()").  Followed by AI Viro's suggestion, fix it in same
manner.

Link: https://lkml.kernel.org/r/[email protected]
Fixes: 3f61631d47 ("take care to handle NULL ->proc_lseek()")
Signed-off-by: wangzijie <[email protected]>
Reviewed-by: Alexey Dobriyan <[email protected]>
Cc: Alexei Starovoitov <[email protected]>
Cc: Al Viro <[email protected]>
Cc: "Edgecombe, Rick P" <[email protected]>
Cc: Kirill A. Shuemov <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

fs/proc/generic.c

@@ -569,6 +569,8 @@ static void pde_set_flags(struct proc_dir_entry *pde)
 	if (pde->proc_ops->proc_compat_ioctl)
 		pde->flags |= PROC_ENTRY_proc_compat_ioctl;
 #endif
+	if (pde->proc_ops->proc_lseek)
+		pde->flags |= PROC_ENTRY_proc_lseek;
 }
 
 struct proc_dir_entry *proc_create_data(const char *name, umode_t mode,

fs/proc/inode.c

@@ -473,7 +473,7 @@ static int proc_reg_open(struct inode *inode, struct file *file)
 	typeof_member(struct proc_ops, proc_open) open;
 	struct pde_opener *pdeo;
 
-	if (!pde->proc_ops->proc_lseek)
+	if (!pde_has_proc_lseek(pde))
 		file->f_mode &= ~FMODE_LSEEK;
 
 	if (pde_is_permanent(pde)) {

fs/proc/internal.h

@@ -99,6 +99,11 @@ static inline bool pde_has_proc_compat_ioctl(const struct proc_dir_entry *pde)
 #endif
 }
 
+static inline bool pde_has_proc_lseek(const struct proc_dir_entry *pde)
+{
+	return pde->flags & PROC_ENTRY_proc_lseek;
+}
+
 extern struct kmem_cache *proc_dir_entry_cache;
 void pde_free(struct proc_dir_entry *pde);
 

include/linux/proc_fs.h

@@ -27,6 +27,7 @@ enum {
 
 	PROC_ENTRY_proc_read_iter	= 1U << 1,
 	PROC_ENTRY_proc_compat_ioctl	= 1U << 2,
+	PROC_ENTRY_proc_lseek		= 1U << 3,
 };
 
 struct proc_ops {