[PATCH 5.4 80/91] m68k: Add missing mmap_read_lock() to sys_cacheflush()

[Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>]

5.4-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Liam Howlett <liam.howlett@xxxxxxxxxx>

commit f829b4b212a315b912cb23fd10aaf30534bb5ce9 upstream.

When the superuser flushes the entire cache, the mmap_read_lock() is not
taken, but mmap_read_unlock() is called.  Add the missing
mmap_read_lock() call.

Fixes: cd2567b6850b1648 ("m68k: call find_vma with the mmap_sem held in sys_cacheflush()")
Signed-off-by: Liam R. Howlett <Liam.Howlett@xxxxxxxxxx>
Reviewed-by: Matthew Wilcox (Oracle) <willy@xxxxxxxxxxxxx>
Link: https://lore.kernel.org/r/20210407200032.764445-1-Liam.Howlett@xxxxxxxxxx
Signed-off-by: Geert Uytterhoeven <geert@xxxxxxxxxxxxxx>
[ mmap_read_lock() open-coded using down_read() as was done prior to v5.8 ]
Signed-off-by: Finn Thain <fthain@xxxxxxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
---
 arch/m68k/kernel/sys_m68k.c |    2 ++
 1 file changed, 2 insertions(+)

--- a/arch/m68k/kernel/sys_m68k.c
+++ b/arch/m68k/kernel/sys_m68k.c
@@ -388,6 +388,8 @@ sys_cacheflush (unsigned long addr, int
 		ret = -EPERM;
 		if (!capable(CAP_SYS_ADMIN))
 			goto out;
+
+		down_read(&current->mm->mmap_sem);
 	} else {
 		struct vm_area_struct *vma;