linux-kernel - [PATCH] mount: fix undefined behavior in bit shift for MS

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Date:   Mon, 31 Oct 2022 19:39:42 +0800
From:   Gaosheng Cui <cuigaosheng1@...wei.com>
To:     <dhowells@...hat.com>, <viro@...iv.linux.org.uk>,
        <cuigaosheng1@...wei.com>
CC:     <linux-kernel@...r.kernel.org>
Subject: [PATCH] mount: fix undefined behavior in bit shift for MS_NOUSER

Shifting signed 32-bit value by 31 bits is undefined, so changing
significant bit to unsigned. The UBSAN warning calltrace like below:

UBSAN: shift-out-of-bounds in fs/namespace.c:3309:14
left shift of 1 by 31 places cannot be represented in type 'int'
Call Trace:
 <TASK>
 dump_stack_lvl+0x7d/0xa5
 dump_stack+0x15/0x1b
 ubsan_epilogue+0xe/0x4e
 __ubsan_handle_shift_out_of_bounds+0x1e7/0x20c
 path_mount+0x93/0xd50
 init_mount+0x6a/0xa3
 devtmpfs_setup+0x47/0x7e
 devtmpfsd+0x1a/0x50
 kthread+0x126/0x160
 ret_from_fork+0x1f/0x30
 </TASK>

Fixes: e262e32d6bde ("vfs: Suppress MS_* flag defs within the kernel unless explicitly enabled")
Signed-off-by: Gaosheng Cui <cuigaosheng1@...wei.com>
---
 include/uapi/linux/mount.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/include/uapi/linux/mount.h b/include/uapi/linux/mount.h
index 4d93967f8aea..035de4bd968f 100644
--- a/include/uapi/linux/mount.h
+++ b/include/uapi/linux/mount.h
@@ -44,7 +44,7 @@
 #define MS_NOSEC	(1<<28)
 #define MS_BORN		(1<<29)
 #define MS_ACTIVE	(1<<30)
-#define MS_NOUSER	(1<<31)
+#define MS_NOUSER	(1U<<31)
 
 /*
  * Superblock flags that can be altered by MS_REMOUNT
-- 
2.25.1