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Subject[PATCH 4.14 14/14] VFS: Fix memory leak caused by concurrently mounting fs with subtype
Date
From: ChenXiaoSong <chenxiaosong2@huawei.com>

If two processes mount same superblock, memory leak occurs:

CPU0 | CPU1
do_new_mount | do_new_mount
fs_set_subtype | fs_set_subtype
kstrdup |
| kstrdup
memrory leak |

The following reproducer triggers the problem:

1. shell command: mount -t ntfs /dev/sda1 /mnt &
2. c program: mount("/dev/sda1", "/mnt", "fuseblk", 0, "...")

with kmemleak report being along the lines of

unreferenced object 0xffff888235f1a5c0 (size 8):
comm "mount.ntfs", pid 2860, jiffies 4295757824 (age 43.423s)
hex dump (first 8 bytes):
00 a5 f1 35 82 88 ff ff ...5....
backtrace:
[<00000000656e30cc>] __kmalloc_track_caller+0x16e/0x430
[<000000008e591727>] kstrdup+0x3e/0x90
[<000000008430d12b>] do_mount.cold+0x7b/0xd9
[<0000000078d639cd>] ksys_mount+0xb2/0x150
[<000000006015988d>] __x64_sys_mount+0x29/0x40
[<00000000e0a7c118>] do_syscall_64+0xc1/0x1d0
[<00000000bcea7df5>] entry_SYSCALL_64_after_hwframe+0x44/0xa9
[<00000000803a4067>] 0xffffffffffffffff

Linus's tree already have refactoring patchset [1], one of them can fix this bug:
c30da2e981a7 ("fuse: convert to use the new mount API")
After refactoring, init super_block->s_subtype in fuse_fill_super.

Since we did not merge the refactoring patchset in this branch, I create this patch.
This patch fix this by adding a write lock while calling fs_set_subtype.

[1] https://patchwork.kernel.org/project/linux-fsdevel/patch/20190903113640.7984-3-mszeredi@redhat.com/

Fixes: 79c0b2df79eb ("add filesystem subtype support")
Cc: David Howells <dhowells@redhat.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: ChenXiaoSong <chenxiaosong2@huawei.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
v1: Can not mount sshfs ([PATCH linux-4.19.y] VFS: Fix fuseblk memory leak caused by mount concurrency)
v2: Use write lock while writing superblock ([PATCH 4.19,v2] VFS: Fix fuseblk memory leak caused by mount concurrency)
v3: Update commit message

fs/namespace.c | 9 ++++++---
1 file changed, 6 insertions(+), 3 deletions(-)

--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -2570,9 +2570,12 @@ static int do_new_mount(struct path *pat
return -ENODEV;

mnt = vfs_kern_mount(type, sb_flags, name, data);
- if (!IS_ERR(mnt) && (type->fs_flags & FS_HAS_SUBTYPE) &&
- !mnt->mnt_sb->s_subtype)
- mnt = fs_set_subtype(mnt, fstype);
+ if (!IS_ERR(mnt) && (type->fs_flags & FS_HAS_SUBTYPE)) {
+ down_write(&mnt->mnt_sb->s_umount);
+ if (!mnt->mnt_sb->s_subtype)
+ mnt = fs_set_subtype(mnt, fstype);
+ up_write(&mnt->mnt_sb->s_umount);
+ }

put_filesystem(type);
if (IS_ERR(mnt))

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