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Subject[PATCH 5.2 137/137] vfs: set fs_context::user_ns for reconfigure
Date
From: Eric Biggers <ebiggers@google.com>

commit 1dd9bc08cf1420d466dd8dcfcc233777e61ca5d2 upstream.

fs_context::user_ns is used by fuse_parse_param(), even during remount,
so it needs to be set to the existing value for reconfigure.

Reproducer:

#include <fcntl.h>
#include <sys/mount.h>

int main()
{
char opts[128];
int fd = open("/dev/fuse", O_RDWR);

sprintf(opts, "fd=%d,rootmode=040000,user_id=0,group_id=0", fd);
mkdir("mnt", 0777);
mount("foo", "mnt", "fuse.foo", 0, opts);
mount("foo", "mnt", "fuse.foo", MS_REMOUNT, opts);
}

Crash:
BUG: kernel NULL pointer dereference, address: 0000000000000000
#PF: supervisor read access in kernel mode
#PF: error_code(0x0000) - not-present page
PGD 0 P4D 0
Oops: 0000 [#1] SMP
CPU: 0 PID: 129 Comm: syz_make_kuid Not tainted 5.3.0-rc5-next-20190821 #3
Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.12.0-20181126_142135-anatol 04/01/2014
RIP: 0010:map_id_range_down+0xb/0xc0 kernel/user_namespace.c:291
[...]
Call Trace:
map_id_down kernel/user_namespace.c:312 [inline]
make_kuid+0xe/0x10 kernel/user_namespace.c:389
fuse_parse_param+0x116/0x210 fs/fuse/inode.c:523
vfs_parse_fs_param+0xdb/0x1b0 fs/fs_context.c:145
vfs_parse_fs_string+0x6a/0xa0 fs/fs_context.c:188
generic_parse_monolithic+0x85/0xc0 fs/fs_context.c:228
parse_monolithic_mount_data+0x1b/0x20 fs/fs_context.c:708
do_remount fs/namespace.c:2525 [inline]
do_mount+0x39a/0xa60 fs/namespace.c:3107
ksys_mount+0x7d/0xd0 fs/namespace.c:3325
__do_sys_mount fs/namespace.c:3339 [inline]
__se_sys_mount fs/namespace.c:3336 [inline]
__x64_sys_mount+0x20/0x30 fs/namespace.c:3336
do_syscall_64+0x4a/0x1a0 arch/x86/entry/common.c:290
entry_SYSCALL_64_after_hwframe+0x49/0xbe

Reported-by: syzbot+7d6a57304857423318a5@syzkaller.appspotmail.com
Fixes: 408cbe695350 ("vfs: Convert fuse to use the new mount API")
Cc: David Howells <dhowells@redhat.com>
Cc: Miklos Szeredi <miklos@szeredi.hu>
Signed-off-by: Eric Biggers <ebiggers@google.com>
Reviewed-by: David Howells <dhowells@redhat.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
fs/fs_context.c | 4 +---
1 file changed, 1 insertion(+), 3 deletions(-)

--- a/fs/fs_context.c
+++ b/fs/fs_context.c
@@ -279,10 +279,8 @@ static struct fs_context *alloc_fs_conte
fc->user_ns = get_user_ns(reference->d_sb->s_user_ns);
break;
case FS_CONTEXT_FOR_RECONFIGURE:
- /* We don't pin any namespaces as the superblock's
- * subscriptions cannot be changed at this point.
- */
atomic_inc(&reference->d_sb->s_active);
+ fc->user_ns = get_user_ns(reference->d_sb->s_user_ns);
fc->root = dget(reference);
break;
}

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