[PATCH dhowells/mount-context] fs: don't call fs_context->free() from fsmount()

[Andrei Vagin <avagin@openvz.org>]

From: Andrei Vagin <avagin@gmail.com>

fs_context->free() will be called from fscontext_release().

Without this patch, a kernel bug is triggered:

BUG: unable to handle kernel NULL pointer dereference at 0000000000000000
PGD 800000012b0a3067 P4D 800000012b0a3067 PUD 131f1a067 PMD 0
Oops: 0000 [#1] SMP PTI
CPU: 0 PID: 612 Comm: test Not tainted 4.18.0-rc1-00037-g60023faf445f #12
Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS ?-20180531_142017-buildhw-08.phx2.fedoraproject.org-1.fc28 04/01/2014
RIP: 0010:proc_fs_context_free+0xd/0x30
Code: 78 dd ff b8 ea ff ff ff eb af e8 fe dd d5 ff 0f 1f 40 00 66 2e 0f 1f 84 00 00 00 00 00 0f 1f 44 00 00 53 48 8b 9f 90 00 00 00 <48> 8b 3b 48 85 ff 74 05 e8 16 24 e3 ff 48 89 df 5b e9 fd f5 f3 ff
RSP: 0018:ffffa9b5c0b53e60 EFLAGS: 00010282
RAX: ffffffffaf332e20 RBX: 0000000000000000 RCX: 0000000000000005
RDX: ffff9bc77abbe1bc RSI: 0000000000000001 RDI: ffff9bc774209908
RBP: ffff9bc77abbe0c8 R08: 0000000000000000 R09: 0000000000000000
R10: ffff9bc77ac08d20 R11: ffffffffb1232378 R12: ffff9bc779af0f28
R13: 0000000000020003 R14: ffff9bc77aba9020 R15: ffff9bc773b6ab48
FS:  00007f725675a4c0(0000) GS:ffff9bc77fc00000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 0000000000000000 CR3: 000000012b0c6003 CR4: 00000000003606f0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
Call Trace:
 put_fs_context+0x4f/0x150
 fscontext_release+0x21/0x30
 __fput+0xc0/0x230
 task_work_run+0xa1/0xd0
 exit_to_usermode_loop+0xbb/0xc0
 do_syscall_64+0x1e5/0x210
 entry_SYSCALL_64_after_hwframe+0x49/0xbe
RIP: 0033:0x7f725627ff24
Code: eb 89 e8 0f f6 01 00 66 2e 0f 1f 84 00 00 00 00 00 0f 1f 44 00 00 8b 05 2a e9 2c 00 48 63 ff 85 c0 75 13 b8 03 00 00 00 0f 05 <48> 3d 00 f0 ff ff 77 44 f3 c3 66 90 48 83 ec 18 48 89 7c 24 08 e8

The following code can be used to reproduce the bug.

int main()
{
	int fsfd,  dfd;

	fsfd = fsopen("proc", 0);
	if (fsfd < 0)
		return 1;

	if (fsconfig(fsfd, fsconfig_cmd_create, NULL, NULL, 0) < 0)
		return 1;

	dfd = fsmount(fsfd, 0, 0);
	if (dfd < 0)
		return 1;

	if (close(dfd) < 0)
		return 1;
	if (close(fsfd) < 0)
		return 1;

	return 0;
}

Signed-off-by: Andrei Vagin <avagin@gmail.com>
---
 fs/namespace.c | 3 ---
 1 file changed, 3 deletions(-)

diff --git a/fs/namespace.c b/fs/namespace.c
index 7e7b1145d15d..1899456194c3 100644
--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -3435,9 +3435,6 @@ SYSCALL_DEFINE3(fsmount, int, fs_fd, unsigned int, flags, unsigned int, ms_flags
 	 * do any memory allocation or anything like that at this point as we
 	 * don't want to have to handle any errors incurred.
 	 */
-	if (fc->ops && fc->ops->free)
-		fc->ops->free(fc);
-	fc->fs_private = NULL;
 	fc->s_fs_info = NULL;
 	fc->sb_flags = 0;
 	fc->sloppy = false;
-- 
2.17.1