[PATCH 5.8 319/633] fs: fix NULL dereference due to data race in prepend_path()

Greg Kroah-Hartman
2020-10-27T15:10:48Z

From: Andrii Nakryiko <andrii@kernel.org>

[ Upstream commit 09cad07547445bf3a41683e4d3abcd154c123ef5 ]

Fix data race in prepend_path() with re-reading mnt->mnt_ns twice
without holding the lock.

is_mounted() does check for NULL, but is_anon_ns(mnt->mnt_ns) might
re-read the pointer again which could be NULL already, if in between
reads one of kern_unmount()/kern_unmount_array()/umount_tree() sets
mnt->mnt_ns to NULL.

This is seen in production with the following stack trace:

BUG: kernel NULL pointer dereference, address: 0000000000000048
...
RIP: 0010:prepend_path.isra.4+0x1ce/0x2e0
Call Trace:
d_path+0xe6/0x150
proc_pid_readlink+0x8f/0x100
vfs_readlink+0xf8/0x110
do_readlinkat+0xfd/0x120
__x64_sys_readlinkat+0x1a/0x20
do_syscall_64+0x42/0x110
entry_SYSCALL_64_after_hwframe+0x44/0xa9

Fixes: f2683bd8d5bd ("[PATCH] fix d_absolute_path() interplay with fsmount()")
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Cc: Alexander Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
fs/d_path.c | 6 +++++-
1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/fs/d_path.c b/fs/d_path.c
index 0f1fc1743302f..a69e2cd36e6e3 100644
--- a/fs/d_path.c
+++ b/fs/d_path.c
@@ -102,6 +102,8 @@ static int prepend_path(const struct path *path,

if (dentry == vfsmnt->mnt_root || IS_ROOT(dentry)) {
struct mount *parent = READ_ONCE(mnt->mnt_parent);
+ struct mnt_namespace *mnt_ns;
+
/* Escaped? */
if (dentry != vfsmnt->mnt_root) {
bptr = *buffer;
@@ -116,7 +118,9 @@ static int prepend_path(const struct path *path,
vfsmnt = &mnt->mnt;
continue;
}
- if (is_mounted(vfsmnt) && !is_anon_ns(mnt->mnt_ns))
+ mnt_ns = READ_ONCE(mnt->mnt_ns);
+ /* open-coded is_mounted() to use local mnt_ns */
+ if (!IS_ERR_OR_NULL(mnt_ns) && !is_anon_ns(mnt_ns))
error = 1; // absolute root
else
error = 2; // detached or not attached yet
--
2.25.1