> We find a kernel NULL pointer dereference problem in overlayfs.
> The problem can appear in the following scene:
>
> mkdir lower upper work merge
> touch lower/old
> touch lower/new
> mount -t overlay overlay -olowerdir=lower,upperdir=upper,workdir=work
> merge rm merge/new
> ------------------------------------------------------------------------------------------
> process A(rename file in merge) process B(delete file in upper)
> renameat2(AT_FDCWD, "old", AT_FDCWD, "new", 0) (new is whiteout file
> in upper)
> do_renameat2
> vfs_rename
> ovl_rename
> (overwrite=true,ovl_lower_positive(old)=true,
> ovl_dentry_is_whiteout(new)=true)
> (we can add some delay after "flags|=RENAME_EXCHANGE",
> it can make the problem appear more easy)
> …… unlink(new)
> …… (delete whiteout in upper)
> (newdentry is negative)
> ovl_do_rename
Is it a real use case? I believe there will be many unexpected behaviors if you delete the files in upper layer deliberately.
Thanks,
Chengguang
As described in the documentation (Documentation/filesystems/overlayfs.rst)
Changes to underlying filesystems
---------------------------------
Changes to the underlying filesystems while part of a mounted overlay
filesystem are not allowed. If the underlying filesystem is changed,
the behavior of the overlay is undefined, though it will not result in
a crash or deadlock.
So, We should probably fix this crash problem.
>
> So,before commencing with ovl_do_rename that the flags maybe attach
> RENAME_EXCHANGE and the newdentry is negative in ovl_rename.If we
> enabled selinux,it will lead to kernel panic.such as the following log:
> PID: 2552045 TASK: ffff8880302faf00 CPU: 2 COMMAND: "fsstress"
> #0 [ffff888080e772a0] machine_kexec at ffffffff856adedc
> #1 [ffff888080e773a8] __crash_kexec at ffffffff8585cd20
> #2 [ffff888080e774c0] panic at ffffffff8572b288
> #3 [ffff888080e77590] oops_end at ffffffff85641f6e
> #4 [ffff888080e775f0] __do_page_fault at ffffffff856cd55b
> #5 [ffff888080e77668] do_page_fault at ffffffff856cd834
> #6 [ffff888080e776a0] async_page_fault at ffffffff8660125e
> [exception RIP: __inode_security_revalidate+34]
> RIP: ffffffff85c43452 RSP: ffff888080e77758 RFLAGS: 00010202
> RAX: 0000000000000000 RBX: 0000000000000000 RCX: ffffffff8593ae7e
> RDX: 0000000000000000 RSI: 0000000000000297 RDI: 0000000000000297
> RBP: ffff8881984e6628 R8: ffffed10115e3f39 R9: ffffed10115e3f39
> R10: 0000000000000001 R11: ffffed10115e3f38 R12: 0000000000000001
> R13: 0000000000000000 R14: ffff88808350a000 R15: 1ffff110101ceef5
> ORIG_RAX: ffffffffffffffff CS: 0010 SS: 0018
> #7 [ffff888080e77780] selinux_inode_rename at ffffffff85c4418d
> #8 [ffff888080e77858] security_inode_rename at ffffffff85c35f24
> #9 [ffff888080e77890] vfs_rename at ffffffff85b01209
> #10 [ffff888080e779a8] ovl_do_rename at ffffffffc0a44c22 [overlay]
> #11 [ffff888080e779d8] ovl_rename at ffffffffc0a46575 [overlay]
> #12 [ffff888080e77b48] vfs_rename at ffffffff85b0155a
> #13 [ffff888080e77c60] do_renameat2 at ffffffff85b06e65
> #14 [ffff888080e77f00] __x64_sys_renameat2 at ffffffff85b06fb2
> #15 [ffff888080e77f30] do_syscall_64 at ffffffff85606243
> #16 [ffff888080e77f50] entry_SYSCALL_64_after_hwframe at
> ffffffff866000ad
>
> We can add some check in ovl_rename for this scene and return error to avoid kernel panic.
>
> Signed-off-by: Zheng Liang <zhengliang6@xxxxxxxxxx>
> ---
> fs/overlayfs/dir.c | 10 +++++++---
> 1 file changed, 7 insertions(+), 3 deletions(-)
>
> diff --git a/fs/overlayfs/dir.c b/fs/overlayfs/dir.c index
> 1fefb2b8960e..93c7c267de93 100644
> --- a/fs/overlayfs/dir.c
> +++ b/fs/overlayfs/dir.c
> @@ -1219,9 +1219,13 @@ static int ovl_rename(struct user_namespace *mnt_userns, struct inode *olddir,
> goto out_dput;
> }
> } else {
> - if (!d_is_negative(newdentry) &&
> - (!new_opaque || !ovl_is_whiteout(newdentry)))
> - goto out_dput;
> + if (!d_is_negative(newdentry)) {
> + if (!new_opaque || !ovl_is_whiteout(newdentry))
> + goto out_dput;
> + } else {
> + if (flags & RENAME_EXCHANGE)
> + goto out_dput;
> + }
> }
>
> if (olddentry == trap)
> --
> 2.25.4
>
