Re: [PATCH AUTOSEL 4.14 090/108] ovl: verify permissions in ovl_path_open()

Naresh Kamboju
2020-06-23T15:25:57Z

On Thu, 18 Jun 2020 at 07:18, Sasha Levin <sashal@kernel.org> wrote:
>
> From: Miklos Szeredi <mszeredi@redhat.com>
>
> [ Upstream commit 56230d956739b9cb1cbde439d76227d77979a04d ]
>
> Check permission before opening a real file.
>
> ovl_path_open() is used by readdir and copy-up routines.
>
> ovl_permission() theoretically already checked copy up permissions, but it
> doesn't hurt to re-do these checks during the actual copy-up.
>
> For directory reading ovl_permission() only checks access to topmost
> underlying layer. Readdir on a merged directory accesses layers below the
> topmost one as well. Permission wasn't checked for these layers.
>
> Note: modifying ovl_permission() to perform this check would be far more
> complex and hence more bug prone. The result is less precise permissions
> returned in access(2). If this turns out to be an issue, we can revisit
> this bug.
>
> Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>
> Signed-off-by: Sasha Levin <sashal@kernel.org>
> ---
> fs/overlayfs/util.c | 27 ++++++++++++++++++++++++++-
> 1 file changed, 26 insertions(+), 1 deletion(-)
>
> diff --git a/fs/overlayfs/util.c b/fs/overlayfs/util.c
> index afdc2533ce74..76d6610767f6 100644
> --- a/fs/overlayfs/util.c
> +++ b/fs/overlayfs/util.c
> @@ -307,7 +307,32 @@ bool ovl_is_whiteout(struct dentry *dentry)
>
> struct file *ovl_path_open(struct path *path, int flags)
> {
> - return dentry_open(path, flags | O_NOATIME, current_cred());
> + struct inode *inode = d_inode(path->dentry);
> + int err, acc_mode;
> +
> + if (flags & ~(O_ACCMODE | O_LARGEFILE))
> + BUG();
> +
> + switch (flags & O_ACCMODE) {
> + case O_RDONLY:
> + acc_mode = MAY_READ;
> + break;
> + case O_WRONLY:
> + acc_mode = MAY_WRITE;
> + break;
> + default:
> + BUG();

This BUG: triggered on stable-rc 5.7, 5.4, 4.19 and 4.14.

steps to reproduce:
- cd /opt/ltp
- ./runltp -s execveat03

Test output:
mke2fs 1.43.8 (1-Jan-2018)
[ 47.739682] ------------[ cut here ]------------
[ 47.744317] kernel BUG at fs/overlayfs/util.c:314!
[ 47.749117] Internal error: Oops - BUG: 0 [#1] PREEMPT SMP
[ 47.754608] Modules linked in: overlay rfkill crc32_ce crct10dif_ce fuse
[ 47.761335] Process execveat03 (pid: 2880, stack limit = 0xffff00000ec60000)
[ 47.768397] CPU: 3 PID: 2880 Comm: execveat03 Not tainted
4.14.186-rc1-00111-gb518002db397 #1
[ 47.776933] Hardware name: ARM Juno development board (r2) (DT)
[ 47.782860] task: ffff8009546ade80 task.stack: ffff00000ec60000
[ 47.788819] pc : ovl_path_open+0xa8/0xb0 [overlay]
[ 47.793641] lr : ovl_check_d_type_supported+0x38/0xf0 [overlay]
[ 47.799567] sp : ffff00000ec63ba0 pstate : 40000145
[ 47.804449] x29: ffff00000ec63ba0 x28: 0000000000000000
[ 47.809770] x27: ffff800955bfb710 x26: ffff800955bfb700
[ 47.815091] x25: ffff00000ec63ce0 x24: 0000000000000000
[ 47.820412] x23: ffff00000ec63cd0 x22: 0000000000000001
[ 47.825733] x21: ffff8009509609b8 x20: ffff00000ec63ce0
[ 47.831054] x19: 0000000000004000 x18: 0000000000000000
[ 47.836375] x17: 0000000000000000 x16: ffff0000080d1800
[ 47.841696] x15: 095a041701101c00 x14: ff00000000000000
[ 47.847017] x13: 0000000000000000 x12: 000000000000000b
[ 47.852338] x11: 0101010101010101 x10: ffff800950960b40
[ 47.857659] x9 : 7f7f7f7f7f7f7f7f x8 : 6f2d6c6473727872
[ 47.862980] x7 : 001c100117045a09 x6 : 095a041701101c00
[ 47.868301] x5 : 0000000000000000 x4 : 0000000000000000
[ 47.873622] x3 : 0000000000000000 x2 : ffff000000b7ec88
[ 47.878943] x1 : ffff800953f78180 x0 : 0000000000004000
[ 47.884264] Call trace:
[ 47.886736] ovl_path_open+0xa8/0xb0 [overlay]
[ 47.891209] ovl_check_d_type_supported+0x38/0xf0 [overlay]
[ 47.896812] ovl_fill_super+0x540/0xc98 [overlay]
[ 47.901528] mount_nodev+0x4c/0xa8
[ 47.904954] ovl_mount+0x14/0x28 [overlay]
[ 47.909056] mount_fs+0x54/0x188
[ 47.912289] vfs_kern_mount.part.0+0x4c/0x118
[ 47.916651] do_mount+0x1cc/0xbe0
[ 47.919970] compat_SyS_mount+0xb0/0x1b8
[ 47.923899] __sys_trace_return+0x0/0x4
[ 47.927742] Code: f94013f5 a8c37bfd d65f03c0 d4210000 (d4210000)
[ 47.933845] ---[ end trace 1b32b515dde8d9db ]---

Test link,
https://lkft.validation.linaro.org/scheduler/job/1517781#L1266

metadata:
git branch: linux-4.14.y
git repo: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux-stable-rc.git
git commit: b518002db397b51173a3e17045bfb3ff0e1aa0ed
kernel-config:
https://builds.tuxbuild.com/B_xCv6-0v8npcEVw6hA_ZQ/kernel.config

--
Linaro LKFT