[PATCH] ext4: check CAP_SYS_ADMIN in ext4_ioctl_get_tune_sb()

[Guanglei Zhu <zhugl3@xiaopeng.com>]

ext4_ioctl_set_tune_sb() requires CAP_SYS_ADMIN before changing the
superblock tunables, but the GET side has no permission check.  Any
user who can open a file on the filesystem read-only can read the
whole tune struct, including the 64-byte mount_opts string that
mount(2) stored in the superblock.

Require CAP_SYS_ADMIN on the GET side as well.

Fixes: 04a91570ac67 ("ext4: implemet new ioctls to set and get superblock parameters")
Signed-off-by: Guanglei Zhu <zhugl3@xiaopeng.com>
---
Verified in a QEMU guest with the ext4 image on a virtio disk: a
process demoted to uid 65534 reads the tune struct of a read-only
opened file unpatched, and gets -EPERM with this patch while root
is unaffected.

 fs/ext4/ioctl.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/fs/ext4/ioctl.c b/fs/ext4/ioctl.c
index c8387e6a2..d7125a8be 100644
--- a/fs/ext4/ioctl.c
+++ b/fs/ext4/ioctl.c
@@ -1284,6 +1284,9 @@ static int ext4_ioctl_get_tune_sb(struct ext4_sb_info *sbi,
 	struct ext4_tune_sb_params ret;
 	struct ext4_super_block *es = sbi->s_es;
 
+	if (!capable(CAP_SYS_ADMIN))
+		return -EPERM;
+
 	memset(&ret, 0, sizeof(ret));
 	ret.set_flags = TUNE_OPS_SUPPORTED;
 	ret.errors_behavior = le16_to_cpu(es->s_errors);
-- 
2.43.0