From 555724a831b4a146e7bdf16ecc989cda032b076d Mon Sep 17 00:00:00 2001 From: Tejun Heo Date: Mon, 12 May 2014 13:56:27 -0400 Subject: kernfs, sysfs, cgroup: restrict extra perm check on open to sysfs The kernfs open method - kernfs_fop_open() - inherited extra permission checks from sysfs. While the vfs layer allows ignoring the read/write permissions checks if the issuer has CAP_DAC_OVERRIDE, sysfs explicitly denied open regardless of the cap if the file doesn't have any of the UGO perms of the requested access or doesn't implement the requested operation. It can be debated whether this was a good idea or not but the behavior is too subtle and dangerous to change at this point. After cgroup got converted to kernfs, this extra perm check also got applied to cgroup breaking libcgroup which opens write-only files with O_RDWR as root. This patch gates the extra open permission check with a new flag KERNFS_ROOT_EXTRA_OPEN_PERM_CHECK and enables it for sysfs. For sysfs, nothing changes. For cgroup, root now can perform any operation regardless of the permissions as it was before kernfs conversion. Note that kernfs still fails unimplemented operations with -EINVAL. While at it, add comments explaining KERNFS_ROOT flags. Signed-off-by: Tejun Heo Reported-by: Andrey Wagin Tested-by: Andrey Wagin Cc: Li Zefan References: http://lkml.kernel.org/g/CANaxB-xUm3rJ-Cbp72q-rQJO5mZe1qK6qXsQM=vh0U8upJ44+A@mail.gmail.com Fixes: 2bd59d48ebfb ("cgroup: convert to kernfs") Signed-off-by: Greg Kroah-Hartman --- include/linux/kernfs.h | 19 ++++++++++++++++++- 1 file changed, 18 insertions(+), 1 deletion(-) (limited to 'include/linux/kernfs.h') diff --git a/include/linux/kernfs.h b/include/linux/kernfs.h index b0122dc6f96..ca1be5c9136 100644 --- a/include/linux/kernfs.h +++ b/include/linux/kernfs.h @@ -50,7 +50,24 @@ enum kernfs_node_flag { /* @flags for kernfs_create_root() */ enum kernfs_root_flag { - KERNFS_ROOT_CREATE_DEACTIVATED = 0x0001, + /* + * kernfs_nodes are created in the deactivated state and invisible. + * They require explicit kernfs_activate() to become visible. This + * can be used to make related nodes become visible atomically + * after all nodes are created successfully. + */ + KERNFS_ROOT_CREATE_DEACTIVATED = 0x0001, + + /* + * For regular flies, if the opener has CAP_DAC_OVERRIDE, open(2) + * succeeds regardless of the RW permissions. sysfs had an extra + * layer of enforcement where open(2) fails with -EACCES regardless + * of CAP_DAC_OVERRIDE if the permission doesn't have the + * respective read or write access at all (none of S_IRUGO or + * S_IWUGO) or the respective operation isn't implemented. The + * following flag enables that behavior. + */ + KERNFS_ROOT_EXTRA_OPEN_PERM_CHECK = 0x0002, }; /* type-specific structures for kernfs_node union members */ -- cgit v1.2.3-70-g09d2