cgroup: verify that source is a string
commit 3b0462726e7ef281c35a7a4ae33e93ee2bc9975b upstream.
The following sequence can be used to trigger a UAF:
int fscontext_fd = fsopen("cgroup");
int fd_null = open("/dev/null, O_RDONLY);
int fsconfig(fscontext_fd, FSCONFIG_SET_FD, "source", fd_null);
close_range(3, ~0U, 0);
The cgroup v1 specific fs parser expects a string for the "source"
parameter. However, it is perfectly legitimate to e.g. specify a file
descriptor for the "source" parameter. The fs parser doesn't know what
a filesystem allows there. So it's a bug to assume that "source" is
always of type fs_value_is_string when it can reasonably also be
This assumption in the cgroup code causes a UAF because struct
fs_parameter uses a union for the actual value. Access to that union is
guarded by the param->type member. Since the cgroup paramter parser
didn't check param->type but unconditionally moved param->string into
fc->source a close on the fscontext_fd would trigger a UAF during
put_fs_context() which frees fc->source thereby freeing the file stashed
in param->file causing a UAF during a close of the fd_null.
Fix this by verifying that param->type is actually a string and report
an error if not.
In follow up patches I'll add a new generic helper that can be used here
and by other filesystems instead of this error-prone copy-pasta fix.
But fixing it in here first makes backporting a it to stable a lot
Fixes: 8d2451f4994f ("cgroup1: switch to option-by-option parsing")
Cc: Christoph Hellwig <email@example.com>
Cc: Alexander Viro <firstname.lastname@example.org>
Cc: Dmitry Vyukov <email@example.com>
Cc: syzkaller-bugs <firstname.lastname@example.org>
Signed-off-by: Christian Brauner <email@example.com>
Signed-off-by: Linus Torvalds <firstname.lastname@example.org>
Signed-off-by: Greg Kroah-Hartman <email@example.com>
1 file changed