[PATCH v6 5/5] fs, fcntl: add F_MAP_DIRECT

Dan Williams
2017-08-23T23:55:47Z

A common pattern for granting a privilege to an unprivileged process is
to pass it an established file descriptor over a unix domain socket.
This enables fine grained access to the MAP_DIRECT mechanism instead of
requiring the mapping process have CAP_LINUX_IMMUTABLE.

Cc: Jan Kara <jack@suse.cz>
Cc: Jeff Moyer <jmoyer@redhat.com>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Dave Chinner <david@fromorbit.com>
Cc: Alexander Viro <viro@zeniv.linux.org.uk>
Cc: "Darrick J. Wong" <darrick.wong@oracle.com>
Cc: Ross Zwisler <ross.zwisler@linux.intel.com>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>
---
fs/fcntl.c | 15 +++++++++++++++
include/linux/fs.h | 5 +++--
include/uapi/linux/fcntl.h | 5 +++++
mm/mmap.c | 3 ++-
4 files changed, 25 insertions(+), 3 deletions(-)

diff --git a/fs/fcntl.c b/fs/fcntl.c
index 3b01b646e528..f2375c406e6f 100644
--- a/fs/fcntl.c
+++ b/fs/fcntl.c
@@ -318,6 +318,18 @@ static long fcntl_rw_hint(struct file *file, unsigned int cmd,
}
}

+static int fcntl_map_direct(struct file *filp)
+{
+ if (!capable(CAP_LINUX_IMMUTABLE))
+ return -EACCES;
+
+ spin_lock(&filp->f_lock);
+ filp->f_map_direct = 1;
+ spin_unlock(&filp->f_lock);
+
+ return 0;
+}
+
static long do_fcntl(int fd, unsigned int cmd, unsigned long arg,
struct file *filp)
{
@@ -425,6 +437,9 @@ static long do_fcntl(int fd, unsigned int cmd, unsigned long arg,
case F_SET_FILE_RW_HINT:
err = fcntl_rw_hint(filp, cmd, arg);
break;
+ case F_MAP_DIRECT:
+ err = fcntl_map_direct(filp);
+ break;
default:
break;
}
diff --git a/include/linux/fs.h b/include/linux/fs.h
index db42da9f98c4..ec2e1d6bf22c 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -855,11 +855,12 @@ struct file {
const struct file_operations *f_op;

/*
- * Protects f_ep_links, f_flags.
+ * Protects f_ep_links, f_flags, f_write_hint, and f_map_direct.
* Must not be taken from IRQ context.
*/
spinlock_t f_lock;
- enum rw_hint f_write_hint;
+ enum rw_hint f_write_hint:3;
+ unsigned int f_map_direct:1;
atomic_long_t f_count;
unsigned int f_flags;
fmode_t f_mode;
diff --git a/include/uapi/linux/fcntl.h b/include/uapi/linux/fcntl.h
index ec69d55bcec7..2a57a503174e 100644
--- a/include/uapi/linux/fcntl.h
+++ b/include/uapi/linux/fcntl.h
@@ -53,6 +53,11 @@
#define F_SET_FILE_RW_HINT (F_LINUX_SPECIFIC_BASE + 14)

/*
+ * Enable MAP_DIRECT on the file without CAP_LINUX_IMMUTABLE
+ */
+#define F_MAP_DIRECT (F_LINUX_SPECIFIC_BASE + 15)
+
+/*
* Valid hint values for F_{GET,SET}_RW_HINT. 0 is "not set", or can be
* used to clear any hints previously set.
*/
diff --git a/mm/mmap.c b/mm/mmap.c
index 32417b2a668c..cf5e0cb7d0e3 100644
--- a/mm/mmap.c
+++ b/mm/mmap.c
@@ -1399,7 +1399,8 @@ unsigned long do_mmap(struct file *file, unsigned long addr,
if (flags & MAP_DIRECT) {
if (!(prot & PROT_WRITE))
return -EACCES;
- if (!capable(CAP_LINUX_IMMUTABLE))
+ if (!file->f_map_direct
+ && !capable(CAP_LINUX_IMMUTABLE))
return -EACCES;
}