[RFC PATCH v1 2/5] fs: Add a MAY_EXECMOUNT flag to infer the noexec mount propertie

From: MickaÃl SalaÃn
Date: Wed Dec 12 2018 - 03:33:48 EST

An LSM doesn't get path information related to an access request to open
an inode. This new (internal) MAY_EXECMOUNT flag enables an LSM to
check if the underlying mount point of an inode is marked as executable.
This is useful to implement a security policy taking advantage of the
noexec mount option.

This flag is set according to path_noexec(), which checks if a mount
point is mounted with MNT_NOEXEC or if the underlying superblock is

Signed-off-by: MickaÃl SalaÃn <mic@xxxxxxxxxxx>
Reviewed-by: Philippe TrÃbuchet <philippe.trebuchet@xxxxxxxxxxx>
Reviewed-by: Thibaut Sautereau <thibaut.sautereau@xxxxxxxxxxx>
Cc: Al Viro <viro@xxxxxxxxxxxxxxxxxx>
Cc: Kees Cook <keescook@xxxxxxxxxxxx>
Cc: MickaÃl SalaÃn <mickael.salaun@xxxxxxxxxxx>
fs/namei.c | 2 ++
include/linux/fs.h | 2 ++
2 files changed, 4 insertions(+)

diff --git a/fs/namei.c b/fs/namei.c
index 0cab6494978c..de4f33b3f464 100644
--- a/fs/namei.c
+++ b/fs/namei.c
@@ -2970,6 +2970,8 @@ static int may_open(const struct path *path, int acc_mode, int flag)

+ /* Pass the mount point executability. */
+ acc_mode |= path_noexec(path) ? 0 : MAY_EXECMOUNT;
error = inode_permission(inode, MAY_OPEN | acc_mode);
if (error)
return error;
diff --git a/include/linux/fs.h b/include/linux/fs.h
index 584c9329ad78..083a31b8068e 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -96,6 +96,8 @@ typedef int (dio_iodone_t)(struct kiocb *iocb, loff_t offset,
#define MAY_NOT_BLOCK 0x00000080
/* the inode is opened with O_MAYEXEC */
#define MAY_OPENEXEC 0x00000100
+/* the mount point is marked as executable */
+#define MAY_EXECMOUNT 0x00000200

* flags in file.f_mode. Note that FMODE_READ and FMODE_WRITE must correspond