[v2,2/5] fs: Add a MAY_EXECMOUNT flag to infer the noexec mount propertie

Commit Message

Mickaël Salaün Sept. 6, 2019, 3:24 p.m. UTC

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
SB_I_NOEXEC.

Signed-off-by: Mickaël Salaün <[e-mail]>
Reviewed-by: Philippe Trébuchet <[e-mail]>
Reviewed-by: Thibaut Sautereau <[e-mail]>
Cc: Al Viro <[e-mail]>
Cc: Kees Cook <[e-mail]>
Cc: Mickaël Salaün <[e-mail]>
---
 fs/namei.c         | 2 ++
 include/linux/fs.h | 2 ++
 2 files changed, 4 insertions(+)

Patch

diff --git a/fs/namei.c b/fs/namei.c
index 209c51a5226c..0a6b9483d0cb 100644
--- a/fs/namei.c
+++ b/fs/namei.c
@@ -2968,6 +2968,8 @@  static int may_open(const struct path *path, int acc_mode, int flag)
 		break;
 	}
 
+	/* 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 848f5711bdf0..e57609dac8dd 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -101,6 +101,8 @@  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