[PATCH v3 2/5] fs: Add a MAY_EXECMOUNT flag to infer the noexec mount property

From: MickaÃl SalaÃn
Date: Tue Apr 28 2020 - 13:51:47 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
SB_I_NOEXEC.

Signed-off-by: MickaÃl SalaÃn <mic@xxxxxxxxxxx>
Reviewed-by: Philippe TrÃbuchet <philippe.trebuchet@xxxxxxxxxxx>
Reviewed-by: Thibaut Sautereau <thibaut.sautereau@xxxxxxxxxxx>
Cc: Aleksa Sarai <cyphar@xxxxxxxxxx>
Cc: Al Viro <viro@xxxxxxxxxxxxxxxxxx>
Cc: Kees Cook <keescook@xxxxxxxxxxxx>
---
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 a320371899cf..33b6d372e74a 100644
--- a/fs/namei.c
+++ b/fs/namei.c
@@ -2849,6 +2849,8 @@ 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 f5be4be7c01d..9213147d8636 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -103,6 +103,8 @@ typedef int (dio_iodone_t)(struct kiocb *iocb, loff_t offset,
#define MAY_NOT_BLOCK 0x00000080
/* the inode is opened with RESOLVE_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
--
2.26.2