[v5,2/6] fs: Add a MAY_EXECMOUNT flag to infer the noexec mount property
diff mbox series

Message ID 20200505153156.925111-3-mic@digikod.net
State New
Headers show
Series
  • Add support for O_MAYEXEC
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Commit Message

Mickaël Salaün May 5, 2020, 3:31 p.m. UTC
This new MAY_EXECMOUNT flag enables 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 <mic@digikod.net>
Reviewed-by: Philippe Trébuchet <philippe.trebuchet@ssi.gouv.fr>
Reviewed-by: Thibaut Sautereau <thibaut.sautereau@ssi.gouv.fr>
Cc: Aleksa Sarai <cyphar@cyphar.com>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Kees Cook <keescook@chromium.org>
---
 fs/namei.c         | 2 ++
 include/linux/fs.h | 2 ++
 2 files changed, 4 insertions(+)

Comments

Kees Cook May 12, 2020, 9:09 p.m. UTC | #1
On Tue, May 05, 2020 at 05:31:52PM +0200, Mickaël Salaün wrote:
> This new MAY_EXECMOUNT flag enables 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 <mic@digikod.net>
> Reviewed-by: Philippe Trébuchet <philippe.trebuchet@ssi.gouv.fr>
> Reviewed-by: Thibaut Sautereau <thibaut.sautereau@ssi.gouv.fr>
> Cc: Aleksa Sarai <cyphar@cyphar.com>
> Cc: Al Viro <viro@zeniv.linux.org.uk>
> Cc: Kees Cook <keescook@chromium.org>
> ---
>  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 313c934de9ee..79435fca6c3e 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 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

I find this name unintuitive, but I cannot think of anything better,
since I think my problem is that "MAY" doesn't map to the language I
want to use to describe what this flag is indicating.

Reviewed-by: Kees Cook <keescook@chromium.org>
Lev R. Oshvang . May 14, 2020, 8:14 a.m. UTC | #2
On Wed, May 13, 2020 at 12:09 AM Kees Cook <keescook@chromium.org> wrote:
>
> On Tue, May 05, 2020 at 05:31:52PM +0200, Mickaël Salaün wrote:
> > This new MAY_EXECMOUNT flag enables 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.
> >
Security policy is expressed by sysadmin by mount -noexec very clear,
I don't think there is a need
in sysctl, wish is system-wide

> > 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 <mic@digikod.net>
> > Reviewed-by: Philippe Trébuchet <philippe.trebuchet@ssi.gouv.fr>
> > Reviewed-by: Thibaut Sautereau <thibaut.sautereau@ssi.gouv.fr>
> > Cc: Aleksa Sarai <cyphar@cyphar.com>
> > Cc: Al Viro <viro@zeniv.linux.org.uk>
> > Cc: Kees Cook <keescook@chromium.org>
> > ---
> >  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 313c934de9ee..79435fca6c3e 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 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
>
> I find this name unintuitive, but I cannot think of anything better,
> since I think my problem is that "MAY" doesn't map to the language I
> want to use to describe what this flag is indicating.
>
> Reviewed-by: Kees Cook <keescook@chromium.org>
>
> --
> Kees Cook


I think that the original patch was perfect, I quite it again
@@ -3167,6 +3167,14 @@ static int may_open(struct path *path, int
acc_mode, int flag)

+
+ if ((acc_mode & MAY_OPENEXEC)
+ && (S_ISREG(inode->i_mode) || S_ISDIR(inode->i_mode))
+ && (path->mnt && (path->mnt->mnt_flags & MNT_NOEXEC)))
+            return -EACCES;
+
+
+
error = inode_permission(inode, MAY_OPEN | acc_mode);

As I said in the inline comment above, sysadmin had already express
security policy in a very clear way,
mount -noexec !
I would only check inside inode_permission() whether the file mode is
any  ---x  permission and deny such
open when file is opened with O_MAYEXEC under MNT_NOEXEC mount point

New sysctl is indeed required to allow userspace that places scripts
or libs under noexec mounts.
fs.mnt_noexec_strict =0 (allow, e) , 1 (deny any file with --x
permission), 2 (deny when O_MAYEXEC absent), for any file with ---x
permissions)
Kees Cook May 14, 2020, 3:48 p.m. UTC | #3
On Thu, May 14, 2020 at 11:14:04AM +0300, Lev R. Oshvang . wrote:
> New sysctl is indeed required to allow userspace that places scripts
> or libs under noexec mounts.

But since this is a not-uncommon environment, we must have the sysctl
otherwise this change would break those systems.

> fs.mnt_noexec_strict =0 (allow, e) , 1 (deny any file with --x
> permission), 2 (deny when O_MAYEXEC absent), for any file with ---x
> permissions)

I don't think we want another mount option -- this is already fully
expressed with noexec and the system-wide sysctl.
Lev R. Oshvang . May 17, 2020, 4:57 p.m. UTC | #4
On Thu, May 14, 2020 at 6:48 PM Kees Cook <keescook@chromium.org> wrote:
>
> On Thu, May 14, 2020 at 11:14:04AM +0300, Lev R. Oshvang . wrote:
> > New sysctl is indeed required to allow userspace that places scripts
> > or libs under noexec mounts.
>
> But since this is a not-uncommon environment, we must have the sysctl
> otherwise this change would break those systems.
>
 But I proposed sysctl on a line below.

> > fs.mnt_noexec_strict =1 (allow, e) , 1 (deny any file with --x
> > permission), 2 (deny when O_MAYEXEC absent), for any file with ---x
> > permissions)
>
> I don't think we want another mount option -- this is already fully
> expressed with noexec and the system-wide sysctl.
>
> --

The intended use of proposed sysctl is to ebable sysadmin to decide
whar is desired semantics  mount with NO_EXEC option.

fs.mnt_noexec_scope =0 |1|2|3
0  - means old behaviour i.e do nor run executables and scripts (default)
1 - deny any file with --x permissions, i.e executables , script and libs
2 - deny any file when O_MAYEXEC is present.

I think this is enough to handle all use cases and to not break
current sysadmin file mounts setting
I oppose the new O_MAY_EXECMOUNT flag, kernel already has MNT_NO_EXEC,
SB_NOEXEC and SB_I_NOEXEC and I frankly do not understand why so many
variants exist.
Lev

Patch
diff mbox series

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 313c934de9ee..79435fca6c3e 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 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