On Wed, May 13, 2020 at 12:09 AM Kees Cook <keescook@xxxxxxxxxxxx> 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@xxxxxxxxxxx> > > Reviewed-by: Philippe Trébuchet <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 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@xxxxxxxxxxxx> > > -- > 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)
