On Thu, Aug 13, 2020 at 03:13:05PM +0100, Marc Zyngier wrote:
> On Fri, 5 Jun 2020 09:00:12 -0700
> Kees Cook <keescook@xxxxxxxxxxxx> wrote:
>
> Hi Kees,
>
> > The execve(2)/uselib(2) syscalls have always rejected non-regular
> > files. Recently, it was noticed that a deadlock was introduced when trying
> > to execute pipes, as the S_ISREG() test was happening too late. This was
> > fixed in commit 73601ea5b7b1 ("fs/open.c: allow opening only regular files
> > during execve()"), but it was added after inode_permission() had already
> > run, which meant LSMs could see bogus attempts to execute non-regular
> > files.
> >
> > Move the test into the other inode type checks (which already look
> > for other pathological conditions[1]). Since there is no need to use
> > FMODE_EXEC while we still have access to "acc_mode", also switch the
> > test to MAY_EXEC.
> >
> > Also include a comment with the redundant S_ISREG() checks at the end of
> > execve(2)/uselib(2) to note that they are present to avoid any mistakes.
> >
> > My notes on the call path, and related arguments, checks, etc:
> >
> > do_open_execat()
> > struct open_flags open_exec_flags = {
> > .open_flag = O_LARGEFILE | O_RDONLY | __FMODE_EXEC,
> > .acc_mode = MAY_EXEC,
> > ...
> > do_filp_open(dfd, filename, open_flags)
> > path_openat(nameidata, open_flags, flags)
> > file = alloc_empty_file(open_flags, current_cred());
> > do_open(nameidata, file, open_flags)
> > may_open(path, acc_mode, open_flag)
> > /* new location of MAY_EXEC vs S_ISREG() test */
> > inode_permission(inode, MAY_OPEN | acc_mode)
> > security_inode_permission(inode, acc_mode)
> > vfs_open(path, file)
> > do_dentry_open(file, path->dentry->d_inode, open)
> > /* old location of FMODE_EXEC vs S_ISREG() test */
> > security_file_open(f)
> > open()
> >
> > [1] https://lore.kernel.org/lkml/202006041910.9EF0C602@keescook/
> >
> > Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
> > ---
> > fs/exec.c | 14 ++++++++++++--
> > fs/namei.c | 6 ++++--
> > fs/open.c | 6 ------
> > 3 files changed, 16 insertions(+), 10 deletions(-)
> >
> > diff --git a/fs/exec.c b/fs/exec.c
> > index 30735ce1dc0e..2b708629dcd6 100644
> > --- a/fs/exec.c
> > +++ b/fs/exec.c
> > @@ -139,8 +139,13 @@ SYSCALL_DEFINE1(uselib, const char __user *, library)
> > if (IS_ERR(file))
> > goto out;
> >
> > + /*
> > + * may_open() has already checked for this, so it should be
> > + * impossible to trip now. But we need to be extra cautious
> > + * and check again at the very end too.
> > + */
> > error = -EACCES;
> > - if (!S_ISREG(file_inode(file)->i_mode))
> > + if (WARN_ON_ONCE(!S_ISREG(file_inode(file)->i_mode)))
> > goto exit;
> >
> > if (path_noexec(&file->f_path))
> > @@ -860,8 +865,13 @@ static struct file *do_open_execat(int fd, struct filename *name, int flags)
> > if (IS_ERR(file))
> > goto out;
> >
> > + /*
> > + * may_open() has already checked for this, so it should be
> > + * impossible to trip now. But we need to be extra cautious
> > + * and check again at the very end too.
> > + */
> > err = -EACCES;
> > - if (!S_ISREG(file_inode(file)->i_mode))
> > + if (WARN_ON_ONCE(!S_ISREG(file_inode(file)->i_mode)))
> > goto exit;
> >
> > if (path_noexec(&file->f_path))
> > diff --git a/fs/namei.c b/fs/namei.c
> > index a320371899cf..0a759b68d66e 100644
> > --- a/fs/namei.c
> > +++ b/fs/namei.c
> > @@ -2835,16 +2835,18 @@ static int may_open(const struct path *path, int acc_mode, int flag)
> > case S_IFLNK:
> > return -ELOOP;
> > case S_IFDIR:
> > - if (acc_mode & MAY_WRITE)
> > + if (acc_mode & (MAY_WRITE | MAY_EXEC))
> > return -EISDIR;
>
> This seems to change (break?) the behaviour of syscalls such as execv,
> which can now return -EISDIR, whereas the existing behaviour was to
> return -EACCES. The man page never hints at the possibility of -EISDIR
> being returned, making it feel like a regression.
>
> POSIX (FWIW) also says:
>
> <quote>
> [EACCES]
> The new process image file is not a regular file and the
> implementation does not support execution of files of its type.
> </quote>
>
> This has been picked up by the Bionic test suite[1], but can just as
> easily be reproduced with the following snippet:
>
> $ cat x.c
> #include <unistd.h>
> #include <stdio.h>
> int main(int argc, char *argv[])
> {
> execv("/", NULL);
> perror("execv");
> return 0;
> }
>
> Before this patch:
> $ ./x
> execv: Permission denied
>
> After this patch:
> $ ./x
> execv: Is a directory
That's a good point, yes. I will submit a fix for this.
--
Kees Cook
