Re: [PATCH v2 2/3] exec: Move S_ISREG() check earlier

[Kees Cook <keescook@chromium.org>]

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@chromium.org> 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@chromium.org>
> > ---
> >  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