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SubjectRe: [PATCH] exec: load_script: kill the onstack interp[BINPRM_BUF_SIZE] array
On Mon, Sep 18, 2017 at 9:34 AM, Oleg Nesterov <oleg@redhat.com> wrote:
> load_script() can simply use i_name instead, it points into bprm->buf[]
> and nobody can change this memory until we call prepare_binprm().

It looks like I missed this after commit b66c59840175 ("exec: do not
leave bprm->interp on stack"), where it became possible to remove the
stack copy in load_script()'s interp variable entirely. Yay, we save a
strcpy in exec. :)

> The only complication is that we need to also change the signature of
> bprm_change_interp() but this change looks good too.

That's totally fine.

> While at it, do whitespace/style cleanups.

Yes please. The code could use some comments too, frankly. The string
manipulation around building the i_name vs i_arg pointers takes some
time to make sense of, IMO.

> NOTE: the real motivation for this change is that people want to increase
> BINPRM_BUF_SIZE, we need to change load_misc_binary() too but this looks
> more complicated because afaics it is very buggy.
>
> Signed-off-by: Oleg Nesterov <oleg@redhat.com>

Acked-by: Kees Cook <keescook@chromium.org>

-Kees

> ---
> fs/binfmt_script.c | 17 +++++++++--------
> fs/exec.c | 2 +-
> include/linux/binfmts.h | 2 +-
> 3 files changed, 11 insertions(+), 10 deletions(-)
>
> diff --git a/fs/binfmt_script.c b/fs/binfmt_script.c
> index afdf4e3..7cde3f4 100644
> --- a/fs/binfmt_script.c
> +++ b/fs/binfmt_script.c
> @@ -19,7 +19,6 @@ static int load_script(struct linux_binprm *bprm)
> const char *i_arg, *i_name;
> char *cp;
> struct file *file;
> - char interp[BINPRM_BUF_SIZE];
> int retval;
>
> if ((bprm->buf[0] != '#') || (bprm->buf[1] != '!'))
> @@ -55,7 +54,7 @@ static int load_script(struct linux_binprm *bprm)
> break;
> }
> for (cp = bprm->buf+2; (*cp == ' ') || (*cp == '\t'); cp++);
> - if (*cp == '\0')
> + if (*cp == '\0')
> return -ENOEXEC; /* No interpreter name found */
> i_name = cp;
> i_arg = NULL;
> @@ -65,7 +64,6 @@ static int load_script(struct linux_binprm *bprm)
> *cp++ = '\0';
> if (*cp)
> i_arg = cp;
> - strcpy (interp, i_name);
> /*
> * OK, we've parsed out the interpreter name and
> * (optional) argument.
> @@ -80,24 +78,27 @@ static int load_script(struct linux_binprm *bprm)
> if (retval)
> return retval;
> retval = copy_strings_kernel(1, &bprm->interp, bprm);
> - if (retval < 0) return retval;
> + if (retval < 0)
> + return retval;
> bprm->argc++;
> if (i_arg) {
> retval = copy_strings_kernel(1, &i_arg, bprm);
> - if (retval < 0) return retval;
> + if (retval < 0)
> + return retval;
> bprm->argc++;
> }
> retval = copy_strings_kernel(1, &i_name, bprm);
> - if (retval) return retval;
> + if (retval)
> + return retval;
> bprm->argc++;
> - retval = bprm_change_interp(interp, bprm);
> + retval = bprm_change_interp(i_name, bprm);
> if (retval < 0)
> return retval;
>
> /*
> * OK, now restart the process with the interpreter's dentry.
> */
> - file = open_exec(interp);
> + file = open_exec(i_name);
> if (IS_ERR(file))
> return PTR_ERR(file);
>
> diff --git a/fs/exec.c b/fs/exec.c
> index 01a9fb9..8a665ec 100644
> --- a/fs/exec.c
> +++ b/fs/exec.c
> @@ -1429,7 +1429,7 @@ static void free_bprm(struct linux_binprm *bprm)
> kfree(bprm);
> }
>
> -int bprm_change_interp(char *interp, struct linux_binprm *bprm)
> +int bprm_change_interp(const char *interp, struct linux_binprm *bprm)
> {
> /* If a binfmt changed the interp, free it first. */
> if (bprm->interp != bprm->filename)
> diff --git a/include/linux/binfmts.h b/include/linux/binfmts.h
> index fb44d61..18d05b5 100644
> --- a/include/linux/binfmts.h
> +++ b/include/linux/binfmts.h
> @@ -131,7 +131,7 @@ extern int setup_arg_pages(struct linux_binprm * bprm,
> int executable_stack);
> extern int transfer_args_to_stack(struct linux_binprm *bprm,
> unsigned long *sp_location);
> -extern int bprm_change_interp(char *interp, struct linux_binprm *bprm);
> +extern int bprm_change_interp(const char *interp, struct linux_binprm *bprm);
> extern int copy_strings_kernel(int argc, const char *const *argv,
> struct linux_binprm *bprm);
> extern int prepare_bprm_creds(struct linux_binprm *bprm);
> --
> 2.5.0
>
>



--
Kees Cook
Pixel Security

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