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SubjectRe: execve(NULL, argv, envp) for nommu?
On 09/12, Oleg Nesterov wrote:
>
> On 09/12, Rob Landley wrote:
> >
> > On 09/11/2017 10:15 AM, Oleg Nesterov wrote:
> > > On 09/08, Rob Landley wrote:
> > >>
> > >> So is exec(NULL, argv, envp) a reasonable thing to want?
> > >
> > > I think that something like prctl(PR_OPEN_EXE_FILE) which does
> > >
> > > dentry_open(current->mm->exe_file->path, O_PATH)
> > >
> > > and returns fd make more sense.
> > >
> > > Then you can do execveat(fd, "", ..., AT_EMPTY_PATH).
> > I'm all for it? That sounds like a cosmetic difference, a more verbose
> > way of achieving the same outcome.
>
> Simpler to implement. Something like the (untested) patch below. Not sure
> it is correct, not sure it is good idea, etc.

OTOH... with the trivial patch below

execveat(AT_FDCWD, "", NULL, NULL, AT_EMPTY_PATH);

should always work, even if the binary is not in scope after chroot, or if
it is no longer executable, or unlinked. But I am not sure what else should
we do to avoid the security problems.

Oleg.


--- x/fs/exec.c
+++ x/fs/exec.c
@@ -832,23 +832,32 @@ static struct file *do_open_execat(int fd, struct filename *name, int flags)
{
struct file *file;
int err;
- struct open_flags open_exec_flags = {
- .open_flag = O_LARGEFILE | O_RDONLY | __FMODE_EXEC,
- .acc_mode = MAY_EXEC,
- .intent = LOOKUP_OPEN,
- .lookup_flags = LOOKUP_FOLLOW,
- };
-
- if ((flags & ~(AT_SYMLINK_NOFOLLOW | AT_EMPTY_PATH)) != 0)
- return ERR_PTR(-EINVAL);
- if (flags & AT_SYMLINK_NOFOLLOW)
- open_exec_flags.lookup_flags &= ~LOOKUP_FOLLOW;
- if (flags & AT_EMPTY_PATH)
- open_exec_flags.lookup_flags |= LOOKUP_EMPTY;

- file = do_filp_open(fd, name, &open_exec_flags);
- if (IS_ERR(file))
- goto out;
+ if (fd == AT_FDCWD && name->name[0] == '\0' && flags == AT_EMPTY_PATH) {
+ file = get_mm_exe_file(current->mm);
+ if (!file) {
+ file = ERR_PTR(-ENOENT);
+ goto out;
+ }
+ } else {
+ struct open_flags open_exec_flags = {
+ .open_flag = O_LARGEFILE | O_RDONLY | __FMODE_EXEC,
+ .acc_mode = MAY_EXEC,
+ .intent = LOOKUP_OPEN,
+ .lookup_flags = LOOKUP_FOLLOW,
+ };
+
+ if ((flags & ~(AT_SYMLINK_NOFOLLOW | AT_EMPTY_PATH)) != 0)
+ return ERR_PTR(-EINVAL);
+ if (flags & AT_SYMLINK_NOFOLLOW)
+ open_exec_flags.lookup_flags &= ~LOOKUP_FOLLOW;
+ if (flags & AT_EMPTY_PATH)
+ open_exec_flags.lookup_flags |= LOOKUP_EMPTY;
+
+ file = do_filp_open(fd, name, &open_exec_flags);
+ if (IS_ERR(file))
+ goto out;
+ }

err = -EACCES;
if (!S_ISREG(file_inode(file)->i_mode))
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