Messages in this thread | | | From | Kees Cook <> | Date | Thu, 14 Feb 2019 08:38:39 -0800 | Subject | Re: [PATCH] exec: load_script: Allow interpreter argument truncation |
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On Thu, Feb 14, 2019 at 8:08 AM Oleg Nesterov <oleg@redhat.com> wrote: > > On 02/13, Kees Cook wrote: > > > > While we want to make sure the kernel doesn't attempt to execute a > > truncated interpreter path, we must allow the interpreter arguments to > > be truncated. Perl, for example, will re-read the script itself to parse > > arguments correctly. > > Heh. I still think that 8099b047ecc4 does the right thing. > > But I can't argue with the fact that it caused the regression, so it should > be reverted. > > > This documents the parsing steps, and will fail to exec if the string was > > truncated with neither an end-of-line nor any trailing whitespace. > > You know, I have already spent 3 hours trying to write something simple and > clear, but failed. Still trying...
That's why I added comments too. It's kind of a weird bit of parsing, and has to protect itself from lack of initial NUL-termination. :P
> Nor I can really understand your fix ;) Will try to read it again, just one > question for now, > > > for (cp = bprm->buf+2;; cp++) { > > - if (cp >= bprm->buf + BINPRM_BUF_SIZE) > > - return -ENOEXEC; > > - if (!*cp || (*cp == '\n')) > > + if (cp == bprm->buf + BINPRM_BUF_SIZE - 1) { > > + truncated = true; > > Off-by-one, no? "bprm->buf + BINPRM_BUF_SIZE - 1" is the very last char, it can > be '\n' or '\0', this should set end_of_interp.
Ah yeah, this bails out one byte too early. As you suggestion, I should move that test to after the !*cp || *cp == '\n' test, like so:
for (cp = bprm->buf+2;; cp++) { if (!*cp || (*cp == '\n')) { end_of_interp = true; break; } if (cp == bprm->buf + BINPRM_BUF_SIZE - 1) { truncated = true; break; } } *cp = '\0';
> > break; > > + } > > + if (!*cp || (*cp == '\n')) { > > + end_of_interp = true; > > + break; > > + } > > so unless I am totally confused you should move this block up before the > "bprm->buf + BINPRM_BUF_SIZE - 1" check or that check should use > "bprm->buf + BINPRM_BUF_SIZE". > > No?
Moving the block is right, dropping the -1 would be lead to the post-loop *cp = '\0' writing past the end of the buffer.
With this change, my tests (after gaining an extra byte of available interp path name) still pass. I'll send a v2...
Thanks!
-- Kees Cook
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