Messages in this thread Patch in this message | | | Date | Tue, 11 Sep 2018 16:13:18 +0200 | From | Oleg Nesterov <> | Subject | Re: get_arg_page() && ptr_size accounting |
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On 09/10, Kees Cook wrote: > > I've looked more closely now. So, while I agree with you about > resource limits, there's a corner case that is better handled here: > once we've called flush_old_exec(), we can no longer send errors back > to the parent. We just segfault. So, I think it's better to give a > resource limit error early, since it is able to do the math early. > > If we move acct_arg_size() earlier, then the "immediate" resource > utilization is checked, but it means it can just segfault later. If we > leave it as-is, we account for later memory allocations "too early", > but we'll still not be able to run: but we can tell the parent why.
I don't follow. Could you spell please?
AFAICS, the trivial patch I proposed changes nothing except it fixes the bprm->pages accounting. The problem is really minor, but this looks confusing and wrong anyway.
> I prefer leave it as-is.
After this discussion, I strongly disagree.
And now I think we should remove this rlim crap from get_arg_page() altogether to make the things more clear.
> > Please forget. I meant that _if_ we actually wanted to account this additional > > memory in bprm->pages, than we would probably need something like > > acct_arg_size(size/PAGE_SIZE + DIV_ROUND_UP(ptr_size, PAGE_SIZE)). > > I'd need to study that more, but that change seems reasonable. :)
Please forget. Not that it matters, but we simply can't account ptr_size 100% correctly if we do this in get_arg_page().
See the patch below. Completely untested, quite possibly wrong, but I think this is what we should do.
Oleg.
diff --git a/fs/exec.c b/fs/exec.c index 1ebf6e5..7804a5c 100644 --- a/fs/exec.c +++ b/fs/exec.c @@ -218,55 +218,10 @@ static struct page *get_arg_page(struct linux_binprm *bprm, unsigned long pos, if (ret <= 0) return NULL; - if (write) { - unsigned long size = bprm->vma->vm_end - bprm->vma->vm_start; - unsigned long ptr_size, limit; - - /* - * Since the stack will hold pointers to the strings, we - * must account for them as well. - * - * The size calculation is the entire vma while each arg page is - * built, so each time we get here it's calculating how far it - * is currently (rather than each call being just the newly - * added size from the arg page). As a result, we need to - * always add the entire size of the pointers, so that on the - * last call to get_arg_page() we'll actually have the entire - * correct size. - */ - ptr_size = (bprm->argc + bprm->envc) * sizeof(void *); - if (ptr_size > ULONG_MAX - size) - goto fail; - size += ptr_size; - - acct_arg_size(bprm, size / PAGE_SIZE); - - /* - * We've historically supported up to 32 pages (ARG_MAX) - * of argument strings even with small stacks - */ - if (size <= ARG_MAX) - return page; - - /* - * Limit to 1/4 of the max stack size or 3/4 of _STK_LIM - * (whichever is smaller) for the argv+env strings. - * This ensures that: - * - the remaining binfmt code will not run out of stack space, - * - the program will have a reasonable amount of stack left - * to work from. - */ - limit = _STK_LIM / 4 * 3; - limit = min(limit, bprm->rlim_stack.rlim_cur / 4); - if (size > limit) - goto fail; - } + if (write) + acct_arg_size(bprm, vma_pages(bprm->vma)); return page; - -fail: - put_page(page); - return NULL; } static void put_arg_page(struct page *page) @@ -410,11 +365,6 @@ static int bprm_mm_init(struct linux_binprm *bprm) if (!mm) goto err; - /* Save current stack limit for all calculations made during exec. */ - task_lock(current->group_leader); - bprm->rlim_stack = current->signal->rlim[RLIMIT_STACK]; - task_unlock(current->group_leader); - err = __bprm_mm_init(bprm); if (err) goto err; @@ -492,6 +442,27 @@ static int count(struct user_arg_ptr argv, int max) return i; } +static int prepare_rlim_stack(struct linux_binprm *bprm) +{ + unsigned long limit, ptr_size; + + task_lock(current->group_leader); + bprm->rlim_stack = current->signal->rlim[RLIMIT_STACK]; + task_unlock(current->group_leader); + + limit = _STK_LIM / 4 * 3; + limit = min(limit, bprm->rlim_stack.rlim_cur / 4); + limit = max(limit, (unsigned long)ARG_MAX); + /* COMMENT */ + ptr_size = (bprm->argc + bprm->envc) * sizeof(void *); + if (limit <= ptr_size) + return -E2BIG; + limit -= ptr_size; + + bprm->p_min = bprm->p - limit; + return 0; +} + /* * 'copy_strings()' copies argument/environment strings from the old * processes's memory to the new process's stack. The call to get_user_pages() @@ -527,6 +498,8 @@ static int copy_strings(int argc, struct user_arg_ptr argv, pos = bprm->p; str += len; bprm->p -= len; + if (bprm->p <= bprm->p_min) + goto out; while (len > 0) { int offset, bytes_to_copy; @@ -1801,6 +1774,10 @@ static int __do_execve_file(int fd, struct filename *filename, if (retval < 0) goto out; + retval = prepare_rlim_stack(bprm); + if (retval < 0) + goto out; + retval = copy_strings_kernel(1, &bprm->filename, bprm); if (retval < 0) goto out; diff --git a/include/linux/binfmts.h b/include/linux/binfmts.h index c05f24f..423e8c1 100644 --- a/include/linux/binfmts.h +++ b/include/linux/binfmts.h @@ -24,7 +24,7 @@ struct linux_binprm { struct page *page[MAX_ARG_PAGES]; #endif struct mm_struct *mm; - unsigned long p; /* current top of mem */ + unsigned long p, p_min; /* current top of mem */ unsigned int /* * True after the bprm_set_creds hook has been called once
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