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SubjectRe: [PATCH] exec: Check stack space more strictly
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minor nit below

On 18.07.2017 01:22, Andy Lutomirski wrote:
> We can currently blow past the stack rlimit and cause odd behavior
> if there are accounting bugs, rounding issues, or races. It's not
> clear that the odd behavior is actually a problem, but it's nicer to
> fail the exec instead of getting out of sync with stack limits.
>
> Improve the code to more carefully check for space and to abort if
> our stack mm gets too large in setup_arg_pages().
>
> Signed-off-by: Andy Lutomirski <luto@kernel.org>
> ---
> fs/exec.c | 44 ++++++++++++++++++++++++++++++++++----------
> 1 file changed, 34 insertions(+), 10 deletions(-)
>
> diff --git a/fs/exec.c b/fs/exec.c
> index 62175cbcc801..0c60c0495269 100644
> --- a/fs/exec.c
> +++ b/fs/exec.c
> @@ -764,23 +764,47 @@ int setup_arg_pages(struct linux_binprm *bprm,
> /* mprotect_fixup is overkill to remove the temporary stack flags */
> vma->vm_flags &= ~VM_STACK_INCOMPLETE_SETUP;
>
> - stack_expand = 131072UL; /* randomly 32*4k (or 2*64k) pages */
> - stack_size = vma->vm_end - vma->vm_start;
> /*
> * Align this down to a page boundary as expand_stack
> * will align it up.
> */
> rlim_stack = rlimit(RLIMIT_STACK) & PAGE_MASK;
> + stack_size = vma->vm_end - vma->vm_start;
> +
> + if (stack_size > rlim_stack) {
> + /*
> + * If we've already used too much space (due to accounting
> + * bugs, alignment, races, or any other cause), bail.
> + */
> + ret = -ENOMEM;
> + goto out_unlock;
> + }
> +
> + /*
> + * stack_expand is the amount of space beyond the space already used
> + * that we're going to pre-allocate in our stack. For historical
> + * reasons, it's 128kB, unless we have less space than that available
> + * in our rlimit.
> + *
> + * This particular historical wart is wrong-headed, though, since
> + * we haven't finished binfmt-specific setup, and the binfmt code
> + * is going to eat up some or all of this space.
> + */
> + stack_expand = min(rlim_stack - stack_size, 131072UL);

nit: Use the SZ_128K from sizes.h.

> +
> #ifdef CONFIG_STACK_GROWSUP
> - if (stack_size + stack_expand > rlim_stack)
> - stack_base = vma->vm_start + rlim_stack;
> - else
> - stack_base = vma->vm_end + stack_expand;
> + if (TASK_SIZE_MAX - vma->vm_end < stack_expand) {
> + ret = -ENOMEM;
> + goto out_unlock;
> + }
> + stack_base = vma->vm_end + stack_expand;
> #else
> - if (stack_size + stack_expand > rlim_stack)
> - stack_base = vma->vm_end - rlim_stack;
> - else
> - stack_base = vma->vm_start - stack_expand;
> + if (vma->vm_start < mmap_min_addr ||
> + vma->vm_start - mmap_min_addr < stack_expand) {
> + ret = -ENOMEM;
> + goto out_unlock;
> + }
> + stack_base = vma->vm_start - stack_expand;
> #endif
> current->mm->start_stack = bprm->p;
> ret = expand_stack(vma, stack_base);
>

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