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fs/exec.c: account for argv/envp pointers

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When limiting the argv/envp strings during exec to 1/4 of the stack limit,
the storage of the pointers to the strings was not included.  This means
that an exec with huge numbers of tiny strings could eat 1/4 of the stack
limit in strings and then additional space would be later used by the
pointers to the strings.

For example, on 32-bit with a 8MB stack rlimit, an exec with 1677721
single-byte strings would consume less than 2MB of stack, the max (8MB /
4) amount allowed, but the pointers to the strings would consume the
remaining additional stack space (1677721 * 4 == 6710884).

The result (1677721 + 6710884 == 8388605) would exhaust stack space
entirely.  Controlling this stack exhaustion could result in
pathological behavior in setuid binaries (CVE-2017-1000365).

[akpm@linux-foundation.org: additional commenting from Kees]
Fixes: b6a2fea393 ("mm: variable length argument support")
Link: http://lkml.kernel.org/r/20170622001720.GA32173@beast
Signed-off-by: Kees Cook <keescook@chromium.org>
Acked-by: Rik van Riel <riel@redhat.com>
Acked-by: Michal Hocko <mhocko@suse.com>
Cc: Alexander Viro <viro@zeniv.linux.org.uk>
Cc: Qualys Security Advisory <qsa@qualys.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
This commit is contained in:
Kees Cook 2017-06-23 15:08:57 -07:00 committed by Linus Torvalds
parent 8818efaaac
commit 98da7d0885
1 changed files with 24 additions and 4 deletions
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28
fs/exec.c
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@ -220,8 +220,26 @@ static struct page *get_arg_page(struct linux_binprm *bprm, unsigned long pos,
if (write) {
unsigned long size = bprm->vma->vm_end - bprm->vma->vm_start;
unsigned long ptr_size;
struct rlimit *rlim;
/*
* 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);
/*
@ -239,13 +257,15 @@ static struct page *get_arg_page(struct linux_binprm *bprm, unsigned long pos,
* to work from.
*/
rlim = current->signal->rlim;
if (size > ACCESS_ONCE(rlim[RLIMIT_STACK].rlim_cur) / 4) {
put_page(page);
return NULL;
}
if (size > READ_ONCE(rlim[RLIMIT_STACK].rlim_cur) / 4)
goto fail;
}
return page;
fail:
put_page(page);
return NULL;
}
static void put_arg_page(struct page *page)