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Subject[PATCH] exec: Limit arg stack to at most _STK_LIM / 4 * 3
To avoid pathological stack usage or the need to special-case setuid
execs, just limit all arg stack usage to at most _STK_LIM / 4 * 3 (6MB).

Signed-off-by: Kees Cook <keescook@chromium.org>
---
fs/exec.c | 8 ++++----
1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/fs/exec.c b/fs/exec.c
index 904199086490..ddca2cf15f71 100644
--- a/fs/exec.c
+++ b/fs/exec.c
@@ -221,7 +221,6 @@ 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
@@ -250,14 +249,15 @@ static struct page *get_arg_page(struct linux_binprm *bprm, unsigned long pos,
return page;

/*
- * Limit to 1/4-th the stack size for the argv+env strings.
+ * 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.
*/
- rlim = current->signal->rlim;
- if (size > READ_ONCE(rlim[RLIMIT_STACK].rlim_cur) / 4)
+ if (size > min_t(unsigned long, rlimit(RLIMIT_STACK) / 4,
+ _STK_LIM / 4 * 3))
goto fail;
}

--
2.7.4

--
Kees Cook
Pixel Security

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