From: Alexey Dobriyan <adobriyan@xxxxxxxxx>
Subject: execve: warn if process starts with executable stack
There were few episodes of silent downgrade to an executable stack over
years:
1) linking innocent looking assembly file will silently add executable
stack if proper linker options is not given as well:
$ cat f.S
.intel_syntax noprefix
.text
.globl f
f:
ret
$ cat main.c
void f(void);
int main(void)
{
f();
return 0;
}
$ gcc main.c f.S
$ readelf -l ./a.out
GNU_STACK 0x0000000000000000 0x0000000000000000 0x0000000000000000
0x0000000000000000 0x0000000000000000 RWE 0x10
^^^
2) converting C99 nested function into a closure
https://nullprogram.com/blog/2019/11/15/
void intsort2(int *base, size_t nmemb, _Bool invert)
{
int cmp(const void *a, const void *b)
{
int r = *(int *)a - *(int *)b;
return invert ? -r : r;
}
qsort(base, nmemb, sizeof(*base), cmp);
}
will silently require stack trampolines while non-closure version will not.
Without doubt this behaviour is documented somewhere, add a warning so
that developers and users can at least notice. After so many years of
x86_64 having proper executable stack support it should not cause too many
problems.
Link: http://lkml.kernel.org/r/20191208171918.GC19716@avx2
Signed-off-by: Alexey Dobriyan <adobriyan@xxxxxxxxx>
Cc: Dan Carpenter <dan.carpenter@xxxxxxxxxx>
Cc: Will Deacon <will@xxxxxxxxxx>
Cc: "Eric W. Biederman" <ebiederm@xxxxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
fs/exec.c | 5 +++++
1 file changed, 5 insertions(+)
--- a/fs/exec.c~execve-warn-if-process-starts-with-executable-stack
+++ a/fs/exec.c
@@ -761,6 +761,11 @@ int setup_arg_pages(struct linux_binprm
goto out_unlock;
BUG_ON(prev != vma);
+ if (unlikely(vm_flags & VM_EXEC)) {
+ pr_warn_once("process '%pD4' started with executable stack\n",
+ bprm->file);
+ }
+
/* Move stack pages down in memory. */
if (stack_shift) {
ret = shift_arg_pages(vma, stack_shift);
_
