[PATCH] prctl: fix PR_SET_MM_AUXV kernel stack leak

Alexey Dobriyan
2021-03-14T20:52:09Z

prctl(PR_SET_MM, PR_SET_MM_AUXV, addr, 1);

will copy 1 byte from userspace to (quite big) on-stack array
and then stash everything to mm->saved_auxv.
AT_NULL terminator will be inserted at the very end.

/proc/*/auxv handler will find that AT_NULL terminator
and copy original stack contents to userspace.

This devious scheme requires CAP_SYS_RESOURCE.

Signed-off-by: Alexey Dobriyan <adobriyan@gmail.com>
---

apply to >=3.5

kernel/sys.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)

--- a/kernel/sys.c
+++ b/kernel/sys.c
@@ -2079,7 +2079,7 @@ static int prctl_set_auxv(struct mm_struct *mm, unsigned long addr,
* up to the caller to provide sane values here, otherwise userspace
* tools which use this vector might be unhappy.
*/
- unsigned long user_auxv[AT_VECTOR_SIZE];
+ unsigned long user_auxv[AT_VECTOR_SIZE] = {};

if (len > sizeof(user_auxv))
return -EINVAL;