From 0a1d52994d440e21def1c2174932410b4f2a98a1 Mon Sep 17 00:00:00 2001
From: Jann Horn <jannh@google.com>
Date: Wed, 27 Feb 2019 21:29:52 +0100
Subject: [PATCH] mm: enforce min addr even if capable() in expand_downwards()
Git-commit: 0a1d52994d440e21def1c2174932410b4f2a98a1
Patch-mainline: v5.0
References: bsc#1128166, CVE-2019-9213

security_mmap_addr() does a capability check with current_cred(), but
we can reach this code from contexts like a VFS write handler where
current_cred() must not be used.

This can be abused on systems without SMAP to make NULL pointer
dereferences exploitable again.

Fixes: 8869477a49c3 ("security: protect from stack expansion into low vm addresses")
Cc: stable@kernel.org
Signed-off-by: Jann Horn <jannh@google.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Acked-by: Michal Hocko <mhocko@suse.com>

---
 mm/mmap.c |    7 +++----
 1 file changed, 3 insertions(+), 4 deletions(-)

--- a/mm/mmap.c
+++ b/mm/mmap.c
@@ -2366,12 +2366,11 @@ int expand_downwards(struct vm_area_stru
 	struct mm_struct *mm = vma->vm_mm;
 	struct vm_area_struct *prev;
 	unsigned long gap_addr;
-	int error;
+	int error = 0;
 
 	address &= PAGE_MASK;
-	error = security_mmap_addr(address);
-	if (error)
-		return error;
+	if (address < mmap_min_addr)
+		return -EPERM;
 
 	/* Enforce stack_guard_gap */
 	gap_addr = address - stack_guard_gap;