[PATCH 1/1] mm: enforce min addr even if capable() in expand_downwards()

[Tyler Hicks]

From: Jann Horn <jannh at google.com>

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 at kernel.org
Signed-off-by: Jann Horn <jannh at google.com>
Signed-off-by: Linus Torvalds <torvalds at linux-foundation.org>

CVE-2019-9213

(cherry picked from commit 0a1d52994d440e21def1c2174932410b4f2a98a1)
Signed-off-by: Tyler Hicks <tyhicks at canonical.com>
---
 mm/mmap.c | 7 +++----
 1 file changed, 3 insertions(+), 4 deletions(-)

diff --git a/mm/mmap.c b/mm/mmap.c
index f189d4b08633..809a5869480f 100644
--- a/mm/mmap.c
+++ b/mm/mmap.c
@@ -2391,12 +2391,11 @@ int expand_downwards(struct vm_area_struct *vma,
 {
 	struct mm_struct *mm = vma->vm_mm;
 	struct vm_area_struct *prev;
-	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 */
 	prev = vma->vm_prev;
-- 
2.7.4