From: Jann Horn <jannh@google.com>
Date: Wed, 27 Feb 2019 20:29:52 +0000 (+0100)
Subject: mm: enforce min addr even if capable() in expand_downwards()
X-Git-Tag: rel_imx_4.19.35_1.1.0~6177
X-Git-Url: https://git.somdevices.com/?a=commitdiff_plain;h=de04d2973a62e4efb3de45e93bd46acd6d510e0a;p=linux.git

mm: enforce min addr even if capable() in expand_downwards()

commit 0a1d52994d440e21def1c2174932410b4f2a98a1 upstream.

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>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---

diff --git a/mm/mmap.c b/mm/mmap.c
index f7cd9cb966c0..43507f7e66b4 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;