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Date:   Sat, 27 Apr 2019 16:13:09 +0100
From:   Ben Hutchings <ben@...adent.org.uk>
To:     linux-kernel@...r.kernel.org, stable@...r.kernel.org
CC:     akpm@...ux-foundation.org, Denis Kirjanov <kda@...ux-powerpc.org>,
        "Linus Torvalds" <torvalds@...ux-foundation.org>,
        "Jann Horn" <jannh@...gle.com>
Subject: [PATCH 3.16 181/202] mm: enforce min addr even if capable() in
 expand_downwards()

3.16.66-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: Jann Horn <jannh@...gle.com>

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")
Signed-off-by: Jann Horn <jannh@...gle.com>
Signed-off-by: Linus Torvalds <torvalds@...ux-foundation.org>
Signed-off-by: Ben Hutchings <ben@...adent.org.uk>
---
 mm/mmap.c | 7 +++----
 1 file changed, 3 insertions(+), 4 deletions(-)

--- a/mm/mmap.c
+++ b/mm/mmap.c
@@ -2244,12 +2244,11 @@ int expand_downwards(struct vm_area_stru
 				   unsigned long address)
 {
 	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;

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