linux-kernel - [PATCH 4.4 053/230] mm: enforce min addr even if capable() in expand

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Date:   Fri, 22 Mar 2019 12:13:11 +0100
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     linux-kernel@...r.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        stable@...r.kernel.org, stable@...nel.org,
        Jann Horn <jannh@...gle.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Subject: [PATCH 4.4 053/230] mm: enforce min addr even if capable() in expand_downwards()

4.4-stable 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")
Cc: stable@...nel.org
Signed-off-by: Jann Horn <jannh@...gle.com>
Signed-off-by: Linus Torvalds <torvalds@...ux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>

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

--- a/mm/mmap.c
+++ b/mm/mmap.c
@@ -2294,12 +2294,11 @@ 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;