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Date:   Fri, 22 Mar 2019 12:14:15 +0100
From:   Greg Kroah-Hartman <>
Cc:     Greg Kroah-Hartman <>,,,
        Jann Horn <>,
        Linus Torvalds <>
Subject: [PATCH 3.18 042/134] mm: enforce min addr even if capable() in expand_downwards()

3.18-stable review patch.  If anyone has any objections, please let me know.


From: Jann Horn <>

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 <>
Signed-off-by: Linus Torvalds <>
Signed-off-by: Greg Kroah-Hartman <>

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

--- a/mm/mmap.c
+++ b/mm/mmap.c
@@ -2292,12 +2292,11 @@ int expand_downwards(struct vm_area_stru
 	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;

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