Commit de04d297 authored Feb 27, 2019 by Jann Horn Committed by Greg Kroah-Hartman Mar 05, 2019

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: 8869477a ("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>

parent ff86bb4d

mm/mmap.c

+3 −4

Original line number	Diff line number	Diff line
		@@ -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;