Commit 0a1d5299 authored Feb 27, 2019 by Jann Horn Committed by Linus Torvalds Feb 27, 2019

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



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>

parent 7d762d69

mm/mmap.c

+3 −4

Original line number	Diff line number	Diff line
		@@ -2426,12 +2426,11 @@ 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;

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