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: 8869477a49c3 ("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>

diff --git a/mm/mmap.c b/mm/mmap.c
index 2837556..3f2314a 100644 (file)
--- a/mm/mmap.c
+++ b/mm/mmap.c
@@ -2345,12 +2345,11 @@ int expand_downwards(struct vm_area_struct *vma,
        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;