4.20-stable review patch. If anyone has any objections, please let me know.
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From: Jann Horn jannh@google.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@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
--- mm/mmap.c | 7 +++---- 1 file changed, 3 insertions(+), 4 deletions(-)
--- a/mm/mmap.c +++ b/mm/mmap.c @@ -2415,12 +2415,11 @@ int expand_downwards(struct vm_area_stru { 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;