From: Jann Horn jannh@google.com
commit 657b5146955eba331e01b9a6ae89ce2e716ba306 upstream.
lock_vma_under_rcu() tries to guarantee that __anon_vma_prepare() can't be called in the VMA-locked page fault path by ensuring that vma->anon_vma is set.
However, this check happens before the VMA is locked, which means a concurrent move_vma() can concurrently call unlink_anon_vmas(), which disassociates the VMA's anon_vma.
This means we can get UAF in the following scenario:
THREAD 1 THREAD 2 ======== ======== <page fault> lock_vma_under_rcu() rcu_read_lock() mas_walk() check vma->anon_vma
mremap() syscall move_vma() vma_start_write() unlink_anon_vmas() <syscall end>
handle_mm_fault() __handle_mm_fault() handle_pte_fault() do_pte_missing() do_anonymous_page() anon_vma_prepare() __anon_vma_prepare() find_mergeable_anon_vma() mas_walk() [looks up VMA X]
munmap() syscall (deletes VMA X)
reusable_anon_vma() [called on freed VMA X]
This is a security bug if you can hit it, although an attacker would have to win two races at once where the first race window is only a few instructions wide.
This patch is based on some previous discussion with Linus Torvalds on the security list.
Cc: stable@vger.kernel.org Fixes: 5e31275cc997 ("mm: add per-VMA lock and helper functions to control it") 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/memory.c | 28 ++++++++++++++++------------ 1 file changed, 16 insertions(+), 12 deletions(-)
--- a/mm/memory.c +++ b/mm/memory.c @@ -5410,27 +5410,28 @@ retry: if (!vma_is_anonymous(vma)) goto inval;
- /* find_mergeable_anon_vma uses adjacent vmas which are not locked */ - if (!vma->anon_vma) - goto inval; - if (!vma_start_read(vma)) goto inval;
/* + * find_mergeable_anon_vma uses adjacent vmas which are not locked. + * This check must happen after vma_start_read(); otherwise, a + * concurrent mremap() with MREMAP_DONTUNMAP could dissociate the VMA + * from its anon_vma. + */ + if (unlikely(!vma->anon_vma)) + goto inval_end_read; + + /* * Due to the possibility of userfault handler dropping mmap_lock, avoid * it for now and fall back to page fault handling under mmap_lock. */ - if (userfaultfd_armed(vma)) { - vma_end_read(vma); - goto inval; - } + if (userfaultfd_armed(vma)) + goto inval_end_read;
/* Check since vm_start/vm_end might change before we lock the VMA */ - if (unlikely(address < vma->vm_start || address >= vma->vm_end)) { - vma_end_read(vma); - goto inval; - } + if (unlikely(address < vma->vm_start || address >= vma->vm_end)) + goto inval_end_read;
/* Check if the VMA got isolated after we found it */ if (vma->detached) { @@ -5442,6 +5443,9 @@ retry:
rcu_read_unlock(); return vma; + +inval_end_read: + vma_end_read(vma); inval: rcu_read_unlock(); count_vm_vma_lock_event(VMA_LOCK_ABORT);