The patch below does not apply to the 6.4-stable tree. If someone wants it applied there, or to any other stable or longterm tree, then please email the backport, including the original git commit id to stable@vger.kernel.org.

To reproduce the conflict and resubmit, you may use the following commands:

git fetch https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/ linux-6.4.y git checkout FETCH_HEAD git cherry-pick -x 657b5146955eba331e01b9a6ae89ce2e716ba306 # <resolve conflicts, build, test, etc.> git commit -s git send-email --to 'stable@vger.kernel.org' --in-reply-to '2023080129-surface-stench-5e24@gregkh' --subject-prefix 'PATCH 6.4.y' HEAD^..

Possible dependencies:

thanks,

greg k-h

------------------ original commit in Linus's tree ------------------

From 657b5146955eba331e01b9a6ae89ce2e716ba306 Mon Sep 17 00:00:00 2001 From: Jann Horn jannh@google.com Date: Wed, 26 Jul 2023 23:41:03 +0200 Subject: [PATCH] mm: lock_vma_under_rcu() must check vma->anon_vma under vma lock

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

diff --git a/mm/memory.c b/mm/memory.c index 01f39e8144ef..603b2f419948 100644 --- a/mm/memory.c +++ b/mm/memory.c @@ -5393,27 +5393,28 @@ struct vm_area_struct *lock_vma_under_rcu(struct mm_struct *mm, if (!vma_is_anonymous(vma) && !vma_is_tcp(vma)) goto inval;

- /* find_mergeable_anon_vma uses adjacent vmas which are not locked */ - if (!vma->anon_vma && !vma_is_tcp(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 && !vma_is_tcp(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) { @@ -5425,6 +5426,9 @@ struct vm_area_struct *lock_vma_under_rcu(struct mm_struct *mm,

rcu_read_unlock(); return vma; + +inval_end_read: + vma_end_read(vma); inval: rcu_read_unlock(); count_vm_vma_lock_event(VMA_LOCK_ABORT);