Re: FAILED: patch "[PATCH] mm: fix a UAF when vma->mm is freed after vma->vm_refcnt got" failed to apply to 6.16-stable tree

On Tue, Aug 12, 2025 at 9:18 AM gregkh@linuxfoundation.org wrote:

The patch below does not apply to the 6.16-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.

Yep, that's expected because 6.6 does not have lock_next_vma() function. I'll send a backport shortly.

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-6.16.y git checkout FETCH_HEAD git cherry-pick -x 9bbffee67ffd16360179327b57f3b1245579ef08 # <resolve conflicts, build, test, etc.> git commit -s git send-email --to 'stable@vger.kernel.org' --in-reply-to '2025081237-buffed-scuba-d3f3@gregkh' --subject-prefix 'PATCH 6.16.y' HEAD^..

Possible dependencies:

thanks,

greg k-h

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

From 9bbffee67ffd16360179327b57f3b1245579ef08 Mon Sep 17 00:00:00 2001 From: Suren Baghdasaryan surenb@google.com Date: Mon, 28 Jul 2025 10:53:55 -0700 Subject: [PATCH] mm: fix a UAF when vma->mm is freed after vma->vm_refcnt got dropped

By inducing delays in the right places, Jann Horn created a reproducer for a hard to hit UAF issue that became possible after VMAs were allowed to be recycled by adding SLAB_TYPESAFE_BY_RCU to their cache.

Race description is borrowed from Jann's discovery report: lock_vma_under_rcu() looks up a VMA locklessly with mas_walk() under rcu_read_lock(). At that point, the VMA may be concurrently freed, and it can be recycled by another process. vma_start_read() then increments the vma->vm_refcnt (if it is in an acceptable range), and if this succeeds, vma_start_read() can return a recycled VMA.

In this scenario where the VMA has been recycled, lock_vma_under_rcu() will then detect the mismatching ->vm_mm pointer and drop the VMA through vma_end_read(), which calls vma_refcount_put(). vma_refcount_put() drops the refcount and then calls rcuwait_wake_up() using a copy of vma->vm_mm. This is wrong: It implicitly assumes that the caller is keeping the VMA's mm alive, but in this scenario the caller has no relation to the VMA's mm, so the rcuwait_wake_up() can cause UAF.

The diagram depicting the race: T1 T2 T3 == == == lock_vma_under_rcu mas_walk <VMA gets removed from mm> mmap <the same VMA is reallocated> vma_start_read __refcount_inc_not_zero_limited_acquire munmap __vma_enter_locked refcount_add_not_zero vma_end_read vma_refcount_put __refcount_dec_and_test rcuwait_wait_event <finish operation> rcuwait_wake_up [UAF]

Note that rcuwait_wait_event() in T3 does not block because refcount was already dropped by T1. At this point T3 can exit and free the mm causing UAF in T1.

To avoid this we move vma->vm_mm verification into vma_start_read() and grab vma->vm_mm to stabilize it before vma_refcount_put() operation.

[surenb@google.com: v3] Link: https://lkml.kernel.org/r/20250729145709.2731370-1-surenb@google.com Link: https://lkml.kernel.org/r/20250728175355.2282375-1-surenb@google.com Fixes: 3104138517fc ("mm: make vma cache SLAB_TYPESAFE_BY_RCU") Signed-off-by: Suren Baghdasaryan surenb@google.com Reported-by: Jann Horn jannh@google.com Closes: https://lore.kernel.org/all/CAG48ez0-deFbVH=E3jbkWx=X3uVbd8nWeo6kbJPQ0KoUD+m... Reviewed-by: Vlastimil Babka vbabka@suse.cz Acked-by: Lorenzo Stoakes lorenzo.stoakes@oracle.com Cc: Jann Horn jannh@google.com Cc: Liam Howlett liam.howlett@oracle.com Cc: stable@vger.kernel.org Signed-off-by: Andrew Morton akpm@linux-foundation.org

diff --git a/include/linux/mmap_lock.h b/include/linux/mmap_lock.h index 1f4f44951abe..11a078de9150 100644 --- a/include/linux/mmap_lock.h +++ b/include/linux/mmap_lock.h @@ -12,6 +12,7 @@ extern int rcuwait_wake_up(struct rcuwait *w); #include <linux/tracepoint-defs.h> #include <linux/types.h> #include <linux/cleanup.h> +#include <linux/sched/mm.h>

#define MMAP_LOCK_INITIALIZER(name) \ .mmap_lock = __RWSEM_INITIALIZER((name).mmap_lock), @@ -154,6 +155,10 @@ static inline void vma_refcount_put(struct vm_area_struct *vma)

• reused and attached to a different mm before we lock it.
• Returns the vma on success, NULL on failure to lock and EAGAIN if vma got
• detached.

• • WARNING! The vma passed to this function cannot be used if the function
• • fails to lock it because in certain cases RCU lock is dropped and then
• • reacquired. Once RCU lock is dropped the vma can be concurently freed.
  */

static inline struct vm_area_struct *vma_start_read(struct mm_struct *mm, struct vm_area_struct *vma) @@ -183,6 +188,31 @@ static inline struct vm_area_struct *vma_start_read(struct mm_struct *mm, }

    rwsem_acquire_read(&vma->vmlock_dep_map, 0, 1, _RET_IP_);

•   /*
  

•    * If vma got attached to another mm from under us, that mm is not
  

•    * stable and can be freed in the narrow window after vma->vm_refcnt
  

•    * is dropped and before rcuwait_wake_up(mm) is called. Grab it before
  

•    * releasing vma->vm_refcnt.
  

•    */
  

•   if (unlikely(vma->vm_mm != mm)) {
  

•           /* Use a copy of vm_mm in case vma is freed after we drop vm_refcnt */
  

•           struct mm_struct *other_mm = vma->vm_mm;
  

•           /*
  

•            * __mmdrop() is a heavy operation and we don't need RCU
  

•            * protection here. Release RCU lock during these operations.
  

•            * We reinstate the RCU read lock as the caller expects it to
  

•            * be held when this function returns even on error.
  

•            */
  

•           rcu_read_unlock();
  

•           mmgrab(other_mm);
  

•           vma_refcount_put(vma);
  

•           mmdrop(other_mm);
  

•           rcu_read_lock();
  

•           return NULL;
  

•   }
  

•   /*
     * Overflow of vm_lock_seq/mm_lock_seq might produce false locked result.
     * False unlocked result is impossible because we modify and check
  

diff --git a/mm/mmap_lock.c b/mm/mmap_lock.c index 729fb7d0dd59..b006cec8e6fe 100644 --- a/mm/mmap_lock.c +++ b/mm/mmap_lock.c @@ -164,8 +164,7 @@ struct vm_area_struct *lock_vma_under_rcu(struct mm_struct *mm, */

    /* Check if the vma we locked is the right one. */

•   if (unlikely(vma->vm_mm != mm ||
  

•                address < vma->vm_start || address >= vma->vm_end))
  

•   if (unlikely(address < vma->vm_start || address >= vma->vm_end))
            goto inval_end_read;
  
    rcu_read_unlock();
  

@@ -236,11 +235,8 @@ struct vm_area_struct *lock_next_vma(struct mm_struct *mm, goto fallback; }

•   /*
  

•    * Verify the vma we locked belongs to the same address space and it's
  

•    * not behind of the last search position.
  

•    */
  

•   if (unlikely(vma->vm_mm != mm || from_addr >= vma->vm_end))
  

•   /* Verify the vma is not behind the last search position. */
  

•   if (unlikely(from_addr >= vma->vm_end))
            goto fallback_unlock;
  
    /*