Re: [PATCH 1/1] mm: fix a UAF when vma->mm is freed after vma->vm_refcnt got dropped

[Suren Baghdasaryan <surenb@google.com>]

On Mon, Jul 28, 2025 at 10:09 AM Suren Baghdasaryan <surenb@google.com> wrote:
>
> 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.
>
> Fixes: 3104138517fc ("mm: make vma cache SLAB_TYPESAFE_BY_RCU")
> Reported-by: Jann Horn <jannh@google.com>
> Closes: https://lore.kernel.org/all/CAG48ez0-deFbVH=E3jbkWx=X3uVbd8nWeo6kbJPQ0KoUD+m2tA@mail.gmail.com/
> Signed-off-by: Suren Baghdasaryan <surenb@google.com>
> Cc: <stable@vger.kernel.org>
> ---
> - Applies cleanly over mm-unstable.
> - Should be applied to 6.15 and 6.16 but these branches do not
> have lock_next_vma() function, so the change in lock_next_vma() should be
> skipped when applying to those branches.

Andrew, if you would like me to post a separate patch for 6.15 and
6.16 please let me know. The merge conflict in those branches should
be trivial: just skip the change in lock_next_vma() which does not
exist in those branches.

>
>  include/linux/mmap_lock.h | 21 +++++++++++++++++++++
>  mm/mmap_lock.c            | 10 +++-------
>  2 files changed, 24 insertions(+), 7 deletions(-)
>
> diff --git a/include/linux/mmap_lock.h b/include/linux/mmap_lock.h
> index 1f4f44951abe..4ee4ab835c41 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),
> @@ -183,6 +184,26 @@ 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)) {
> +               /*
> +                * __mmdrop() is a heavy operation and we don't need RCU
> +                * protection here. Release RCU lock during these operations.
> +                */
> +               rcu_read_unlock();
> +               mmgrab(vma->vm_mm);
> +               vma_refcount_put(vma);
> +               mmdrop(vma->vm_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..aa3bc42ecde0 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 of the last search position. */
> +       if (unlikely(from_addr >= vma->vm_end))
>                 goto fallback_unlock;
>
>         /*
>
> base-commit: c617a4dd7102e691fa0fb2bc4f6b369e37d7f509
> --
> 2.50.1.487.gc89ff58d15-goog
>