Message-ID: <3f8c28f4-6935-4581-83ec-d3bc1e6c400e@suse.cz>
Date: Mon, 28 Jul 2025 19:19:15 +0200
From: Vlastimil Babka <vbabka@...e.cz>
To: Suren Baghdasaryan <surenb@...gle.com>, akpm@...ux-foundation.org
Cc: jannh@...gle.com, Liam.Howlett@...cle.com, lorenzo.stoakes@...cle.com,
 pfalcato@...e.de, linux-mm@...ck.org, linux-kernel@...r.kernel.org,
 stable@...r.kernel.org
Subject: Re: [PATCH 1/1] mm: fix a UAF when vma->mm is freed after
 vma->vm_refcnt got dropped

On 7/28/25 19:09, Suren Baghdasaryan 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@...gle.com>
> Closes: https://lore.kernel.org/all/CAG48ez0-deFbVH=E3jbkWx=X3uVbd8nWeo6kbJPQ0KoUD+m2tA@mail.gmail.com/
> Signed-off-by: Suren Baghdasaryan <surenb@...gle.com>
> Cc: <stable@...r.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.
> 
>  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);

The vma can go away here.

> +		mmdrop(vma->vm_mm);

So we need to copy the vma->vm_mm first?

> +		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

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