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Message-ID: <39fbeb2c-ef42-4898-b0f8-9340a6e9988e@suse.cz>
Date: Mon, 28 Jul 2025 23:42:47 +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 v2 1/1] mm: fix a UAF when vma->mm is freed after
vma->vm_refcnt got dropped
On 7/28/25 19:53, 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>
Reviewed-by: Vlastimil Babka <vbabka@...e.cz>
> ---
> Changes since v1 [1]
> - Made a copy of vma->mm before using it in vma_start_read(),
> per Vlastimil Babka
>
> Notes:
> - 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.
>
> [1] https://lore.kernel.org/all/20250728170950.2216966-1-surenb@google.com/
>
> include/linux/mmap_lock.h | 23 +++++++++++++++++++++++
> mm/mmap_lock.c | 10 +++-------
> 2 files changed, 26 insertions(+), 7 deletions(-)
>
> diff --git a/include/linux/mmap_lock.h b/include/linux/mmap_lock.h
> index 1f4f44951abe..da34afa2f8ef 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 +184,28 @@ 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.
> + */
> + 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..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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