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AJvYcCUdRT+XUBmVJYNj3z/4FwfsFuJIAPKDKZ91lFMv12FianiHrJnsLByI0hnxB9vcPuMLhiC88eFZYg==@kvack.org X-Gm-Message-State: AOJu0YzqfEflwcdxC9vPVhZNiwrk6KV0EOEaur5l/DVYSuk72fayvEaQ WexEsl0nchEetZZHFeY5/naWrpFW99RgBd+/jGIK3MLI77YN8XB7soZJWwT+EbmZNgD392yzOYO fwCe7Fw== X-Google-Smtp-Source: AGHT+IEyb7DFh1Ctr01+r2+P02e+x/ttb54C/yw3CRQSNoviHIgWG2NyMvi3eNyzh9bqPdN2qlAcWUvfG4c= X-Received: from pfbhm20.prod.google.com ([2002:a05:6a00:6714:b0:748:f270:c438]) (user=surenb job=prod-delivery.src-stubby-dispatcher) by 2002:a05:6a21:9999:b0:235:2cd8:6cd1 with SMTP id adf61e73a8af0-23d701886f8mr24958186637.34.1753801032262; Tue, 29 Jul 2025 07:57:12 -0700 (PDT) Date: Tue, 29 Jul 2025 07:57:09 -0700 Mime-Version: 1.0 X-Mailer: git-send-email 2.50.1.487.gc89ff58d15-goog Message-ID: <20250729145709.2731370-1-surenb@google.com> Subject: [PATCH v3 1/1] mm: fix a UAF when vma->mm is freed after vma->vm_refcnt got dropped From: Suren Baghdasaryan To: akpm@linux-foundation.org Cc: jannh@google.com, Liam.Howlett@oracle.com, lorenzo.stoakes@oracle.com, vbabka@suse.cz, pfalcato@suse.de, linux-mm@kvack.org, linux-kernel@vger.kernel.org, surenb@google.com, stable@vger.kernel.org Content-Type: text/plain; charset="UTF-8" X-Rspam-User: X-Rspamd-Server: rspam04 X-Rspamd-Queue-Id: 93075C000B X-Stat-Signature: jt6pniqdzspqttfon388ti8u7ucqpt1d X-HE-Tag: 1753801033-472614 X-HE-Meta: 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 FdhJ6U6V AOvrzp3yOStfvWyV/eri6XfWbbKk/VcXbulFiPAX7elDEtq52P4x6jQRy65ssd0Y8n5Ab6BO+eSuqcmHh+KYX1rw4Owk5S6jBoEKo1EmOO/7ZScG7EVaKicd/9/2XjwJxRUQRq+1y5ugu1hPxOvc1jkHWu6+iLsNJm2onRp6dyjfozf/VgsikU1XSXRHHJFmjwrNasDQnDTqZHsl1efOZJJJA0OyM+UdyiN0+nW9KkWHdducl0hlSX6sdFvtAuQA+/MHRJ2MoEtOYjGHuOqkRhUrbr/HvvV/9uGpP4iQCfNiBNF8GuPWWNWn7e6bkxcBY2ujc7XkHEesuTnYU0VSreiulJg== X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: 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 mmap 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 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 Closes: https://lore.kernel.org/all/CAG48ez0-deFbVH=E3jbkWx=X3uVbd8nWeo6kbJPQ0KoUD+m2tA@mail.gmail.com/ Signed-off-by: Suren Baghdasaryan Reviewed-by: Vlastimil Babka Acked-by: Lorenzo Stoakes Cc: --- Changes since v2 [1] - Addressed Lorenzo's nits, per Lorenzo Stoakes - Added a warning comment for vma_start_read() - Added Reviewed-by and Acked-by, per Vlastimil Babka and Lorenzo Stoakes Notes: - Applies cleanly over mm-unstable after reverting previous version of this patch [1]. - 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/20250728175355.2282375-1-surenb@google.com/ include/linux/mmap_lock.h | 30 ++++++++++++++++++++++++++++++ mm/mmap_lock.c | 10 +++------- 2 files changed, 33 insertions(+), 7 deletions(-) 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 #include #include +#include #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; /* -- 2.50.1.487.gc89ff58d15-goog