From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from mail-pf1-f201.google.com (mail-pf1-f201.google.com [209.85.210.201]) (using TLSv1.2 with cipher ECDHE-RSA-AES128-GCM-SHA256 (128/128 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 04CA21EA7E4 for ; Tue, 29 Jul 2025 14:57:12 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=209.85.210.201 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1753801034; cv=none; b=Fu0t2XSRjHvYD88o+/PUSwOMQR+5UST9gUX9//RtrFlZCXaIAHr6uJcFJDXwYQpE6PD9SyVee8EuZJErKmtlRgr3twFM2J2hVDqH8qIf1hRh6kOR/hoHZOcLe/dlKGnnZtlYOgR1craXFwbe0FdzbUdqMvZuG3ExWNBUbaOr3Kk= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1753801034; c=relaxed/simple; bh=UYlrDG+p90cEotYS2k8pU4954aF5Dz3y9FS1+wHx0AI=; h=Date:Mime-Version:Message-ID:Subject:From:To:Cc:Content-Type; b=NlY/pOPsXw1QmWAuIZwY5/LRAtj+GvXIVHrl30QL6xh/IEjgWv1QWF4twzA1ch5nHkAsf0FJZc1TYNPsCauvawqQZJ/llXpz1pKWE4yP9ApNuESYMZvMX4viUg21U5qD5QGiw9/vIxHkTaTX4aXx8GaUu7HTg+1JSIz//ziTIPM= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=reject dis=none) header.from=google.com; spf=pass smtp.mailfrom=flex--surenb.bounces.google.com; dkim=pass (2048-bit key) header.d=google.com header.i=@google.com header.b=St2+gAfV; arc=none smtp.client-ip=209.85.210.201 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=reject dis=none) header.from=google.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=flex--surenb.bounces.google.com Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=google.com header.i=@google.com header.b="St2+gAfV" Received: by mail-pf1-f201.google.com with SMTP id d2e1a72fcca58-7641e7c233cso3362451b3a.1 for ; Tue, 29 Jul 2025 07:57:12 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=20230601; t=1753801032; x=1754405832; darn=vger.kernel.org; h=cc:to:from:subject:message-id:mime-version:date:from:to:cc:subject :date:message-id:reply-to; bh=rE5QaqGtBbgEZH/uZ7FzXjyerj3ccqofJgA61gtYGaw=; b=St2+gAfVGm+S/409YlUIE8i3x51IUDOCncDH4PWlPrZdlIaQsy3HOUIUCiwrkP7sei eCZgl4XgeZDvxwkAY8JJ3WJZYE9rAmU5t0jwrzvPgJxG1UQc89j2Wm1Gp3syIuRz+Y18 sqsTujzXMt96YU8037bh0bEl1f/1rdq8Yvauk9S+EuK0pwKIJXbpP4J30NYvBPaceSL5 TfAKL/eQOfJrQAQ+fYcxUQ+zHDzNH7nBYao2Tj/TN6U7U03kTffSGzZf+/M1nyK1WcCd iQYK5Mz7Gc94QRD5CBF+ay0kDsDGjpMe/luzEy7Jo7MBGZFR2Q3xl2N3KC+thIte9Gts Dh3w== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20230601; t=1753801032; x=1754405832; h=cc:to:from:subject:message-id:mime-version:date:x-gm-message-state :from:to:cc:subject:date:message-id:reply-to; bh=rE5QaqGtBbgEZH/uZ7FzXjyerj3ccqofJgA61gtYGaw=; b=LP1BaOKB30WbqHSolCXw3sSCmUuCo9GLJ37zESl3E2/h+Ql+q69TKpxJs2WxwU55PY FKrWQTikpxu26Z8j+k244EMzsJf3ADANza0S00x+DD3RHxxFVeHqZObKVP4cxHMnhRpq Odc2zD6PvTFrHPjFhoXKebPFSxvosyHc5NacN2K3bQazczkZ7hnSXM6CU8+fv/7L/bmp O1htMp/OY5WPAfaEkiWV/FCcy8oVgYWQZEkUfm4ktuc4nPUmEDSCQz1VyjlYz0ItinD7 OsKX4Y5E/YDdr5H4MaMQUEWU+Wi3I+x3MxqFwTlAeQMDLjIxh0M+BOKQY3RqkBH9rIDg vhQw== X-Forwarded-Encrypted: i=1; AJvYcCVbqQC79QNsvoHQSoM/SlvZQPXqvHRILKViKTxzVD0pNbVq01hXY1K/Dj95J085ah70t3JxC1M=@vger.kernel.org X-Gm-Message-State: AOJu0YwICN9q7t2kTzgCi6RbDYwkqu3cAaVB+b2PP6Dyn+tNme/7nFOu ec/zDo/3v4+wcRPsnvU6c8G03fKQmpqpLuWkBvT5kUNm0D81he8L7mJ5Wpe4rpD/o30568IsXHZ nL6iOWQ== 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 Precedence: bulk X-Mailing-List: stable@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: 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" 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