From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from mail-pj1-f73.google.com (mail-pj1-f73.google.com [209.85.216.73]) (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 9405D280018 for ; Tue, 12 Aug 2025 16:54:07 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=209.85.216.73 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1755017649; cv=none; b=sQB0BcNDuuxNojc3BxC2p1fJwqTTgrCwAdsCqjR64dMEaoaKMrtU1SgeZjCa0QwWTCXv6MMezkNRvrlARtAvXwxVcOXhl9ZQpaRNt1q2uDRjkMZUMo/OcauAExuf2/6WtSNMCvD5MHQhPVwxMY9IFs5bHiMsmsIV3wLPs/9AvCg= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1755017649; c=relaxed/simple; bh=Pik7QmBfKb78Emi5FDZHO9/g29Tap05Gm42Xm91m1lw=; h=Date:In-Reply-To:Mime-Version:References:Message-ID:Subject:From: To:Cc:Content-Type; b=O+ufqj06C+Xhbidf+U7g/rh1LRhuWISQbPPLZh5o/7RCfydPwoRTB61BZ2lpek4vGZE06ETEiSOJLevCitRnHY9PuPtgsAik1KNAXAxB6wPbJPNYLrMncxybccDc+USIjho0vneOhHbkNVxEVa0vWAxsMZWaUkwZxHEeEplUNSc= 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=FPawbaYE; arc=none smtp.client-ip=209.85.216.73 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="FPawbaYE" Received: by mail-pj1-f73.google.com with SMTP id 98e67ed59e1d1-31f322718faso4484671a91.0 for ; Tue, 12 Aug 2025 09:54:07 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=20230601; t=1755017647; x=1755622447; darn=vger.kernel.org; h=cc:to:from:subject:message-id:references:mime-version:in-reply-to :date:from:to:cc:subject:date:message-id:reply-to; bh=3m5TDKUNnmPU78OK/YBQv4hX6zYqGOEdsXQE+EsuZa0=; b=FPawbaYEZc1vdZkEQFmEROAC9Thrbree3diclu/bliBc6CL5b80qQLUCaDCW3ICedi 5fzoSitMKmASTuHIEytvi5s/putyLgwB6Q17WwMeARfHxiFuRiQr0ezj6w3kwAz1DVk6 bp0vOHGPeVTRwIg8/SPHNFTTg+8uK+ge/HSianMmfvt1+3UN7/2LSaXZFiNG5XHfj3oY N0ubaJ7MKKeMhHgODYcHBAx70zicH80rHIW3vEw/EZsuVJcqoaJx4TshJT2rxm4mQhvG UquwHTgAZwWIOXYchzPdFWhGO2PSU0liCNUMobizqNFCTHMbjiZ3QKQ5maPoPk4VZeDE A+xw== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20230601; t=1755017647; x=1755622447; h=cc:to:from:subject:message-id:references:mime-version:in-reply-to :date:x-gm-message-state:from:to:cc:subject:date:message-id:reply-to; bh=3m5TDKUNnmPU78OK/YBQv4hX6zYqGOEdsXQE+EsuZa0=; b=ZFtebCn0WJMTrj8/hrXuqlQ9zdUBHvpazGIn6uExgf8wwoZOliNA0lt3nSksNYjpRn oMmv0t57IW70p/mhb25KmRat3kRW5RjbBUZu3lPv5PsCUA2iZFVylW5vDxNaMLro9gNL lHUR5nkUbrDIvkLCDjf8U1osAsJh5jswSoWYrjgE1RTY91jN8csZJbKBriQjdQYGBpFv yBIZIQ9lwn2kI5fq+w2i16ePgW66B0MhmuYWScyODHlkmYQuG1hzdUzVAcuKw0xTFUhO wi/z7h6oPJ0M52/L56VQQHCFWHMPiuUc0NKDJneJO+m9oACLxL3fW6vs83zkM8lTBEw2 l/6A== X-Gm-Message-State: AOJu0YxELHiWethL+dQNxPA8XeU/Ft1k3YJ26dZsDGi71RzfMgWJQs0a +lomGCA2nXzQ/UI+/HPLAwG5X+ybiMtXjoWuc0pkx63gyjqagECcgEnax0mSOX8RTAUsct1w8R2 RkDwnx6DQxYftFNv6jQFQRZPXaXAQE+JELFkQ6z7ssFTqbQvnbMWM+HyFtnXJXVt5FjCmGWcfBx uTLkUZNepxXiQedG/qxTr+/3829xnLPKbiiusz X-Google-Smtp-Source: AGHT+IHx2eY3ruWvXtshUJvgJbilPLoFRasEzHA9+aft/6uAZQh7lGUaoHXy54K8bw4SquFTMMraIfE+bnU= X-Received: from pjqf22.prod.google.com ([2002:a17:90a:a796:b0:31e:fac4:4723]) (user=surenb job=prod-delivery.src-stubby-dispatcher) by 2002:a17:90a:d606:b0:31c:36f5:d95 with SMTP id 98e67ed59e1d1-321cf8fd07bmr429769a91.2.1755017646663; Tue, 12 Aug 2025 09:54:06 -0700 (PDT) Date: Tue, 12 Aug 2025 09:54:02 -0700 In-Reply-To: <2025081237-buffed-scuba-d3f3@gregkh> Precedence: bulk X-Mailing-List: stable@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: Mime-Version: 1.0 References: <2025081237-buffed-scuba-d3f3@gregkh> X-Mailer: git-send-email 2.51.0.rc0.205.g4a044479a3-goog Message-ID: <20250812165402.1011612-1-surenb@google.com> Subject: [PATCH 6.16.y] mm: fix a UAF when vma->mm is freed after vma->vm_refcnt got dropped From: Suren Baghdasaryan To: stable@vger.kernel.org Cc: Suren Baghdasaryan , Jann Horn , Vlastimil Babka , Lorenzo Stoakes , Liam Howlett , Andrew Morton 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. [surenb@google.com: v3] Link: https://lkml.kernel.org/r/20250729145709.2731370-1-surenb@google.com Link: https://lkml.kernel.org/r/20250728175355.2282375-1-surenb@google.com Fixes: 3104138517fc ("mm: make vma cache SLAB_TYPESAFE_BY_RCU") Signed-off-by: Suren Baghdasaryan Reported-by: Jann Horn Closes: https://lore.kernel.org/all/CAG48ez0-deFbVH=E3jbkWx=X3uVbd8nWeo6kbJPQ0KoUD+m2tA@mail.gmail.com/ Reviewed-by: Vlastimil Babka Acked-by: Lorenzo Stoakes Cc: Jann Horn Cc: Liam Howlett Cc: Signed-off-by: Andrew Morton (cherry picked from commit 9bbffee67ffd16360179327b57f3b1245579ef08) Signed-off-by: Suren Baghdasaryan --- include/linux/mmap_lock.h | 30 ++++++++++++++++++++++++++++++ mm/mmap_lock.c | 3 +-- 2 files changed, 31 insertions(+), 2 deletions(-) diff --git a/include/linux/mmap_lock.h b/include/linux/mmap_lock.h index 5da384bd0a26..ae9f89672574 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 5f725cc67334..5cd2b0789500 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(); -- 2.51.0.rc0.205.g4a044479a3-goog