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AJvYcCVd3b5EqQJitzMMIsImwzy/bdMfZaMQGkjmRaCbzhGJBF43iTb3FreZ5rtAHoWJRaSG220v32qFig==@kvack.org X-Gm-Message-State: AOJu0Yy03Ih56dA2jqCtnfnnjlNf3x2aXnfBfpJxqyv4+c4ugIrPhTWy AILeqmDnevXRlr4hHUzgcflNk+isDWyjBvxt2ZNJIDmn3/l1+V8T9gVP+5qulf8v0RSpZJmDXq7 CX/e0uA== X-Google-Smtp-Source: AGHT+IFd2+VRWI3M+Kfoauwo/cT4HXIsIJL90+W0WrGWufYg6PKRrmk/3I6NP6HrbQ2n89hPQHxfP8Zj3x4= X-Received: from plwp15.prod.google.com ([2002:a17:903:248f:b0:240:1be2:19ee]) (user=surenb job=prod-delivery.src-stubby-dispatcher) by 2002:a17:903:4b07:b0:240:4b3b:334f with SMTP id d9443c01a7336-2404b3b34a8mr26618875ad.34.1753722595188; Mon, 28 Jul 2025 10:09:55 -0700 (PDT) Date: Mon, 28 Jul 2025 10:09:50 -0700 Mime-Version: 1.0 X-Mailer: git-send-email 2.50.1.487.gc89ff58d15-goog Message-ID: <20250728170950.2216966-1-surenb@google.com> Subject: [PATCH 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-Queue-Id: 5C3F340014 X-Rspamd-Server: rspam06 X-Stat-Signature: 3s8cd5hmioqidsdxuke67astdo65k7mo X-HE-Tag: 1753722596-982413 X-HE-Meta: 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 mEkoqyqX 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 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 Cc: --- - 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 #include #include +#include #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); + mmdrop(vma->vm_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 -- 2.50.1.487.gc89ff58d15-goog